Maintaining a Vbe Multiplier's bias value
Jon Kirwan
I'd like to consider the Vbe multiplier often used in audio
amplifiers to maintain a bias voltage for the output stage.
The purpose is to better mitigate against ripple in the
unregulated power supply rails and against the the VAS
voltage output resulting from amplified signal voltages.
(The only active device under consideration is a BJT, though.
No JFETs or MOSFETS or opamps or other ICs.)
The basic starting form for a Vbe multiplier is shown in Fig.
1 and the bias voltage output is indicated there. Assume Q1
is thermally coupled in some magic way, for now, in just the
right way so that if the current through the Vbe multiplier
were perfectly stable, that the bias voltage would track just
as needed (The 'Eg' of Q1 is exactly what's needed for the
output stage's temperature tracking in some nice way and the
values of R1 and R2 are set correctly and the thermal
coupling and location is somehow where it needs to be.) The
focus is on the Vbe multiplier's variation of bias in the
face of changes in sourcing current at the top of Fig. 1.
>: +V
>: |
>: resistor or
>: current source
>: |
>: ,---+---,
>: | |
>: \ |
>: / R2 |
>: \ +-----> upper quadrant
>: / | ^
>: | | |
>: | |/c Q1 BIAS
>: +-----| VOLTAGE
>: | |>e |
>: \ | |
>: / R1 | v
>: \ +-----> lower quadrant
>: / |
>: | |
>: '---+---'
>: |
>: VAS ---'
>:
>: FIGURE 1
If I use a resistor as the load for the VAS, it's obvious to
me that the Vbe multiplier will need to cope with varying
currents. But even if I use a BJT (or two) to make a current
source sitting above the Vbe multiplier, it's still not going
to hold entirely still with +V ripple and with varying VAS
drive voltages. That variation will ultimately manifest
itself in a varying Vbe bias voltage. That will change the
operating point for the output stage.
If it is class-A, I suppose it doesn't matter that much. But
I don't want to be forced into class-A operation. Nor do I
want to be forced into regulated rails. So it becomes a
little more important, I think, to get this nailed down
better.
There's the problem, anyway.
To quantify how bad all this really is, I tried my hand at
figuring out the small signal analysis of the Vbe multiplier.
If I got a first approximation about right, it is based
squarely upon the small-re of the BJT. The very familiar
value for (kT/q)/Ic.
There is also the value of R2 shown in Fig. 1, but since its
effect is only affected by the change in base current, I
believe it's contribution is divided by Q1's beta. So the
actual equation is something like:
R_ac = (1/Ic)*(kT/q)*(1+R2/R1) + R2/beta
For a 2X multiplier where R2 is about R1, this is:
R_ac = (2/Ic)*(kT/q) + R2/beta
The Vbe multipler value is:
V_bias = Vbe*(1+(R2/R1)) + R2*Ic/beta
(The latter term being a correction for base current.)
Ignoring base current for now and assuming I had Ic set
around 5mA and placed R1=R2=1k for the 2X factor, this R_ac
value works out to about 15.4 ohms.
A variation of half an mA in Ic yields about 7.7mV change in
the bias point.
I decided to see if the Early effect made much of a
difference. The adjustment appears to be something like
this:
R_early = dV/dI = -Ic/VA*R^2
If I'm interpreting it right, it really does show as negative
resistance added to R_ac. The fuller equation, then,
including the Early effect, would be:
R_ac = (2/Ic)*(kT/q) + R2/beta - Ic/VA*R^2
(Which requires a quadratic solution to solve for R.)
If R_ac is 15.4 ohms and Ic is around 5mA, a VA of 100V would
suggest about R_early=-10mOhms. Which is roughly a factor of
1500 less than 15.4 Ohms. Since it now appears to be on the
order of 0.1% or so for typical Ic, VA, and, R_ac values, I
think I can ignore it for these considerations.
So drop it, I will.
I had scouted around a few weeks back (not for this reason)
and found what is shown in Fig. 2. I remembered it, but
didn't understand it then.
>: +V
>: |
>: resistor or
>: current source
>: |
>: ,---+---, <-- node A
>: | |
>: | \
>: | / R3
>: \ \
>: / R2 /
>: \ |
>: / +-----> upper quadrant
>: | | ^
>: | |/c Q1 |
>: +-----| BIAS
>: | |>e VOLTAGE
>: \ | |
>: / R1 | v
>: \ +-----> lower quadrant
>: / |
>: | |
>: '---+---'
>: |
>: VAS ---'
>:
>: FIGURE 2
I think I now understand why R3 was there. Changes in Ic
create changes in Q1's collector voltage, per Ic*R3. The
result is that dV=dI*R3. If R3 is on the order of the above
computed R_ac, then variations at node A caused by changing
currents through the Vbe multipler (most of which are seen as
Ic changes) will be neatly compensated for the change in the
voltage drop caused by R3.
However, that can only be set for some assumed Ic. Nearby
changes will work pretty well. But further deviations will
start to show problems again. Also, the Fig. 2 version will
use a slightly higher multiplier value to get node A up high
enough for the R3 drop to hit the right place required to
bias the output stage. That higher multiplier means that
while, let's say, the two (or four, if that's it) output
BJT's Vbe values vary over temp and the thermally coupled Q1
above also varies it's own Vbe value, the multiplier other
than 2 (or 4) will mean the variation of the bias will match
at only one place -- if it ever did more than one spot. How
important that is, I've not considered yet.
I'm wondering about additional topology changes to improve
the performance still more. Obviously, if they are crazy and
wild, I'm probably going to live with the above and be done
with it. But I think there's got to be something still
better. Another BJT as a bypass route across Q1 and R3?
Getting this nailed down should help mitigate against both
unreg supply ripple (on one side, anyway) putting hum into
the output and also against large scale changes in the VAS
amplified signal voltage (which means distortion.)
Jon
John Larkin
Hang a big capacitor across it.
John
Jamie
actually, I was going to suggest a diode in the base circuit to VAS to
help with thermo issues with that type of circuit..
oh well.
Jim Thompson
<jo...@infinitefactors.org> wrote:
What's a "VAS"?
What exactly are you trying to do?
My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
Thompson, because I could pull so much magic with Vbe compensation
methods ;-)
(Vbe multipliers generally are used just to create a smaller dead-band
that is temperature stable. Class AB bias is an art form of which I
am expert, but cannot divulge publicly at this time :-)
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
mi...@sushi.com
Less words and real schematics would get you more readers. [The only
thing worse than ascii equations are ascii schematics.]
In any event, just google improved vbe multiplier. I've seen all sorts
of circuits published to get lower impedance at the nodes.
George Herold
>
> - Show quoted text -- Hide quoted text -
>
> - Show quoted text -
"I'm wondering about additional topology changes to improve
the performance still more."
Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
wonder if you abandoned class A operation too early? Why not keep
things linear evreywhere and avoid the ‘dead band’? So what if you
need a bigger heat sink. It’s certainly a lot simpler.
George H.
Jon Kirwan
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>Hang a big capacitor across it.
Nice try.
Jon
John Larkin
No, seriously, that solves a bunch of problems.
John
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>What's a "VAS"?
Sorry. I read it somewhere regarding audio amplifiers and
the term stuck in my mind, I suppose. It's short-hand for
Voltage Amplifier Stage. It's almost so simple that no one
would bother creating a term for it, except that it seems as
though someone did and folks have used it in places where
I've been reading.
By the way, if you look at the semi-conceptual schematic at
the top of this page:
http://en.wikipedia.org/wiki/Electronic_amplifier
You will see Q3 acting as the VAS. Together with R6 it
converts the beta multiplied current into drive voltage.
(The Vbe/Ic transfer nasties this up, but I think it may be
survivable. Everything is important, but I'm leaving
worrying about this till later.)
That schematic isn't entirely realistic, either. R3/R4 are
better replaced with a mirror, regular, Wilson, or otherwise.
R5 is often itself a current source or sink (depending on
which way you flip the schematic polarities) and may be a BJT
and diodes or two BJTs, etc.
>What exactly are you trying to do?
If you look again at the schematic mentioned above, note the
function of D1 and D2. They stack to create a bias voltage.
That's used to set the point of operation for the output
stage (two-quadrant emitter follower -- which may be just two
BJTs as in that picture, or more.) Often, this is replaced
with an adjustable BJT configured as a Vbe multiplier. That's
what I'm trying to do. Except that I'd like to have the +V
and -V supply rails (ground is also present in the system) be
unregulated.
Part of the function of the Vbe multiplier is to also track
the Vbe requirements for the output stage as it heats up and
cools down. The variation of Vbe is quite large, as you
know, where the controlling Eg term in the Is(T) equation
overwhelms the otherwise oppositely-signed dV/dT of the
Shockley equation. Above -2mV/K. And with the exponential
dependance of Ic on Vbe... well, it serves that function as
well. So the Vbe value needs to track temperature in just
such a way that it maintains the design operating point for
the output stage, over temperature, while also ignoring
variations in the current that sources through it.
I'm trying to keep my options open, regarding the amplifier's
class. If it were operating class-A all the time, my limited
understanding suggests that some variation across the Vbe
multiplier isn't nearly as important as it clearly would be
for, say, class-B operation. I'm not exactly sure where I
want to wind up biasing things.
So I am slowly learning this stuff and, assuming the Vbe
multiplier has some part within it thermally coupled as
appropriate to some well-chosen part of the output stage,
trying to gather how I'd: (1) stabilize the voltage at some
fixed temperature T against variations in the current flowing
through it, and (2) calibrate it's Vbe multiplication factor
in just the right way so that it tracks well with the
effective Eg found in the Is(T) function of the output stage
needed to hold the operating point steady vs temperature.
My question here was regarding (1), not (2). I'm not far
enough along on that one to even begin on that one, yet. To
be honest, I just started learning about audio amplifier
design, including terms like VAS, starting around the 26th
last month. So I may be far off the mark in a few places.
I'm finding it a very interesting education, though, and I'm
glad I started down the road a small bit. But "being exact"
about what I want remains part of the learning process,
itself. So what you see here is as far as I've gotten to.
>My nickname, as a kid engineer at Motorola (48 years ago), was "Vbe"
>Thompson, because I could pull so much magic with Vbe compensation
>methods ;-)
Well, I can believe it. And I mean that as a sincere
compliment. If you can suggest something still better than
what I've already posted, I'd like to look at it.
>(Vbe multipliers generally are used just to create a smaller dead-band
>that is temperature stable.
In this case, I want it to track the output stage so I'm
going to have to couple it thermally in some useful way. What
I'm considering, right now, is how to make it immune to
unregulated supply variations and VAS output voltage swings.
>Class AB bias is an art form of which I
>am expert, but cannot divulge publicly at this time :-)
Well, I want to examine class-AB at some point. It may be
where I want to settle, though class-B would be quite fine
for my needs.
If you can't help with class-AB, then you can't. I will have
to struggle along. However, anywhere else you can send me a
clue I'd certainly appreciate it.
There is no interest other than personal. Certainly nothing
commercial in mind. I'm just a hobbyist trying to learn.
Jon
Jon Kirwan
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin
>><jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>>
>>>Hang a big capacitor across it.
>>
>>Nice try.
>>
>>Jon
>
>No, seriously, that solves a bunch of problems.
>
>John
Which problems does a slew-dependent, C*dV/dt bypass current
solve?
Jon
John Larkin
<jo...@infinitefactors.org> wrote:
>On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin
><jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan
>><jo...@infinitefactors.org> wrote:
>>
>>>On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin
>>><jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>>>
>>>>Hang a big capacitor across it.
>>>
>>>Nice try.
>>>
>>>Jon
>>
>>No, seriously, that solves a bunch of problems.
>>
>>John
>
>Which problems does a slew-dependent, C*dV/dt bypass current
>solve?
>
>Jon
A big cap across the biasing gadget keeps the voltage drop across it
fairly constant, of course. That nukes some of the problems you
referred to. More peak current is available to the output bases, for
example.
John
Tim Williams
news:okr1n55h5dvjjklg7...@4ax.com...
> Part of the function of the Vbe multiplier is to also track
> the Vbe requirements for the output stage as it heats up and
> cools down.
The general idea is to put the Vbe transistor on the same heatsink as the
outputs, if not glued to a transistor directly.
Unfortunately, for widely mismatched current densities, this doesn't work.
http://webpages.charter.net/dawill/Images/Ampere.gif
In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
tempco to compensate the far beefier (= lower current density??) output
darlingtons.
I was thinking of adding another CCS so a constant voltage drop appears on
the Vbe's base divider resistor. Algebraically subtracting a fairly stable
voltage results in the effective tempco (percentwise) increasing. The base
divider ratio has to be changed to compensate.
