Wien bridge oscillator with diodes in feedback loops
jozamm
I built a Wien bridge oscillator on a breadboard according to the guidelines in the LM324 datasheet (Fig. 11) http://tinyurl.com/6ejjvhz . The output is a distorted sine wave. I set to oscillate less than 1kHz. For diodes I used the 1n4148.
Can someone give me some hints on what I am doing wrong?
Regards,
Jozamm
George Herold
>
> Can someone give me some hints on what I am doing wrong?
>
> Regards,
> Jozamm
Yeah diode AGC is like that. You might check out App note AN-43 from
Linear Technologies. There is a nice discussion of the Wein bridge
there that tells you how the circuit works. You might try adding a
variable pot to the 50k feed back resistor (Rf). If I remember the
numbers correctly the circuit will just start to oscillate when Rf is
a bit more than 30 kohms. That should give you less distortion. But
still not great... it is only a diode after all.
George H.
Jim Thompson
wrote:
Wien Bridge Oscillators are a PITA to amplitude stabilize.
Try this....
http://www.analog-innovations.com/SED/Sedra-Espinosa-Oscillator.pdf
...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 |
Remember: Once you go over the hill, you pick up speed
John Larkin
wrote:
>Hi all,
In addition to other comments, LM324s have a lot of distortion all on
their own. You can even see it at 60 Hz. Horrible opamp.
John
Tim Wescott
> On Tue, 29 Mar 2011 11:32:30 -0700 (PDT), jozamm<joz...@gmail.com>
> wrote:
>
>> Hi all,
>>
>> I built a Wien bridge oscillator on a breadboard according
>> to the guidelines in the LM324 datasheet (Fig. 11)
>> http://tinyurl.com/6ejjvhz . The output is a distorted sine
>> wave. I set to oscillate less than 1kHz. For diodes I used
>> the 1n4148.
>>
>> Can someone give me some hints on what I am doing wrong?
>>
>> Regards,
>> Jozamm
>
> Wien Bridge Oscillators are a PITA to amplitude stabilize.
>
> Try this....
>
> http://www.analog-innovations.com/SED/Sedra-Espinosa-Oscillator.pdf
>
> ...Jim Thompson
Hey Jim:
I'm sure I could figure that out if I weren't lazy. Could you save me
some effort and give a run down on the theory? Obviously you're
engaging in some trickery with that upper op-amp, and I assume that the
diodes are limiting the output at least semi-softly, but that's all I
get before the urge to grab some paper and a pencil battles with the
urge to have another sip of tea.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
Tim Wescott
Using a circuit out of a data sheet? Designed by an applications
engineer who either really loves his job or couldn't get a 'real job' as
a circuit designer? And whose job is to puff off chips, not make things
work?
Traditionally, using diodes to stabilize a Wien bridge oscillator has
been asking for a little bit of distortion. The