> In this case, I want it to track the output stage so I'm
> going to have to couple it thermally in some useful way. What
> I'm considering, right now, is how to make it immune to
> unregulated supply variations and VAS output voltage swings.
Don't worry about stability -- as John said, bypass and forget about it.
Most of the dynamic VAS/CCS current flows into the output stage, since
that's what it's there for anyway. The capacitor helps turn on the N side /
turn off the P side for rising edges and vice versa.
As for PSRR, the CCS's and gobs of feedback keep that in check. Of course,
in principle you need something to start the CCS's. ICs do this with a JFET
(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
sometimes both, to set a master current, from which everything else is
mirrored. Most discrete circuits just use a resistor, which is "0%" PSRR,
but it's not all that bad because the currents are balanced (*on average*,
which means you'll see IMD products when it's moving).
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
John Larkin
This topology, thermally coupled Vbe multiplier, was mediocre 50 years
ago. And still is.
John
Jon Kirwan
A trek of a thousand miles starts with but the first step.
Jon
Robert Baer
(depletion FETs are good here)?
Robert Baer
same die better still..National makes them.
Robert Baer
Ban
"Jon Kirwan" <jo...@infinitefactors.org> schrieb im Newsbeitrag
news:jg91n5d684ru5imsq...@4ax.com...
>I think this fits in sci.electronics.design, not .basics.
>
> I'd like to consider the Vbe multiplier often used in audio
> amplifiers to maintain a bias voltage for the output stage.
> The purpose is to better mitigate against ripple in the
> unregulated power supply rails and against the the VAS
> voltage output resulting from amplified signal voltages.
>
and is used to create and temperature compensate the bias voltage of the
output stage. A pur DC function. Since the power Transistors draw quite a
bit of quiescent current the ripple of the power supply will be higher then
without. The supply ripple is reduced mainly by negative feedback from
output to the input stage.
snip>
In class A you do not use this kind of bias generator
snip
I think you should understand before calculating. It is not much of a use to
view this stage in isolation without the output stage and the associated NFB
paths.
ciao Ban
Robert Baer
processes that greatly alter the pseudo-log I/V relationship in the
power device.
for small signal transistors, one can get over 10 decades matching to
ideal; with some power devices one can be lucky to get that over 3-5
decades.
Robert Baer
integer (2?).
mi...@sushi.com
> On Mon, 08 Feb 2010 19:09:28 -0700, Jim Thompson
>
Have you read Randy Slone's power amplifier book? This stuff really
isn't rocket science. Nor is AB. ;-)
<http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
dp/0071599258/ref=dp_ob_title_bk>
The black art is all in assembly, protection circuitry, and making
sure it starts up cleanly. [Most engineers never look at start up, so
you get designs that thump when you power them. I have lots of gear
with power-on thumps.]
I'd pick one of his MOS designs. Bipolar designs often have good
intentions, but ring like a bell. MOS is mushy, but predictably mushy.
Jon Kirwan
>Have you read Randy Slone's power amplifier book? This stuff really
>isn't rocket science. Nor is AB. ;-)
I haven't. It's not rocket science. But it is interesting
at my level.
><http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
>dp/0071599258/ref=dp_ob_title_bk>
I'll look, but the title appears more on the contruction
side. I am using this to educate myself a little better.
>The black art is all in assembly, protection circuitry, and making
>sure it starts up cleanly. [Most engineers never look at start up, so
>you get designs that thump when you power them. I have lots of gear
>with power-on thumps.]
I recall reading of a recommendation suggesting that all
electronic devices use less than 1W when on, but not
performing their intended application. This would also seem
to require a little added effort to achieve, as well. But I
take your point.
>I'd pick one of his MOS designs. Bipolar designs often have good
>intentions, but ring like a bell. MOS is mushy, but predictably mushy.
I'm still learning about BJTs. In fact, that's what this is
about for me. MOS later. ICs later. BJTs now.
Jon
Jon Kirwan
<rober...@localnet.com> wrote:
><snip>
> Have you considered making R2 and/or R3 constant current devices
>(depletion FETs are good here)?
I've not. Could you be more specific?
Jon
Jon Kirwan
wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> schrieb im Newsbeitrag
>news:jg91n5d684ru5imsq...@4ax.com...
>>I think this fits in sci.electronics.design, not .basics.
>>
>> I'd like to consider the Vbe multiplier often used in audio
>> amplifiers to maintain a bias voltage for the output stage.
>> The purpose is to better mitigate against ripple in the
>> unregulated power supply rails and against the the VAS
>> voltage output resulting from amplified signal voltages.
>>
>No this is not the purpose of this stage. It is used as an adjustable Zener
>and is used to create and temperature compensate the bias voltage of the
>output stage. A pur DC function. Since the power Transistors draw quite a
>bit of quiescent current the ripple of the power supply will be higher then
>without. The supply ripple is reduced mainly by negative feedback from
>output to the input stage.
I think your perspective is more comprehensive than mine,
obviously. And I am still building up bits, piecewise. It's
how I have to approach this.
That said, and I may yet be getting this wrong, but it seems
to me that I've seen some serious attention in amplifier
schematics; not only looking at global NFB to solve such
problems -- though that seems central, of course.
But I need to take things one at a time, right now. This is
education for me, after all. Not constructing an amplifier
to solve some problem I have. I've no problem focusing upon
this, a bit, until I subsume it, and then not wind up using
all the options I looked at. Learning doesn't only come from
taking all the right steps, but also from taking many others
that aren't entirely in the right direction. I could only
hope to be so perfect as to never choose wrongly. And if so,
I probably wouldn't be learning.
Anyway, the understanding I wrote can certainly be wrong.
However, your later comment seems to say it is wrong because
it's "better" to do it using the NFB from output to the diff
amp. Yet I still wonder if doing some of this locally is
appropriate. In any case, it seems certain that temp comp is
one of its functions. Unless I've somehow completely missed
things altogether.
If I set it "wide enough" it seems to operate that way.
Perhaps I've got that wrong, too?
>snip
>
>I think you should understand before calculating. It is not much of a use to
>view this stage in isolation without the output stage and the associated NFB
>paths.
I think it is useful for me to learn by studying the small
building blocks, right now, and considering some thoughts
(but not necessarily all the right ones) about larger issues
these may need to cope with. I'm in no way ready for the
"larger view" you are talking about. Not yet. And only a
rare few can start there. I'm not such. For me, it goes
from small to large, then back to small, then back out again,
and so on until it "gels."
I think I will take this structure just a little further in
thinking... perhaps a 3rd BJT, I'm thinking. But not more
than that. Diminishing returns. I was just wondering if
there was more I hadn't come across. Perhaps not.
Thanks,
Jon
Jon Kirwan
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>On Mon, 08 Feb 2010 22:11:51 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>On Mon, 08 Feb 2010 20:49:24 -0800, John Larkin
>><jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>>
>>>On Mon, 08 Feb 2010 20:43:03 -0800, Jon Kirwan
>>><jo...@infinitefactors.org> wrote:
>>>
>>>>On Mon, 08 Feb 2010 17:54:13 -0800, John Larkin
>>>><jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>>>>
>>>>>Hang a big capacitor across it.
>>>>
>>>>Nice try.
>>>>
>>>>Jon
>>>
>>>No, seriously, that solves a bunch of problems.
>>>
>>>John
>>
>>Which problems does a slew-dependent, C*dV/dt bypass current
>>solve?
>>
>>Jon
>
>A big cap across the biasing gadget keeps the voltage drop across it
>fairly constant, of course. That nukes some of the problems you
>referred to. More peak current is available to the output bases, for
>example.
What size cap would help with power supply ripple? Seems the
dV/dt is so small that a fair sized cap would be required to
make any difference. Similarly for low frequency amplified
signal out of the VAS. When you say "big," maybe you mean
it.
Ban is suggesting global NFB from output back to input.
You've said as much when you say to apply "lots of NFB." I
don't doubt the sincerity of either of you and I'm certain it
will do a lot. But right now I'm interested in seeing what
can be done right on this local subcircuit and at LF as well
as higher frequencies. Unless someone wants to walk me
through the thinking towards the larger concepts. I'm good
either way, as it's the learning that takes place I'm looking
for. But without such guidance, I need to move along at the
pace I can handle while guiding myself.
Jon
Jon Kirwan
<gghe...@gmail.com> wrote:
>><snip>
>"I'm wondering about additional topology changes to improve
>the performance still more."
>
>Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
>wonder if you abandoned class A operation too early? Why not keep
>things linear evreywhere and avoid the �dead band�? So what if you
>need a bigger heat sink. It�s certainly a lot simpler.
>
>George H.
Well, George... No, I've not abandoned it. Actually, it's my
hope to wind up building the amplifier and then operating it
(by hopefully choosing a design where that is possible) in
different modes for the learning experience of it. I hope
that is in the cards. I really do.
But to make a sharp point on it, although it's probably just
an extreme case, I remember reading about a 10W amplifier,
single channel, dissipating 120W! Creeps me out. So I
definitely _want_ to consider other classes of operation. And
cripes, I want to learn, anyway. So why not keep my options
open?
Jon
Jon Kirwan
<mi...@sushi.com> wrote:
><snip>
>Less words and real schematics would get you more readers. [The only
>thing worse than ascii equations are ascii schematics.]
ASCII is what I'll post. It's the only way to get them
archived or properly posted to a text newsgroup. I no longer
have access to the binary for schematics, sadly. If I lose
some people because they cannot manage fixed-spaced fonts, I
guess I lose them. I could place links up on my domain, I
suppose. But in this case, the schematics are really very
basic and not overly burdensome in ASCII. Besides, Win Hill
posted some really nice examples here, before. Folks seemed
to live with that. Not sure why you are picking on me, here.
>In any event, just google improved vbe multiplier. I've seen all sorts
>of circuits published to get lower impedance at the nodes.
Okay. I'll do that if folks here aren't interested at all in
talking about it.
Jon
Jon Kirwan
<tmor...@charter.net> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:okr1n55h5dvjjklg7...@4ax.com...
>> Part of the function of the Vbe multiplier is to also track
>> the Vbe requirements for the output stage as it heats up and
>> cools down.
>
>The general idea is to put the Vbe transistor on the same heatsink as the
>outputs, if not glued to a transistor directly.
>
>Unfortunately, for widely mismatched current densities, this doesn't work.
>http://webpages.charter.net/dawill/Images/Ampere.gif
>In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
>tempco to compensate the far beefier (= lower current density??) output
>darlingtons.
Which makes sense to me. I think I already understood this,
generally, if not in intimate detail. One of the reasons I
included in the opening salvo, talking about Eg matching.
>I was thinking of adding another CCS so a constant voltage drop appears on
>the Vbe's base divider resistor. Algebraically subtracting a fairly stable
>voltage results in the effective tempco (percentwise) increasing. The base
>divider ratio has to be changed to compensate.
I need to think about this, more. As you write above,
several alternatives appear in my mind and if you wouldn't
mind including a short example, I'd appreciate it.
>> In this case, I want it to track the output stage so I'm
>> going to have to couple it thermally in some useful way. What
>> I'm considering, right now, is how to make it immune to
>> unregulated supply variations and VAS output voltage swings.
>
>Don't worry about stability -- as John said, bypass and forget about it.
I can't agree, yet. Slow changes, without a crazy-sized cap
there, will have the same effective R_ac I'd mentioned
before. The cap's Z just won't change it. And I'm not ready
to chalk everything up and pile it all onto the global NFB,
either -- not because I disagree with you or John, because I
can't... I just don't know enough either way. But because
this whole post is about _learning_ something.
In particular, I was very specific about what I'd like to
study right now. Vbe multipliers and various incarnations
that may help to deal with current ripple (from 20Hz to
20kHz, I suppose.) I'm wanting to make sure my analysis so
far isn't grossly wrongly made, accepting corrections as they
arrive, and I'd like to consider interesting ideas, too.
Hopefully, my question here on this narrow subject won't be
taken as "Well, what does he know about the field of audio
amplifier design?" If that's the question to be asked, the
answer is easy. "Not much."
Cripes, I just started looking at the whole idea about two
weeks ago. If I knew enough to ask all the right questions
on this topic in barely more than 10 days, I'd likely be
headed into being the next Bobby Fischer of audio amps!
I'm just a hobbyist, for gosh sake. I found a few circuits
on the web that included a collector resistor in the Vbe
multiplier and, at first, had no idea why it was there. I
asked in .basics and no one else seemed firm about knowing,
either. I grew more curious about it and sat down and lo,
and behold, the scales fell from my eyes and I could _see_! I
could actually see why it was there. Not only why, but how
to estimate quantitative values for it and what to expect as
a result. It's that sense of discovery that sometimes pushes
one further.
So I want to understand a little better how one might do even
more about compensating vs current variations? In this
focus, I don't even need to care about amplifier design, at
all. It is purely about the Vbe multiplier right now and
nothing else. Sure, audio amplifier design questions caused
me to look more closely at this structure. That was my
inspiration to set down this short path, right now.
But is it wrong to want to explore this area a little more
before moving on?
>Most of the dynamic VAS/CCS current flows into the output stage, since
>that's what it's there for anyway.
I think I see that, though I'll see it a lot better later on.
Hopefully where I'll be able to put quantities to it.
>The capacitor helps turn on the N side /
>turn off the P side for rising edges and vice versa.
I think I gather that much. It's got very low impedance when
the dV/dt is there.
>As for PSRR, the CCS's and gobs of feedback keep that in check.
Yes, and yes. John L. mentioned this, too, a week ago and
more. No question I've got the point, there. The CCS's
aren't perfect and where they don't do so well, it gets all
nicely lumped into the global NFB and left for it to deal
with. I don't mind, though, investigating things just a
little more. And I will very soon start taking on the CCS's
themselves. I know a few and I know there are a lot more
than I'm not even remotely aware of, too. So that is going
to be fun. But I'm not one to just borrow and run. I need
to _understand_ the mathematics and try my hand at deriving
certain features in quantitative ways, not just qualitative
ones. I expect to analyze at least four or five different
CCS structures before I move on, in what quantitative detail
I can manage at the time.
>Of course,
>in principle you need something to start the CCS's. ICs do this with a JFET
>(i.e. current regulating diode) or bandgap reference (e.g., TL431), or
>sometimes both, to set a master current, from which everything else is
>mirrored.
I've seen that done time and time again in ICs. I can
remember tracing my fingers from one to another to another as
I spent time understanding them better.
Last July, in fact, here in this very group, I posted this
about the LM334:
>: By the way, I just looked at the general schematic for the LM334 on
>: National's datasheet and with a quick sweep of my arms came up with a
>: design Iset/Ibias of 8, not 16 as they show on page 5. I'm off by a
>: factor of two.
>:
>: My logic went like this. 1/2 of the I from V+ flows via Q6 to the R
>: rail. 1/4 via Q4 and 1/4 via Q5. Q5's 1/4*I flows via Q1 to the R
>: rail, too. So now up to 3/4*I into the R rail. Q4's 1/4*I passes
>: through two paths. The Ic(Q2)=Ic(Q1)/2... but Ic(Q1)=1/4*I, so that
>: is 1/8*I, leaving the other 1/8*I for Q3's Vbe conduction, which also
>: flows to the R rail. So the R rail gets 7/8*I and the V- picks up
>: 1/8*I. Multiplying through by 8 to get rid of the divisor, I see a
>: factor of 8 for Iset/Ibias... not 16.
>:
>: Can someone do a quick description about how to arrive at something
>: more like 16? I'm missing a clue (or two.)
No one here _did_ fully answer my question, Tim. There it
is, and I did try to follow it.
I'm aware of the frequent practice, at least.
>Most discrete circuits just use a resistor, which is "0%" PSRR,
>but it's not all that bad because the currents are balanced (*on average*,
>which means you'll see IMD products when it's moving).
>
>Tim
Intermodulation distortion?? I never saw the term IMD
before, but that seems as though it must be what you just
said. Fits, anyway. Which brings me back to the MC1495,
again. And yes, I think I see why you bring it up.
Jon
Bitrex
Hey Jon, I found a derivation of the input impedance of the two-resistor
/transistor Vbe multiplier you might be interested in looking at:
http://paginas.fe.up.pt/~fff/eBook/MDA/Mult_Vbe.html
For bypassing purposes the rule of thumb I've always heard is to make
the impedance of the capacitor 1/10th the value of the impedance looking
in to the circuit at the lowest audio frequency.
Jim Thompson
Miso is all blabber and no content... best if ignored ;-)
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
John Larkin
A trek of 23,000 miles starts with but the first step in the wrong
direction.
John
John Larkin
Can you explain "mushy" in any more technical terms?
John
Jim Thompson
(1) Split R1, bypass that junction to ground
(2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and
bypassed.
(3) As you said, replace R4:R3 with a mirror, I don't think a compound
device mirror, such as Wilson, is necessary. Study this if you want
more info....
http://analog-innovations.com/SED/EnhancedCurrentMirrors.pdf
(4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe,
losing about 1/5 of the Q3 quiescent current in the resistors.
Bypassing base-to-base (of Q4-Q5) will help at all but very low
frequencies.
(5) Long haul as you "oomph" the power:
Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more
complicated (Darlington extension of Vbe multiplier).
Start simple, then grow it, that way you learn before you flame it ;-)
Jim Thompson
Bob Monsen
news:c7u2n5ldssoc0drl2...@4ax.com...
Not while I'm drinking my coffee, please... :)
Regards,
Bob Monsen
George Herold
> On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold
>
It might have been here,
http://www.passdiy.com/default.html
I got to reading about amplifiers on the above site... Do in part to
your interest.
George H.
Bob Monsen
> I think this fits in sci.electronics.design, not .basics.
>
>
Sorry, I didn't read the entire message...
However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
Somewhat more expensive, but it'll be VERY stiff.
However, you don't really want to hold that value constant. You want the
voltage to compensate for the temperature of the output transistors. You
might be able to use a diode to track the temperature change, and then use
that in the feedback loop to compensate the TLV431.
A honking big capacitor, one that has very low impedance at your frequencies
of interest, is probably the best idea I've seen on the thread.
On a related note, there was an article in a recent EDN about a self biasing
preamp which was kinda cool. Instead of trying to track the difference using
diodes or a multiplier, it used a couple of transistors and an opamp to set
the correct values at the bases of the pass transistors. It was so novel (at
least to me) that I typed it into LTSpice. Here it is:
Version 4
SHEET 1 948 680
WIRE -288 -304 -608 -304
WIRE -16 -304 -288 -304
WIRE 144 -304 -16 -304
WIRE 320 -304 144 -304
WIRE 512 -304 320 -304
WIRE -288 -272 -288 -304
WIRE -608 -240 -608 -304
WIRE 320 -240 320 -304
WIRE 320 -128 320 -160
WIRE 320 -128 240 -128
WIRE 512 -128 512 -304
WIRE 448 -80 384 -80
WIRE 240 -64 240 -128
WIRE -608 -16 -608 -160
WIRE -160 -16 -608 -16
WIRE -128 32 -464 32
WIRE 48 32 -48 32
WIRE 112 32 48 32
WIRE 512 32 512 -32
WIRE 512 32 192 32
WIRE 560 32 512 32
WIRE 656 32 624 32
WIRE 672 32 656 32
WIRE -464 96 -464 32
WIRE 48 96 48 32
WIRE 128 96 48 96
WIRE 240 96 240 0
WIRE 240 96 192 96
WIRE 320 96 320 -32
WIRE 512 96 512 32
WIRE 672 128 672 32
WIRE -608 144 -608 -16
WIRE -608 144 -704 144
WIRE 32 144 -96 144
WIRE 448 144 384 144
WIRE 144 160 144 -304
WIRE -464 176 -464 160
WIRE -16 176 -16 -304
WIRE 48 176 48 96
WIRE 112 176 48 176
WIRE -96 192 -96 144
WIRE -48 192 -96 192
WIRE 240 192 240 96
WIRE 240 192 176 192
WIRE 320 192 240 192
WIRE -464 208 -464 176
WIRE 32 208 32 144
WIRE 32 208 16 208
WIRE 112 208 32 208
WIRE -160 224 -160 -16
WIRE -48 224 -160 224
WIRE -608 240 -608 144
WIRE -704 256 -704 144
WIRE -704 352 -704 320
WIRE -608 352 -608 320
WIRE -608 352 -704 352
WIRE -464 352 -464 288
WIRE -464 352 -608 352
WIRE -288 352 -288 -192
WIRE -288 352 -464 352
WIRE -272 352 -288 352
WIRE -16 352 -16 240
WIRE -16 352 -272 352
WIRE 144 352 144 224
WIRE 144 352 -16 352
WIRE 512 352 512 192
WIRE 512 352 144 352
WIRE 672 352 672 208
WIRE 672 352 512 352
FLAG -272 352 0
FLAG 656 32 out
FLAG -464 176 in
SYMBOL npn 384 96 M0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn 448 -128 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL pnp 384 -32 R180
SYMATTR InstName Q4
SYMATTR Value 2N3906
SYMBOL pnp 448 192 M180
SYMATTR InstName Q5
SYMATTR Value 2N3906
SYMBOL voltage -288 -288 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL cap 224 -64 R0
SYMATTR InstName C1
SYMATTR Value 10�F
SYMBOL res 208 16 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res -32 16 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R4
SYMATTR Value 100
SYMBOL voltage -464 192 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value SINE(0 .2 1k)
SYMBOL cap -480 96 R0
SYMATTR InstName C2
SYMATTR Value 10�F
SYMBOL cap 624 16 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 470�
SYMBOL res 656 112 R0
SYMATTR InstName R5
SYMATTR Value 8
SYMBOL res 304 -256 R0
SYMATTR InstName R1
SYMATTR Value 1k5
SYMBOL Opamps\\LT6234 144 192 R0
SYMATTR InstName U1
SYMBOL res -624 224 R0
SYMATTR InstName R9
SYMATTR Value 1k
SYMBOL cap 192 80 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C5
SYMATTR Value 10pF
SYMBOL res -624 -256 R0
SYMATTR InstName R3
SYMATTR Value 1k
SYMBOL Opamps\\LT6234 -16 208 R0
SYMATTR InstName U2
SYMBOL cap -720 256 R0
SYMATTR InstName C4
SYMATTR Value 1�F
TEXT 552 -296 Left 0 !.tran 0 .1 0 1u
TEXT 552 -256 Left 0 !.four 1k 10 v(out)
TEXT 552 -216 Left 0 ;.noise V(out) V2 oct 1001 1 100k
Regards,
Bob Monsen
Jon Kirwan
That one takes an approach that I'm not familiar with and
didn't take. I'll have to consider the approach more.
However, I did take a look at the end of it. It says:
R = (R1+(R2||re)) / (1+(1/R1+gm)*(R2||re))
If I understand the value gm, and I may not, it's just 1/re
or else re=1/gm. Basically, just the (kT/q)/Ic I'd mentioned
when I wrote. If that is the case, I used these to see how
that page predicts:
ic=.005
vt=k*300/q
gm=ic/vt
re=1/gm
r1=1000
r2=1000
r2p=r2*re/(r2+re)
and then computed:
(r1+r2p)/(1+(1/r1+gm)*r2p)
and got:
502.5719049 Ohms.
This is so far from my own calculations of about 15.4 Ohms
that I just _had_ to put this into LTspice and test it. To
do that, I simply set up the basic circuit with the two
resistors and BJT and then hooked up a variable current
source to the topside. I set it up as an AC source of 5mA
with peaks of 500uA, and then ran a .TRAN on it and plotted
the upper rail of the structure's voltage. I used a 2N2222
BJT, as well. Convenient, and I have them laying about.
Anyway, so I ran the sims and got 17.44mV, peak to peak.
Divided by the peak to peak current variation of 1mA gives an
apparent R of 17.44 Ohms. My calculations arrived at 15.4
Ohms, or so.
All this could be operator error. I may be operating the web
page you suggested incorrectly, so that the 503 Ohms I get is
because I didn't know what I was plugging in and where. I
may be operating LTspice incorrectly, so that it's results
aren't usable and it's just luck that the numbers worked out
in my favor.
But there it is.
Here is the LTspice file:
Version 4
SHEET 1 880 680
WIRE 128 0 16 0
WIRE 224 0 128 0
WIRE 288 0 224 0
WIRE 128 32 128 0
WIRE 16 112 16 0
WIRE 224 112 224 0
WIRE 128 160 128 112
WIRE 160 160 128 160
WIRE 128 208 128 160
WIRE 16 224 16 192
WIRE 128 320 128 288
WIRE 224 320 224 208
WIRE 224 320 128 320
WIRE 128 336 128 320
FLAG 128 336 0
FLAG 288 0 V_rail
FLAG 16 224 0
SYMBOL npn2 160 112 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL res 112 192 R0
SYMATTR InstName R1
SYMATTR Value 1k
SYMBOL res 112 16 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL current 16 192 R180
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value SINE(5m 500u 50)
TEXT -76 296 Left 0 !.tran 1
>For bypassing purposes the rule of thumb I've always heard is to make
>the impedance of the capacitor 1/10th the value of the impedance looking
>in to the circuit at the lowest audio frequency.
Well, let's assume that I got lucky and LTspice and I agree
on the figure of about 16 Ohms. With a signal at 20Hz, we
are talking:
C = 1/(2 PI f (R_ac/10)) = 5000uF
Yikes. John L. wasn't kidding when he wrote "big." Luckily,
in steady state it could be a low voltage cap!
Jon
Jon Kirwan
<jjla...@highNOTlandTHIStechnologyPART.com> wrote:
There is no wrong direction at the start. It's all good.
Jon
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>(1) Split R1, bypass that junction to ground
Understood.
>(2) Make R5 and R6 into mirrors, resistor feed from VDD, but split and
> bypassed.
I had been thinking more like this structure:
>: to input to voltage
>: stage mirror amp stage
>: | ,---, |
>: | | | |
>: | | | |
>: | gnd | |
>: | \ |
>: | / R4 |
>: | \ |
>: | / |
>: | | |
>: | ,------+ |
>: | | | |
>: | --- C1 | |
>: | --- | |
>: | | \ |
>: | | / R3 |
>: | | \ |
>: | | / |
>: | Vdd | |
>: Q2 c\| | R5 |/c Q3
>: |-----------+----/\/\-----|
>: e<| | |>e
>: | | |
>: | R1 |/c Q1 |
>: +---/\/\----| |
>: | |>e \
>: | | / R6
>: | | \
>: \ | /
>: / R2 | |
>: \ | |
>: / | |
>: | | |
>: Vdd Vdd Vdd
However, I take your point.
>(3) As you said, replace R4:R3 with a mirror, I don't think a compound
>device mirror, such as Wilson, is necessary.
Understood. Although I'm not able to make my own decisions
on this, yet, I've read repeatedly that the distortions to
deal with are not at the input stage. The input stage can be
made better, the improvements are small in comparison to what
remains in the rest of a well-designed system. Point taken.
>Study this if you want more info....
>
> http://analog-innovations.com/SED/EnhancedCurrentMirrors.pdf
Thanks, I will!
>(4) Since you're on a learning curve, just replace D1/D2 with 1.5*Vbe,
>losing about 1/5 of the Q3 quiescent current in the resistors.
Thanks for taking a moment to confirm the "1/5th" division.
I'd already figured that was commonly done and had some ideas
of my own about why that makes sense. (I could talk about
that, but I'm sure you already know and I think I know, too.)
>Bypassing base-to-base (of Q4-Q5) will help at all but very low
>frequencies.
Okay. That's how I see it, too.
>(5) Long haul as you "oomph" the power:
> Q3 goes to Darlington, as do Q4 and Q5; D1/D2 becomes more
>complicated (Darlington extension of Vbe multiplier).
This is what I'd like to explore, right now. Extensions.
It's because it is where my mind is at, right now. And I
want to explore this more fully before walking away from it
and moving on.
>Start simple, then grow it, that way you learn before you flame it ;-)
hehe. Good advice, of course. As I'm still struggling to
make sure I understand each piece, right now, I'm just not
yet ready to put it all together -- not even in a low power
system. I might be able to vaguely grasp what I would be
doing, but I prefer taking each part and thoroughly looking
at its function before moving on. Then, when I look once
again at the whole, I can better "read" what I see and that
helps a lot in terms of gaining a global view. I'm still in
the trenches, right now, and not allowing myself to raise my
head much above that until I get some of the details nailed
down.
Speaking of that, can you confirm (or correct) the equation I
developed for the simple Vbe multiplier's small signal R? Or
the relative scale and _sign_ of the Early effect correction
to it, which I peg near -1 part per thousand in the case I
cited?
All this is good for me to go through.
Thanks,
Jon
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
><snip>
>Useless nonsense....
Are you talking about my comments, those of others here, or
the link you posted below?
>http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
I'll take a look, today.
Thanks,
Jon
bg
Jon Kirwan wrote in message ...
>I think this fits in sci.electronics.design, not .basics.
>
>amplifiers to maintain a bias voltage for the output stage.
>The purpose is to better mitigate against ripple in the
>unregulated power supply rails and against the the VAS
>voltage output resulting from amplified signal voltages.
>
>No JFETs or MOSFETS or opamps or other ICs.)
>
>The basic starting form for a Vbe multiplier is shown in Fig.
>1 and the bias voltage output is indicated there. Assume Q1
>is thermally coupled in some magic way, for now, in just the
>right way so that if the current through the Vbe multiplier
>were perfectly stable, that the bias voltage would track just
>as needed (The 'Eg' of Q1 is exactly what's needed for the
>output stage's temperature tracking in some nice way and the
>values of R1 and R2 are set correctly and the thermal
>coupling and location is somehow where it needs to be.) The
>focus is on the Vbe multiplier's variation of bias in the
>face of changes in sourcing current at the top of Fig. 1.
>
>>: |
>>: resistor or
>>: current source
>>: |
>>: | |
>>: | \
>>: | / R3
>>: \ \
>>: / R2 /
>>: \ |
>>: / +-----> upper quadrant
>>: | | ^
>>: | |/c Q1 |
>>: +-----| BIAS
>>: | |>e VOLTAGE
>>: \ | |
>>: \ +-----> lower quadrant
>>: / |
>>: | |
>>: '---+---'
>>: |
>>: VAS ---'
>>:
>
>I think I now understand why R3 was there. Changes in Ic
>create changes in Q1's collector voltage, per Ic*R3. The
>result is that dV=dI*R3. If R3 is on the order of the above
>computed R_ac, then variations at node A caused by changing
>currents through the Vbe multipler (most of which are seen as
>Ic changes) will be neatly compensated for the change in the
>voltage drop caused by R3.
>
>However, that can only be set for some assumed Ic. Nearby
>changes will work pretty well. But further deviations will
>start to show problems again. Also, the Fig. 2 version will
>use a slightly higher multiplier value to get node A up high
>enough for the R3 drop to hit the right place required to
>bias the output stage. That higher multiplier means that
>while, let's say, the two (or four, if that's it) output
>BJT's Vbe values vary over temp and the thermally coupled Q1
>above also varies it's own Vbe value, the multiplier other
>than 2 (or 4) will mean the variation of the bias will match
>at only one place -- if it ever did more than one spot. How
>important that is, I've not considered yet.
>
>wild, I'm probably going to live with the above and be done
>with it. But I think there's got to be something still
>better. Another BJT as a bypass route across Q1 and R3?
>
>Getting this nailed down should help mitigate against both
>unreg supply ripple (on one side, anyway) putting hum into
>the output and also against large scale changes in the VAS
>amplified signal voltage (which means distortion.)
>
>Jon
not work well with large signal swings and it lowers the input impedance. A
lower input impedance means that the drivers will also need a lower output
impedance. The bottom line is that you will not see this bias method used in
a power output stages. Another option is to use emitter feedback but for the
emitter resistors to be effective, they will drop alot of signal output and
waste power, which explains why those resistors are usually very low values,
They have very little effect unless the emitter current is large. At the DC
bias current level, they don't do a thing.
Local feedback (emitter feedback or collector feedback) both create more
problems then they solve for stabilizing the power output stage bias point.
To keep the output stage bias point stabilized, the use of overall feedback
is the standard practise. A simple typical amplifier might have a
differential input stage, followerd by a voltage amplifier, followed by the
power output stage. The output from the power stage is fed back to the
differential input stage. High open loop gain with large feedback is the key
to better stabilization of the operating points. Fix it with feedback is a
term to remember.
The typical bias chain using diodes can be made with resistors as well, but
diodes have the advantage of dropping the bias voltage while having a lower
impedance to the signal. Sometimes you will see those bypassed with a large
cap if the impedance causes to much signal loss. Diodes can also offer
temperature compensation. In any case, an output stage will have way more
current flowing in that bias chain than is actually needed as base bias
current. The voltage drops developed in the bias chain will not be greatly
affected by changes in the base emitter junction because the base bias
current is small compared to the current in the bias chain. And remember,
that " Fix it with feedback " applies here too. So variations in the power
supply have a very reduced effect on the bias point. The feedback signal is
a voltage, and enough feedback will compensate to keep the output voltage
offset at zero. It will not compensate for for excessve collector currents
or power dissaption if the offset voltage remains low. That is why
temperature compensation is used too.
In the early years of transistors, it was common to see transistor stages
using many of the techniques used with vacuum tubes. Dc coupled amplifiers
were rare, because any bias shift was amplified in further stages. Feedback
was applied locally, and overall feedback had no effect on the DC operating
points. The trend now is to stabilize everything with feedback. It works,
and it works well. Unless you are a purist and have some religious reason to
avoid this technique, there is no sense in reinventing the wheel.
Jon Kirwan
Egads. Loads of PDF files. Now I have to create a
directory, download them one by one, and then call them up
with my slow machine to look. Any particular page or file
where you saw it? (No, that isn't where I saw the comment.)
But thanks for the link. I'll add it to those I read, also.
Jon
Jim Thompson
The link. Do the math, it's a hoax, good only at one current and
temperature pair... besides being Beta sensitive.
Jon Kirwan
<rcmo...@gmail.com> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:jg91n5d684ru5imsq...@4ax.com...
>> I think this fits in sci.electronics.design, not .basics.
>>
>>
>> Jon
>
>Sorry, I didn't read the entire message...
>
>However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
>Somewhat more expensive, but it'll be VERY stiff.
I'm still in "discrete" mode. For example, I am _less_
interested in opamp topologies and design techniques than I
am in _how_ to design opamps. There is nothing like knowing
the details about how they are designed inside to understand
the gotchas that aren't readily accessible to someone using
them.
A comparison here might be like "using a handgun" vs
"understanding how handguns are designed and built." A
gunsmith requires a very detailed knowledge and while this
level of detailed knowledge may not make them a better
shooter, that knowledge still informs them about the handgun
in ways that most shooters have little idea about. And I
think it prepares them for certain unusual circumstances a
little better.
I'm at the gunsmith level, right now. I am NOT wanting to go
shooting, just yet.
>However, you don't really want to hold that value constant. You want the
>voltage to compensate for the temperature of the output transistors.
Yes.
>You might be able to use a diode to track the temperature change, and then use
>that in the feedback loop to compensate the TLV431.
No ICs. I might like to thoroughly _understand_ the internal
design of the TLV431, first. Then I'm willing to use it.
>A honking big capacitor, one that has very low impedance at your frequencies
>of interest, is probably the best idea I've seen on the thread.
Well, I'm interested in focusing on the crafted design of Vbe
multipliers, right now. I can _always_ slap a cap on
whatever that winds up being, later on. So set that aside.
What also bugs me is how that darned thing is going to
interact with the larger system, eventually. I don't like
ignorantly littering a schematic with poles and zeros and
phase delays where right now I have very little idea right
what then happens when I close the outer NFB loop. I'm still
"in the trenches" and trying to understand each piece in
detail and think at that level. The capacitor is at the next
level above and is outside my "view."
Besides, it doesn't do much for LF. The Z is too high and in
parallel, ignorable.
>On a related note, there was an article in a recent EDN about a self biasing
>preamp which was kinda cool. Instead of trying to track the difference using
>diodes or a multiplier, it used a couple of transistors and an opamp to set
>the correct values at the bases of the pass transistors. It was so novel (at
>least to me) that I typed it into LTSpice.
><snip>
Okay. I'm going to save it, too. I'm not ready to
assimilate it, of course. But I definitely want it around
when I _am_ ready for it.
Thanks,
Jon
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Tue, 09 Feb 2010 12:59:01 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>On Tue, 09 Feb 2010 08:33:45 -0700, Jim Thompson
>><To-Email-Use-Th...@My-Web-Site.com> wrote:
>>
>>><snip>
>>>Useless nonsense....
>>
>>Are you talking about my comments, those of others here, or
>>the link you posted below?
>>
>>>http://home.comcast.net/~mercerd/MobileStudioProject/Activity_6_zero_gain_amp.pdf
>>
>>I'll take a look, today.
>>
>>Thanks,
>>Jon
>
>The link. Do the math, it's a hoax, good only at one current and
>temperature pair... besides being Beta sensitive.
>
> ...Jim Thompson
Ah! Thanks! I can use lessons like this, too! If I can see
what you see there, then that means something. Very good way
to teach. Will keep your points in mind as I read it.
Jon
Phil Allison
"Tim Williams"
>
> The general idea is to put the Vbe transistor on the same heatsink as the
> outputs, if not glued to a transistor directly.
>
> Unfortunately, for widely mismatched current densities, this doesn't work.
** Huh ??
More gobbledegook presented as fact.
> http://webpages.charter.net/dawill/Images/Ampere.gif
> In this boringly typical circuit, the 2N3904 Vbe mult. doesn't have enough
> tempco to compensate the far beefier (= lower current density??) output
> darlingtons.
** Proof by assertion and an isolated example known only to the poster.
Gotta love that on usenet.
.... Phil
Tim Williams
news:12657460...@sj-nntpcache-3.cisco.com...
> On a related note, there was an article in a recent EDN about a self
> biasing preamp which was kinda cool. Instead of trying to track the
> difference using diodes or a multiplier, it used a couple of transistors
> and an opamp to set the correct values at the bases of the pass
> transistors. It was so novel (at least to me) that I typed it into
> LTSpice. Here it is:
Could you take a screenshot of the schematic?
Tim
--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Jon Kirwan
wrote:
><snip>
>Your circuit is an example of collector feedback. Collector feedback does
>not work well with large signal swings and it lowers the input impedance. A
>lower input impedance means that the drivers will also need a lower output
>impedance. The bottom line is that you will not see this bias method used in
>a power output stages. Another option is to use emitter feedback but for the
>emitter resistors to be effective, they will drop alot of signal output and
>waste power, which explains why those resistors are usually very low values,
>They have very little effect unless the emitter current is large. At the DC
>bias current level, they don't do a thing.
>Local feedback (emitter feedback or collector feedback) both create more
>problems then they solve for stabilizing the power output stage bias point.
>To keep the output stage bias point stabilized, the use of overall feedback
>is the standard practise. A simple typical amplifier might have a
>differential input stage, followerd by a voltage amplifier, followed by the
>power output stage. The output from the power stage is fed back to the
>differential input stage. High open loop gain with large feedback is the key
>to better stabilization of the operating points. Fix it with feedback is a
>term to remember.
This point is made, again and again, so it must be true! And
if it weren't true, why else would opamps have been such a
successful building block?? So I completely buy the idea.
I am still trying to study each part, though. At some point,
I will raise my head a bit above that level and take a larger
look. But I'm not yet prepared for it, as the pieces
themselves are still too fuzzily understood. I want to
quantify those, in detail, before expanding my view. In
doing so, I hope to have a somewhat better understanding of
opamps, themselves, too. Not just from a large scale view,
but also in understanding problems within them and how to
choose among various approaches when struggling with a
specific application in mind.
I take your point. But it doesn't change, at all, my
interest in seeing what can be reasonably done with at teh
Vbe multiplier level to accomodate variations in current.
That remains interesting in and of its own right.
>The typical bias chain using diodes can be made with resistors as well, but
>diodes have the advantage of dropping the bias voltage while having a lower
>impedance to the signal.
That makes sense.
>Sometimes you will see those bypassed with a large
>cap if the impedance causes to much signal loss.
I had thought of that, as well. Though, of course, I hadn't
put values to it.
>Diodes can also offer temperature compensation.
Their Eg and N would seem to suggest some difficulties with
curve matching, but I agree broadly.
>In any case, an output stage will have way more
>current flowing in that bias chain than is actually needed as base bias
>current.
This seems to argue with something I read John L. saying, but
I didn't accept it (or reject it), yet. I will need to get
there in due course. But I'm still taking pieces one at a
time.
>The voltage drops developed in the bias chain will not be greatly
>affected by changes in the base emitter junction because the base bias
>current is small compared to the current in the bias chain.
I read this sentence a few times to try and make sure I
followed it well. If I do, and I may not, I think I
addressed this when I tried to calculate the R_ac figure.
The numbers I come up with for a 5mA "bias chain" current
with 1mA in the base bias current and 4mA in the collector,
come out as around 15 Ohms, or so. If I'm right about that,
it seems almost certain that there is _some_ response to even
modest variations in current through it. A 500uA change
yields a 7.5mV change. When I LTspice it, I get a simulation
that matches what I calculate, too.
>And remember,
>that " Fix it with feedback " applies here too.
Yes, the mantra is slowly deepening within me.
>So variations in the power
>supply have a very reduced effect on the bias point. The feedback signal is
>a voltage, and enough feedback will compensate to keep the output voltage
>offset at zero. It will not compensate for for excessve collector currents
>or power dissaption if the offset voltage remains low.
Good point for me to remember!! Thanks.
>That is why temperature compensation is used too.
I begin to see, better. Thanks, again.
>In the early years of transistors, it was common to see transistor stages
>using many of the techniques used with vacuum tubes. Dc coupled amplifiers
>were rare, because any bias shift was amplified in further stages. Feedback
>was applied locally, and overall feedback had no effect on the DC operating
>points.
I seem to recall that vacuum tube amplifiers even let the
consumer modify the global NFB. But, as you say, since it
wasn't so critical to the design that was probably why it was
allowed in the first place. With BJTs, it seems now to me
that global NFB is _so_ important that such things cannot be
left as "tweeks" by some consumer playing with a knob!
>The trend now is to stabilize everything with feedback. It works,
>and it works well. Unless you are a purist and have some religious reason to
>avoid this technique, there is no sense in reinventing the wheel.
It's not a religious reason, unless _learning_ is a religion,
I suppose. I don't mind being told that "one day when you
are ready, you will use global NFB to take care of this." I
can gather and accept it, of course. But I also cannot
believe an amplifier can be designed with bags of random
bolts tossed together and "fixed with global NFB" in the end.
There are parts in there and they need to perform some
intended function to some reasonable approximation. And I am
still working on understanding each piece as well as some
thoughts about various approaches at that level to improve
the ideas.
For example, it's important to understand not just vaguely,
but quantitatively on various scores, how a diff-amp behaves
and why I may want to have a current mirror on the tails. I
don't want to just hear "put a current mirror there" and
learn nothing then about why. Later on, when I'm looking
globally at an amplifier, I can look backwards and say, "Hmm.
That Wilson mirror is great, but I really don't need it. The
bog standard 2-BJT mirror is fine enough." But I want to say
that from _understanding_ the details, not from others merely
assuring me about it.
See the difference?
Meanwhile, I'm still interested in seeing if my quantitative
analysis was correct (or wrong) and if there are some other
topologies for it, other than the two I mentioned, that may
be interesting to look at.
Thanks,
Jon
Jon Kirwan
<tmor...@charter.net> wrote:
>"Bob Monsen" <rcmo...@gmail.com> wrote in message
>news:12657460...@sj-nntpcache-3.cisco.com...
>> On a related note, there was an article in a recent EDN about a self
>> biasing preamp which was kinda cool. Instead of trying to track the
>> difference using diodes or a multiplier, it used a couple of transistors
>> and an opamp to set the correct values at the bases of the pass
>> transistors. It was so novel (at least to me) that I typed it into
>> LTSpice. Here it is:
>
>Could you take a screenshot of the schematic?
I'll include an ASCII version here:
>: R2
>: +V = 12V ,------/\/\---------------------,
>: | 1k |
>: | +V |
>: | | |
>: | \ |
>: | / R1 |
>: | \ 1k5 |
>: | / |
>: | | +V |
>: C2 | ,----+ | |
>: || 10uF R4 | | | 2N3904| |
>: ,------||------/\/\---------+ | | | |
>: I| || 100 | | Q4 e>| |/c Q3 |
>: N| | | |-------| |
>: | | | c/| |>e |
>: | | C1 --- | | |
>: --- | 10uF--- |2N3906 | |
>: - V2 | | | | |
>: --- SINE(0 .2 1k) | | | | | C3
>: - | C5 | | | | || 470uF
>: | | || 10p| | +----+-||----,O
>: | +V +---||----+ | | || |U
>: | | | || | | | |T
>: gnd | | | | | \
>: | | | |2N3904 | / R5
>: \ ,-------, | | | | \ 8
>: / R3 | | | +V | Q1 c\| |<e Q5 /
>: \ 1k | +V | | | 2N| |-------| 2N3906 |
>: / | | | | |\| | e<| |\c |
>: | | |\| | '-|-\ | | | |
>: | '--|-\ | | >----+----' | gnd
>: | | >-+-----|+/ |
>: +-------|+/ |/| LT6234 |
>: ,-----+ |/| LT6234 | gnd
>: | | | gnd
>: --- C4 \ gnd
>: --- 1uF/ R9
>: | \ 1k
>: | /
>: | |
>: gnd gnd
(This was auto-generated from my LTspice to ASCII program.)
Jon
Ban
"Jon Kirwan" <jo...@infinitefactors.org> schrieb im Newsbeitrag
news:8te2n5llb72tgv03g...@4ax.com...
Jon, you should read this book, bit torrentwise
Audio Power Amplifier Design Handbook, 4th Ed. - (Malestrom)
by Doug Self, one of the deeper going but still very practical publications,
you'll love it.
Ban
Jim Thompson
What in the world ?:-)
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>What in the world ?:-)
View in fixed-spaced font. And it's a rendition of the
schematic that Bob Monson had posted, earlier, from EDN. He
wrote, "On a related note, there was an article in a recent
EDN about a self biasing preamp which was kinda cool. Instead
of trying to track the difference using diodes or a
multiplier, it used a couple of transistors and an opamp to
set the correct values at the bases of the pass transistors.
It was so novel (at least to me) that I typed it into
LTSpice."
I merely re-arranged it in LTspice to be a little more to my
taste and then passed it through a program that generates
ASCII from that.
Jon
Jon Kirwan
wrote:
>
>"Jon Kirwan" <jo...@infinitefactors.org> schrieb im Newsbeitrag
>news:8te2n5llb72tgv03g...@4ax.com...
>> On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "mi...@sushi.com"
>> <mi...@sushi.com> wrote:
>>
>>><>
>> Okay. I'll do that if folks here aren't interested at all in
>> talking about it.
>>
>BS, a couple of good answers have come.
Agreed. We are past that question.
>Jon, you should read this book, bit torrentwise
>Audio Power Amplifier Design Handbook, 4th Ed. - (Malestrom)
>by Doug Self, one of the deeper going but still very practical publications,
>you'll love it.
>Ban
I just received a copy of the 5th edition, today. I'll
start, though the author says that it assumes a certain level
of prior training. And skimming through, I agree.
Jon
Jon Kirwan
Okay!!! It has a great section on Vbe multipliers under
Chapter 15 on Thermal Compensation!! This is helpful. And
it includes a discussion on that collector resistor there and
in Chapter 7, where a chart is presented with various values
for my R3 shown and the curves over current. Nice!! It also
appears, on first glance, to confirm my impressions!! This
is very good.
Jon
Jim Thompson
Burr-Brown was famous for using bias compensation like that in the
front ends of some of their operational amplifiers, but I doubt its
efficacy in power output stages.
The Burr-Brown scheme is similar to a discussion here a few (seven :-)
years ago...
http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Tue, 09 Feb 2010 14:50:50 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>On Tue, 09 Feb 2010 15:42:48 -0700, Jim Thompson
>><To-Email-Use-Th...@My-Web-Site.com> wrote:
>>
>>>What in the world ?:-)
>>
>>View in fixed-spaced font. And it's a rendition of the
>>schematic that Bob Monson had posted, earlier, from EDN. He
>>wrote, "On a related note, there was an article in a recent
>>EDN about a self biasing preamp which was kinda cool. Instead
>>of trying to track the difference using diodes or a
>>multiplier, it used a couple of transistors and an opamp to
>>set the correct values at the bases of the pass transistors.
>>It was so novel (at least to me) that I typed it into
>>LTSpice."
>>
>>I merely re-arranged it in LTspice to be a little more to my
>>taste and then passed it through a program that generates
>>ASCII from that.
>>
>>Jon
>
>Burr-Brown was famous for using bias compensation like that in the
>front ends of some of their operational amplifiers, but I doubt its
>efficacy in power output stages.
I am still struggling to understand it. There are very
obvious parts that I completely understand. For example, the
divider for a "center" voltage followed by a unity gain
buffer for drive current compliance. I would guess that the
gain is determined by the NFB resistor divided by the input
impedance, which is mostly R4 in this case... so 10. I see
an opamp looking like an integrator, but I'm frankly
unfamiliar with the 4-BJT arrangement structure and need to
think about that one.
>The Burr-Brown scheme is similar to a discussion here a few (seven :-)
>years ago...
>
> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf
I'll download it now and look when I get a moment to engage a
little thought.
Thanks,
Jon
Jon Kirwan
><snip>
>I'm frankly
>unfamiliar with the 4-BJT arrangement structure and need to
>think about that one.
><snip>
I do "see" the emitter followers of Q3/Q5 on the schematic,
of course. It's the R1/Q1/Q4/C1 parts that I'm assuming is
the bias compensation and is the part I don't follow. The C5
looks like a very lightly applied integrator cap, which I
take is needed to avoid oscillation. And that's about where
I'm stuck.
Jon
Jim Thompson
<jo...@infinitefactors.org> wrote:
Study up on writing loop and nodal equations and either solving by
simultaneous equations or matrix manipulation.
Jim Thompson
Did something get lost in the ASCII? Otherwise collector-to-collector
as in Q1-Q4 is a no-no... one of those devices will saturate.
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
I'm familiar with Norton and Thevenin and the use of three
different perspectives, branch-current, mesh, and nodal
analyses. I very much prefer to "think" with nodal analysis
and have pretty much set aside the other two approaches, now.
I'm also familiar with matrix solutions and have developed my
own programs for solving them a little easier than my TI
calculator allows for and with better accuracy in difficult
cases. I can also do Laplace, but frankly I have NOT yet
learned the shortcuts often used. So I wind up with pages of
partial fractions in the end, converting back to time domain
with tables, and seeing how things look there.
Being capable at a detailed level does not let me "see" at
the top level, just yet. Sometimes, it takes a while. The
immediate example making this point is where I didn't
_understand_ what the collector resistor _might_ do when I
first saw it in a Vbe multiplier. And I initially tried to
analyze the circuit with it, included, and I realized then
that there was a negative feedback present at the tap-off
point. But it was only when I analyzed the simpler circuit,
without it, and found the approximate R_ac for it that I then
_saw_ that this calculated R_ac closely matched the collector
resistor values I saw in the examples. That then immediately
told me the _why_!!
It's how things sometimes work for me, I guess.
Anyway, I am very glad for the suggested examples.
Thanks,
Jon
Jon Kirwan
I just double-checked. It's just that way in Bob's posted
LTspice schematic. And when I simulate the thing, it
produces a 2V p-p output into R5 from a .2V p-p input. Takes
a few cycles to settle on a DC center level, though.
Jon
Bob Monsen
> On Tue, 9 Feb 2010 11:40:45 -0800, "Bob Monsen"
> <rcmo...@gmail.com> wrote:
>
>>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>>news:jg91n5d684ru5imsq...@4ax.com...
>>> I think this fits in sci.electronics.design, not .basics.
>>>
>>>
>>> Jon
>>
>>Sorry, I didn't read the entire message...
>>
>>However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
>>Somewhat more expensive, but it'll be VERY stiff.
>
> I'm still in "discrete" mode. For example, I am _less_
> interested in opamp topologies and design techniques than I
> am in _how_ to design opamps. There is nothing like knowing
> the details about how they are designed inside to understand
> the gotchas that aren't readily accessible to someone using
> them.
TLV431s are very simple. They are a bandgap that sucks current until the
'ref' input voltage matches the bandgap output. I've modeled the TLV431
using the datasheet, and it is a fun exercise.
BTW, do you have a link to that cool LTSpice -> ASCII program? I'd forgotten
that you wrote it. I've been laboring over a hot 'andy's ascii' program for
schematics that I already have in LTSpice...
Well, the thing about these horrible power output stages is that they can
get dicey if they get too hot. When they heat up, the Vbe goes down for
both, which tends to pass more 'shoot through' current, which heats them
more... You get this.
So, using three diodes may actually be better than a single transistor,
assuming that they are thermally coupled with the output devices. Then, the
diodes will have a higher TC than the devices (since there are three rather
than two). So, the output current goes down when it gets hot.
Here is a simulation that shows it. Look at the shoot-through current as the
temperature goes from 0 to 150C:
Version 4
SHEET 1 880 680
WIRE 32 -48 -160 -48
WIRE 272 -48 32 -48
WIRE 32 -32 32 -48
WIRE 272 32 272 -48
WIRE 32 80 32 48
WIRE 208 80 32 80
WIRE 96 128 64 128
WIRE 160 128 128 128
WIRE 304 128 272 128
WIRE 400 128 384 128
WIRE 32 144 32 80
WIRE 96 144 96 128
WIRE 160 144 160 128
WIRE -160 176 -160 -48
WIRE 400 192 400 128
WIRE 496 192 400 192
WIRE 496 224 496 192
WIRE 32 240 32 208
WIRE 64 240 64 128
WIRE 64 240 32 240
WIRE 96 240 96 208
WIRE 96 240 80 240
WIRE 128 240 128 128
WIRE 128 240 96 240
WIRE 304 256 272 256
WIRE 400 256 400 192
WIRE 400 256 384 256
WIRE 160 304 160 208
WIRE 208 304 160 304
WIRE -64 320 -112 320
WIRE 160 320 160 304
WIRE 80 352 80 240
WIRE 80 352 32 352
WIRE 96 352 80 352
WIRE -64 368 -64 320
WIRE -32 368 -64 368
WIRE -160 400 -160 256
WIRE -112 400 -160 400
WIRE 160 400 -112 400
WIRE 272 400 272 352
WIRE 272 400 160 400
WIRE 336 400 272 400
WIRE 496 400 496 304
WIRE 496 400 400 400
FLAG 400 192 a
FLAG -32 336 a
FLAG -160 400 0
SYMBOL npn 208 32 R0
SYMATTR InstName Q1
SYMBOL pnp 208 352 M180
SYMATTR InstName Q2
SYMBOL res 400 112 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 1
SYMBOL res 400 240 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R2
SYMATTR Value 1
SYMBOL voltage -160 160 R0
SYMATTR InstName V1
SYMATTR Value 20
SYMBOL diode 144 144 R0
SYMATTR InstName D1
SYMBOL diode 80 144 R0
SYMATTR InstName D2
SYMBOL diode 16 144 R0
SYMATTR InstName D3
SYMBOL current 32 -32 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value 100m
SYMBOL Opamps\\opamp 0 288 R0
SYMATTR InstName U1
SYMBOL voltage -112 304 R0
WINDOW 3 24 44 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V2
SYMATTR Value 10
SYMBOL res 480 208 R0
SYMATTR InstName R3
SYMATTR Value 2
SYMBOL cap 400 384 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 1
SYMBOL current 160 320 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I2
SYMATTR Value 100m
TEXT -32 424 Left 0 !.include opamp.sub
TEXT -194 444 Left 0 !.dc TEMP 0 150 .01
Tim Williams
> I'm also familiar with matrix solutions and have developed my
> own programs for solving them a little easier than my TI
> calculator allows for and with better accuracy in difficult
> cases. I can also do Laplace, but frankly I have NOT yet
> learned the shortcuts often used. So I wind up with pages of
> partial fractions in the end, converting back to time domain
> with tables, and seeing how things look there.
Find "residues" in your TI, it's exactly what you need. Also, polynomial
factorization, if you aren't using it already.
Jim Thompson
<jo...@infinitefactors.org> wrote:
A demonstration of why not to trust simulators. What does LTspice
show for the voltages at Q1:c and Q1:e? Q4:c and Q4:e?
Tim Williams
message news:mhr3n5pv3mnp4dlfo...@4ax.com...
> Did something get lost in the ASCII? Otherwise collector-to-collector
> as in Q1-Q4 is a no-no... one of those devices will saturate.
It's a current mirror, based on hFE instead of Vbe (yuck!). When Q1 or Q4
saturates, bias current (or op-amp current) is diverted to the output
transistors, driving the load.
Tim Williams
>>The trend now is to stabilize everything with feedback. It works,
>>and it works well. Unless you are a purist and have some religious reason
>>to
>>avoid this technique, there is no sense in reinventing the wheel.
>
> It's not a religious reason, unless _learning_ is a religion,
> I suppose. I don't mind being told that "one day when you
> are ready, you will use global NFB to take care of this."
There are valid reasons for doing so. High bandwidth is one: when you need
high GBW, you don't have time to wait for the signal to propagate through
the 5 or 10 or 100 transistors you have[1]. Those kinds of systems are
usually built with a lot of low-gain, high bandwidth stages -- the feedback
is local, so the phase shift per stage is small, meanwhile the overall phase
shift (from input to output) can be arbitrarily large. The disadvantage is
there's nothing global to account for distortion or DC offset. However, you
can add low frequency servos to stabilize it, getting the best of both
worlds.
[1] Unless you happen to be Linear Technology. For instance, their LT1016
stupid-fast comparator claims an internal 60GHz GBW which is unity-gain
stable (you can use it as an op-amp). Better not have anything near that
feedback node.
Jon Kirwan
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:sgr3n5tb6hamiocr5...@4ax.com...
>> I'm also familiar with matrix solutions and have developed my
>> own programs for solving them a little easier than my TI
>> calculator allows for and with better accuracy in difficult
>> cases. I can also do Laplace, but frankly I have NOT yet
>> learned the shortcuts often used. So I wind up with pages of
>> partial fractions in the end, converting back to time domain
>> with tables, and seeing how things look there.
>
>Find "residues" in your TI, it's exactly what you need.
That's something I know I have yet to study and so I have NOT
looked them up in the TI manual. I'll need to study and
understand them much better than now, before trying that.
>Also, polynomial
>factorization, if you aren't using it already.
I use it sometimes when I have the TI nearby. I don't carry
it, everywhere, though. And I do think about problems lots
more places than where I have a TI nearby.
Besides, it's nice to have it in my head and to keep on
working on that part with practice. I remember some of the
conversion table and can actually _do_ some derivations the
hard way when I am lacking both tables _and_ a calculator. I
do like keeping up a personal skill, so that I'm not overly
frustrated when the mood strikes and the rare tools are not
handy at the moment.
I always have a finger, my brain, and some dirt no matter
where I am, unless I wind up chained to some prison wall.
Then I'll have to work on my memory, too. ;)
With your recommendation, it's now off to find a good book on
complex analysis.
Jon
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Among many other such demonstrations -- and a good part about
why _I_ need to understand these things, for myself.
>What does LTspice
>show for the voltages at Q1:c and Q1:e? Q4:c and Q4:e?
>
> ...Jim Thompson
I know your own preference to include diagrams, rather than
just words. So here is a link:
>: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1 e+c.gif
Forgive the coloring. I didn't mess with it to make it
prettier.
Jon
Jon Kirwan
>I know your own preference to include diagrams, rather than
>just words. So here is a link:
>
>>: http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1 e+c.gif
Sorry. I missed a %20 in there, so it may or may not work in
your browser. Try:
http://www.infinitefactors.org/misc/images/EDN%20schematic%20Q1%20e+c.gif
Jon
Jim Thompson
The original worked, pasted rather than directly clicked.
Something smelly there. I'll try it tomorrow... the Moo Goo Gai Pan
is just about ready to eat... plus I've already had some Dry Sack ;-)
Jon Kirwan
<rcmo...@gmail.com> wrote:
>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>news:hhj3n5lpqucim8nmi...@4ax.com...
>> On Tue, 9 Feb 2010 11:40:45 -0800, "Bob Monsen"
>> <rcmo...@gmail.com> wrote:
>>
>>>"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
>>>news:jg91n5d684ru5imsq...@4ax.com...
>>>> I think this fits in sci.electronics.design, not .basics.
>>>>
>>>>
>>>> Jon
>>>
>>>Sorry, I didn't read the entire message...
>>>
>>>However, if you want a stiff multiplier, use a TLV431 instead of a BJT.
>>>Somewhat more expensive, but it'll be VERY stiff.
>>
>> I'm still in "discrete" mode. For example, I am _less_
>> interested in opamp topologies and design techniques than I
>> am in _how_ to design opamps. There is nothing like knowing
>> the details about how they are designed inside to understand
>> the gotchas that aren't readily accessible to someone using
>> them.
>
>TLV431s are very simple. They are a bandgap that sucks current until the
>'ref' input voltage matches the bandgap output. I've modeled the TLV431
>using the datasheet, and it is a fun exercise.
I'll take a crack at it. AofE also talks about bandgaps and
I think I understand them, given the nice discussion there.
>BTW, do you have a link to that cool LTSpice -> ASCII program? I'd forgotten
>that you wrote it. I've been laboring over a hot 'andy's ascii' program for
>schematics that I already have in LTSpice...
Hehe. Sure.
Yes, I _get_ it. I earlier ignored temperature when thinking
about BJT analysis, as a rule. Too much to include all at
once, I suppose. But now I keep it in some part of my mind.
>So, using three diodes may actually be better than a single transistor,
>assuming that they are thermally coupled with the output devices. Then, the
>diodes will have a higher TC than the devices (since there are three rather
>than two). So, the output current goes down when it gets hot.
>
>Here is a simulation that shows it. Look at the shoot-through current as the
>temperature goes from 0 to 150C:
><snip>
I love the fact that you are shooting these schematics to me.
But I'm not sure what "shoot-through" current to look at.
What I _do_ see is your point about the output current going
downwards as temperature increases, supporting what you
suggest about stacking the diodes 3-deep.
And I like the general lesson in the schematic, too, allowing
you to focus on the output stage. The opamp basically
represents the input stage up through, but not including, the
final stage. I need to do that for other sections as I
proceed around understanding each detail individually and
then together as a whole. Kind of a behavioral thing for
everything but the section under the microscope. I need to
take that approach more often than I do.
Jon
Jon Kirwan
>>BTW, do you have a link to that cool LTSpice -> ASCII program? I'd forgotten
>>that you wrote it. I've been laboring over a hot 'andy's ascii' program for
>>schematics that I already have in LTSpice...
>
>Hehe. Sure.
Sorry... I didn't stick in the link. Because it isn't set
up well, right now. Let me fix that and post it up and then
I will add the link.
Jon
Bob Monsen
"Jon Kirwan" <jo...@infinitefactors.org> wrote in message
news:1g34n513mhonq98pr...@4ax.com...
No hurry. Thanks, Bob Monsen
Bob Monsen
>
> Something smelly there. I'll try it tomorrow... the Moo Goo Gai Pan
The original is from EDN, Oct 22, 2009, page 45. "Class B amplifier has
automatic bias".
Regards,
Bob Monsen
mi...@sushi.com
Site.com> wrote:
> On Tue, 09 Feb 2010 02:44:31 -0800, Jon Kirwan
>
>
>
> <j...@infinitefactors.org> wrote:
> >On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "m...@sushi.com"
> ><m...@sushi.com> wrote:
>
> >><snip>
> >>Less words and real schematics would get you more readers. [The only
> >>thing worse than ascii equations are ascii schematics.]
>
> >ASCII is what I'll post. It's the only way to get them
> >archived or properly posted to a text newsgroup. I no longer
> >have access to the binary for schematics, sadly. If I lose
> >some people because they cannot manage fixed-spaced fonts, I
> >guess I lose them. I could place links up on my domain, I
> >suppose. But in this case, the schematics are really very
> >basic and not overly burdensome in ASCII. Besides, Win Hill
> >posted some really nice examples here, before. Folks seemed
> >to live with that. Not sure why you are picking on me, here.
>
> >>In any event, just google improved vbe multiplier. I've seen all sorts
> >>of circuits published to get lower impedance at the nodes.
>
> >Okay. I'll do that if folks here aren't interested at all in
> >talking about it.
>
>
> Miso is all blabber and no content... best if ignored ;-)
>
> ...Jim Thompson
> --
> | James E.Thompson, CTO | mens |
> | Analog Innovations, Inc. | et |
> | Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
> | Phoenix, Arizona 85048 Skype: Contacts Only | |
> | Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
> | E-mail Icon athttp://www.analog-innovations.com| 1962 |
>
Right. Maybe I'll get another job of yours to fix.
mi...@sushi.com
> On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "m...@sushi.com"
>
> <m...@sushi.com> wrote:
> ><snip>
> >Less words and real schematics would get you more readers. [The only
> >thing worse than ascii equations are ascii schematics.]
>
> ASCII is what I'll post. It's the only way to get them
> archived or properly posted to a text newsgroup. I no longer
> have access to the binary for schematics, sadly. If I lose
> some people because they cannot manage fixed-spaced fonts, I
> guess I lose them. I could place links up on my domain, I
> suppose. But in this case, the schematics are really very
> basic and not overly burdensome in ASCII. Besides, Win Hill
> posted some really nice examples here, before. Folks seemed
> to live with that. Not sure why you are picking on me, here.
>
> >In any event, just google improved vbe multiplier. I've seen all sorts
> >of circuits published to get lower impedance at the nodes.
>
> Okay. I'll do that if folks here aren't interested at all in
> talking about it.
>
> Jon
There are all sorts of free places to post images. Imageshack comes to
mind. I just lose interest if I have to look at ascii circuits.
You realize you can just string diodes. Nobody says you have to VBE
multiply. It's just one of many biasing techinques.
mi...@sushi.com
> On Mon, 8 Feb 2010 23:47:24 -0800 (PST), "m...@sushi.com"
>
> <m...@sushi.com> wrote:
> >Have you read Randy Slone's power amplifier book? This stuff really
> >isn't rocket science. Nor is AB. ;-)
>
> I haven't. It's not rocket science. But it is interesting
> at my level.
>
> ><http://www.amazon.com/High-Power-Audio-Amplifier-Construction-Manual/
> >dp/0071599258/ref=dp_ob_title_bk>
>
> I'll look, but the title appears more on the contruction
> side. I am using this to educate myself a little better.
>
> >The black art is all in assembly, protection circuitry, and making
> >sure it starts up cleanly. [Most engineers never look at start up, so
> >you get designs that thump when you power them. I have lots of gear
> >with power-on thumps.]
>
> I recall reading of a recommendation suggesting that all
> electronic devices use less than 1W when on, but not
> performing their intended application. This would also seem
> to require a little added effort to achieve, as well. But I
> take your point.
>
> >I'd pick one of his MOS designs. Bipolar designs often have good
> >intentions, but ring like a bell. MOS is mushy, but predictably mushy.
>
> I'm still learning about BJTs. In fact, that's what this is
> about for me. MOS later. ICs later. BJTs now.
>
> Jon
There is plenty of circuit design in the book. It is the assembly
stuff (ground loops, thermal tracking, etc.) that people screw up.
Anyway, I'd wait for the new edition.
mi...@sushi.com
> On Tue, 9 Feb 2010 23:38:06 +0100, "Ban" <bans...@web.de>
> wrote:
>
>
>
> >"Jon Kirwan" <j...@infinitefactors.org> schrieb im Newsbeitrag
> >news:8te2n5llb72tgv03g...@4ax.com...
> >> On Mon, 8 Feb 2010 18:23:16 -0800 (PST), "m...@sushi.com"
> >> <m...@sushi.com> wrote:
>
> >>><>
> >> Okay. I'll do that if folks here aren't interested at all in
> >> talking about it.
>
> >BS, a couple of good answers have come.
>
> Agreed. We are past that question.
>
> >Jon, you should read this book, bit torrentwise
> >Audio Power Amplifier Design Handbook, 4th Ed. - (Malestrom)
> >by Doug Self, one of the deeper going but still very practical publications,
> >you'll love it.
> >Ban
>
> I just received a copy of the 5th edition, today. I'll
> start, though the author says that it assumes a certain level
> of prior training. And skimming through, I agree.
>
> Jon
Self has published many papers too if you have access to an
engineering library. I think his stuff is worth reading. Nelson Pass
runs a DIY forum
http://www.passdiy.com/
Pass is up there with Levinson, and miles above any Arizona designer.
Jon Kirwan
Okay, will do. Turns out the 5th edition of Self's book on
amplifiers contains many typos. Lots of references to parts
that aren't in the schematic or clearly are not the one under
discussion. Same with some of the terms in equations. I'm
correcting as I go.
Jon
Tim Williams
news:692726fc-bf7b-4fc7...@z39g2000vbb.googlegroups.com...
> You realize you can just string diodes. Nobody says you have to VBE
> multiply. It's just one of many biasing techinques.
They don't make diodes with pots varying Eg. ;-)
Jon Kirwan
I have a domain I can use. It is just that it hardly seemed
necessary, given the simplicity involved and the fact that
this group is quite used to ASCII schematics, given the years
I've been watching here. It has advantages in that it is
archived for very long times, this way, as well.
However, I will try and take your concerns into account and
see about organizing a directory and smoothing through issues
of dropping files there which have been massaged and arranged
for easier viewing.
>You realize you can just string diodes. Nobody says you have to VBE
>multiply. It's just one of many biasing techinques.
Yes, I think that's been well-posted in the thread and I was
aware of it, before. It's one of the first things I saw when
starting on this trek 2 weeks back. Hard to miss. The issue
is more about learning, though. Not picking a specific
solution and ignoring the others. I'd like to have some
spectrum of options I've looked at well and discarded (as
well as retained.)
Jon
Jon Kirwan
<tmor...@charter.net> wrote:
><mi...@sushi.com> wrote in message
>news:692726fc-bf7b-4fc7...@z39g2000vbb.googlegroups.com...
>> You realize you can just string diodes. Nobody says you have to VBE
>> multiply. It's just one of many biasing techinques.
>
>They don't make diodes with pots varying Eg. ;-)
>
>Tim
:)
Jon
Bitrex
> On Tue, 09 Feb 2010 14:50:50 -0800, Jon Kirwan
>
>> On Tue, 09 Feb 2010 15:42:48 -0700, Jim Thompson
>> <To-Email-Use-Th...@My-Web-Site.com> wrote:
>>
>>> What in the world ?:-)
>> View in fixed-spaced font. And it's a rendition of the
>> schematic that Bob Monson had posted, earlier, from EDN. He
>> EDN about a self biasing preamp which was kinda cool. Instead
>> of trying to track the difference using diodes or a
>> multiplier, it used a couple of transistors and an opamp to
>> set the correct values at the bases of the pass transistors.
>> It was so novel (at least to me) that I typed it into
>> LTSpice."
>>
>> taste and then passed it through a program that generates
>> ASCII from that.
>>
>> Jon
>
> Burr-Brown was famous for using bias compensation like that in the
> front ends of some of their operational amplifiers, but I doubt its
> efficacy in power output stages.
>
> years ago...
>
> http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps.pdf
>
Could you give a quick explanation of how the circuit in the link works?
I'm having a bit of trouble following what's going on...
Jon Kirwan
It's at:
http://www.infinitefactors.org/misc/asc.zip
It contains just two files, the EXE and a library symbol
file. Place both in some directory that is in the path. You
need to use DOS, sadly. I didn't set these up for Windows --
wanted to focus on the task, not get mired in Windows
sideshows.
If you run the program without a filename, it will say:
>: asc version 1.2.1, (library C:\TOOLS\BIN\ASC.SYM found)
>:
>: This program converts LTSpice schematics into ASCII schematic output (or
>: files.) If you specify no files at all, it accepts the LTSpice schematic
>: from its standard input device. If you specify exactly one file, it dis-
>: plays the ASCII schematic output to the standard output device. If you
>: specify more than one file, it then generates .TXT files otherwise having
>: the same name as the specified schematics.
>:
>: These options are supported:
>: +h requests this help message, -h disables it.
>: +r enables rectangle drawing, -r disables it (default is -r)
>: +c enables clipboard copying, -c disables it (default is -c)
>: +c<char> enables clipboard copying and prepends <char> to each line
>:
>: Usage: asc <options> <filename> [<filename>]...
There are some options, like the clipboard. But post
Win2000, that mechanism was broken and I haven't set up the
additional DLL I'd need to remedy it. (Something I may yet
take care of.) So under WinXP, for example, I just run it
into a file and use notepad to call it up. Something like:
ASC amplify.asc >amplify.txt
NOTEPAD amplify.txt
It gets the job done. Under Win98SE, I just use the +c
option and then paste the text, as desired, in Windows.
The library is semi-okay. There's some symbols I've probably
not yet added to it because I don't use the parts that much.
I won't mind extending it (it's not hard to do) if there is
anything you use and would like put in. I just use a text
editor and hack in the new ASCII and then save it. The
program automatically parses it every time it runs.
Jon
Jim Thompson
<jo...@infinitefactors.org> wrote:
>On Tue, 9 Feb 2010 20:55:07 -0800 (PST), "mi...@sushi.com"
><mi...@sushi.com> wrote:
>
>>I just lose interest if I have to look at ascii circuits.
You really can't convey much more than a "basics" circuit with ASCII.
Post links or use LTspice listings... everyone seems to have that ;-)
>
>I have a domain I can use. It is just that it hardly seemed
>necessary, given the simplicity involved and the fact that
>this group is quite used to ASCII schematics, given the years
>I've been watching here. It has advantages in that it is
>archived for very long times, this way, as well.
>
>However, I will try and take your concerns into account and
>see about organizing a directory and smoothing through issues
>of dropping files there which have been massaged and arranged
>for easier viewing.
>
>>You realize you can just string diodes. Nobody says you have to VBE
>>multiply. It's just one of many biasing techinques.
>
>Yes, I think that's been well-posted in the thread and I was
>aware of it, before. It's one of the first things I saw when
>starting on this trek 2 weeks back. Hard to miss. The issue
>is more about learning, though. Not picking a specific
>solution and ignoring the others. I'd like to have some
>spectrum of options I've looked at well and discarded (as
>well as retained.)
>
>Jon
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Jim Thompson
Start with IB coming out (*) of IN+ and IN- of EACH section of the
LM324.
(*) LM324 has as inputs PNP diff pairs.
Then simply compose loop and node equations.
If any of you are struggling with this kind of analysis, I'll mark it
up and show how to solve it.
Jim Thompson
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Examine this mark-up...
http://analog-innovations.com/SED/IB-Cancellation-WithTwoOpAmps_MarkedUp.pdf
George Herold
> On Tue, 9 Feb 2010 11:35:36 -0800 (PST), George Herold
>
>
>
>
>
> <ggher...@gmail.com> wrote:
> >On Feb 9, 5:39 am, Jon Kirwan <j...@infinitefactors.org> wrote:
> >> On Mon, 8 Feb 2010 19:16:24 -0800 (PST), George Herold
>
> >> <ggher...@gmail.com> wrote:
> >> >><snip>
> >> >"I'm wondering about additional topology changes to improve
> >> >the performance still more."
>
> >> >Hi Jon, I've been 'sorta' following your thread on s.e.basics. I
> >> >wonder if you abandoned class A operation too early? Why not keep
> >> >things linear evreywhere and avoid the ‘dead band’? So what if you
> >> >need a bigger heat sink. It’s certainly a lot simpler.
>
> >> >George H.
>
> >> Well, George... No, I've not abandoned it. Actually, it's my
> >> hope to wind up building the amplifier and then operating it
> >> (by hopefully choosing a design where that is possible) in
> >> different modes for the learning experience of it. I hope
> >> that is in the cards. I really do.
>
> >> But to make a sharp point on it, although it's probably just
> >> an extreme case, I remember reading about a 10W amplifier,
> >> single channel, dissipating 120W! Creeps me out. So I
> >> definitely _want_ to consider other classes of operation. And
> >> cripes, I want to learn, anyway. So why not keep my options
> >> open?
>
> >> Jon
>
> >" I remember reading about a 10W amplifier,
> >> single channel, dissipating 120W! "
>
> >It might have been here,
> >http://www.passdiy.com/default.html
> >I got to reading about amplifiers on the above site... Do in part to
> >your interest.
>
> >George H.
>
> Egads. Loads of PDF files. Now I have to create a
> directory, download them one by one, and then call them up
> with my slow machine to look. Any particular page or file
> where you saw it? (No, that isn't where I saw the comment.)
>
> But thanks for the link. I'll add it to those I read, also.
>
> Jon- Hide quoted text -
>
> - Show quoted text -
Ohh sorry about that... Don't bother reading them... He (Nelson Pass)
has designs for Class A amps using a FET. (named Zen) One of his
variations (son of Zen?) was made with no NFB and I think wasted your
stated 120 Watts of power for 10W into the speaker. But this seemed
pretty pointless to me... some audio guys wanted an amp with out
NFB.... Don't aks me why!
Say can you make a push-pull stage run class A? (Or is that just a
silly idea?)
George H.
Tim Williams
news:b603649e-8d7c-4349...@a5g2000yqi.googlegroups.com...
> Say can you make a push-pull stage run class A? (Or is that just a
> silly idea?)
You can, but it stops being class A for large signal swings or low load
impedances.
There's a monolithic structure which bends the output transistors'
characteristics so they never cut off. I forget who owns it. I suppose
that would count as class A at any signal/load condition.
John Larkin
If quiescent current exceeds peak load current, it's pretty much Class
A. It could be symmetric, like a classic PNP-NPN class AB with tons of
bias current, or asymmetric, like an emitter follower with a
constant-current sink to ground.
It's silly as much as any class A amp is silly: it wastes a lot of
power.
John
John Larkin
<tmor...@charter.net> wrote:
>"George Herold" <gghe...@gmail.com> wrote in message
>news:b603649e-8d7c-4349...@a5g2000yqi.googlegroups.com...
>> Say can you make a push-pull stage run class A? (Or is that just a
>> silly idea?)
>
>You can, but it stops being class A for large signal swings or low load
>impedances.
>
>There's a monolithic structure which bends the output transistors'
>characteristics so they never cut off. I forget who owns it. I suppose
>that would count as class A at any signal/load condition.
>
>Tim
The current mirror config, the one that uses the opamp rail currents
to drive mirrors, can do that. If the top side mirror is pulling up
the load, you can arrange for the bottom side to keep running at its
quiescent current, as opposed to turning off and even off-er as the
top side drives the load harder. Keeping both side on all the time has
some nice dynamics when driving reactive loads.
I don't know if that's still "class A". "A" is just a letter, after
all.
John
Jim Thompson
And then there's SLIDING-class-A, which I've used successfully in
hearing aids... better efficiency than class-B at really low voltages.
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Tue, 09 Feb 2010 22:21:12 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>>On Tue, 9 Feb 2010 20:55:07 -0800 (PST), "mi...@sushi.com"
>><mi...@sushi.com> wrote:
>>
>>>I just lose interest if I have to look at ascii circuits.
>
>You really can't convey much more than a "basics" circuit with ASCII.
>Post links or use LTspice listings... everyone seems to have that ;-)
Agreed. Good advice, actually. In this case, the schematic
_was_ pretty basic. But then the LTspice file would also
have been similarly short, too. So it probably just makes
sense, period.
...
I had earlier been imagining what would be multiple views
offered, so that others can choose their own poison more
freely:
(1) ASCII, where not crazy-minded,
(2) GIF, JPG, or PNG schematic for browsing via http
(3) as (2), but for associated signal/freq plots
(4) .ASC file, for LTspice schematic/netlist
(5) .NET file, where schematic layout isn't important
(6) associated other inputs not in standard LTspice
(7) PDF file documenting why and considerations
(8) ZIP file boxing all this up in a package
(9) .html page with clickable links and PDF info
Looked like _work_ to me, though. ;)
Jon
Jim Thompson
<jo...@infinitefactors.org> wrote:
>On Wed, 10 Feb 2010 07:46:16 -0700, Jim Thompson
><To-Email-Use-Th...@My-Web-Site.com> wrote:
>
>>On Tue, 09 Feb 2010 22:21:12 -0800, Jon Kirwan
>><jo...@infinitefactors.org> wrote:
>>
>>>On Tue, 9 Feb 2010 20:55:07 -0800 (PST), "mi...@sushi.com"
>>><mi...@sushi.com> wrote:
>>>
>>>>I just lose interest if I have to look at ascii circuits.
>>
>>You really can't convey much more than a "basics" circuit with ASCII.
>>Post links or use LTspice listings... everyone seems to have that ;-)
>
>Agreed. Good advice, actually. In this case, the schematic
>_was_ pretty basic. But then the LTspice file would also
>have been similarly short, too. So it probably just makes
>sense, period.
>
>...
>
>I had earlier been imagining what would be multiple views
>offered, so that others can choose their own poison more
>freely:
>
> (1) ASCII, where not crazy-minded,
> (2) GIF, JPG, or PNG schematic for browsing via http
PNG and PDF are most readable and one or the other is marginally
smaller depending on creating software tool.
> (3) as (2), but for associated signal/freq plots
> (4) .ASC file, for LTspice schematic/netlist
> (5) .NET file, where schematic layout isn't important
> (6) associated other inputs not in standard LTspice
> (7) PDF file documenting why and considerations
> (8) ZIP file boxing all this up in a package
> (9) .html page with clickable links and PDF info
>
>Looked like _work_ to me, though. ;)
>
>Jon
It always is ;-)
bg
Jon Kirwan wrote in message <9kk3n5tjcntp1b1it...@4ax.com>...
>On Tue, 9 Feb 2010 14:05:32 -0700, "bg" <b...@nospam.com>
>wrote:
>
>><snip>
Have a look -
http://www.xmission.com/~bmge/Feedback.htm
Jon Kirwan
wrote:
I noted the author's comment, "If I knew what I was doing, I
could probably optimize this circuit to produce even better
results with feedback..." The author the also talks about
"bias stability", which I might have taken to mean keeping
the bias point at the same voltage as Vs1 varies, but could
instead mean making it linear, without any "bow" to it. (As
I understand it, for simple amplifier stages the collector is
usually set somewhere near the midpoint and thus one might
actually _want_ the change but want it to be linear in some
known fashion and certainly not bending over in a bow as in
the second circuit's case!) So the 'goal' isn't clearly
stated to me.
By the way, that first circuit _is_ a Vbe multiplier with a
resistor limiting current from Vs1. So computing point A on
it isn't much different than what I did, earlier, when I
first posted.
The first circuit shown there on that web site (and I don't
know if you intended this, or not) is basically a Vbe
multiplier sourced by a 10k resistor with point A being the
Vbe multiplier output. The second circuit shown there is,
once again, basically a Vbe multiplier with the collector
resistor in place, if you think of point A in that case as
being the output, but now with the topside tied directly to a
voltage source instead of a resistor or current source --
which obviously isn't the way a Vbe multiplier would operate.
However, the curve he shows for it remains interesting.
Although the author is talking about something else, the
importance of NFB instead, the first case he makes actually
presents the _problem_ I was talking. In his first circuit
case, the variation in the Vbe multiplier's output vs
sourcing current through R1 is shown clearly.
I pointed this out in the first post in this thread (with
terms now changed to match his first circuit's usage):
V_bias = Vbe*(1+(R2/R3)) + R2*Ic/beta
(Which ignores the tiny kT/q 26mV always present in the
emitter.)
He shows a Vbe of 0.633V, R2=98k, R3=5.6k and I think the
beta of his Q1 is over 200. Ic is about 770uA from his
values. From these, I compute V_bias = 12.0878V. His
circuit shows 12.3V there.
Perhaps close enough, but I was interested in seeing what
LTspice would show. After duplicating his schematic and
running it, I see 12.081V at point A. Much closer to my
computed value.
I'd also gone to the trouble, that the author does not, of
computing R_ac for the system. In his case, the value works
out to around 1200 Ohms. Roughly speaking then, we have a
10k/1.2k divider for small __changes__ in voltage. This
suggests about .11V/V while his graph shows something more
like .75/5 or .15V/V. However, once again my schematic in
LTspice shows instead .61/5 or .122V/V, which is closer to
the value I calculated using R_ac as an approximation.
I'd already done some useful analysis for his circuits and
I'd not even read his web site, yet.
Another interesting point. In the second circuit's case,
although it uses a voltage source at the top -- which is
decidedly NOT what I'm considering, obviously -- the _shape_
of his curve is exactly what I _want_ to have.
Obviously driven differently than shown, I would set the
collector resistor value to be approximately the R_ac
computed without it and that nice curve should appear -- just
not the very large magnitude excursions since the drive is
different and the collector resistor is smaller in magnitude
(it would be set to around 1.1k, not 10k, other things being
similar.) In fact, I think I mentioned this either in this
thread or the one over in .basics, last week. That curve
helps to allow me to tweak for an optimal spot and then
minimize voltage output variation over current that is
sourcing through it.
So I again modified the schematic to vary a current source
instead of a voltage source, from 500uA to 1000uA (roughly
centered over the estimated 770uA drive from before), and
plotted the voltage curve. Using that 1.1k collector
resistor in place, it is a very nice bow centered very
sweetly around the target of about 750uA, drooping by only
38mV out at the skirts. Exactly as I predicted the shape
should be with that value.
Interesting page, sadly lacking in equation development. What
I took away from it may have been different from what the
author (or you) perhaps intended. But there it is.
I still _get_ the idea of NFB!! So I don't mean to argue
against that! I just went somewhere else with that page.
Jon
Jim Thompson
<jo...@infinitefactors.org> wrote:
[snip]
>
>I still _get_ the idea of NFB!! So I don't mean to argue
>against that! I just went somewhere else with that page.
>
>Jon
First rule of "NFB": Make it as good as you possibly can without NFB,
_then_ apply NFB ;-)
But it's sort of a trick and a lie... you use _local_ feedback to make
the individual pieces as linear as you can, then add overall _global_
feedback.
Jim Thompson
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>[snip]
>>
>>I still _get_ the idea of NFB!! So I don't mean to argue
>>against that! I just went somewhere else with that page.
>>
>>Jon
>
>First rule of "NFB": Make it as good as you possibly can without NFB,
>_then_ apply NFB ;-)
>
>But it's sort of a trick and a lie... you use _local_ feedback to make
>the individual pieces as linear as you can, then add overall _global_
>feedback.
>
Like this _very_linear_ differential pair....
http://analog-innovations.com/SED/TL431DiffPair.pdf
:-P
Jon Kirwan
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Wed, 10 Feb 2010 15:37:00 -0800, Jon Kirwan
><jo...@infinitefactors.org> wrote:
>
>[snip]
>>
>>I still _get_ the idea of NFB!! So I don't mean to argue
>>against that! I just went somewhere else with that page.
>>
>>Jon
>
>First rule of "NFB": Make it as good as you possibly can without NFB,
>_then_ apply NFB ;-)
>
>But it's sort of a trick and a lie... you use _local_ feedback to make
>the individual pieces as linear as you can, then add overall _global_
>feedback.
>
> ...Jim Thompson
Now _this_ is what I wanted to hear.
Many seem to just tell me "use global NFB to fix things"
almost, it seems, to simply stop me from bothering to
struggle at all or even care about understanding things.
Maybe it is just because it _takes work_ to actually engage a
quantitative discussion and the lazy way out is to just hand
wave and tell me to "move on by."
But it was my sense at the outset, and it is my motivation
for starting this thread as well, to do exactly what you are
talking about here. I'm so glad to see it said. "Make it as
good as you can without NFB, then apply NFB." Yes!
For example, the Sziklai pair is really a BJT wrapped with a
local NFB using the other BJT for that purpose. Nice.
I couldn't state it this clearly because I'm just learning
things. But what you said is what my instincts tell me,
despite attempts to say "move on, there's nothing to see
here."
Jon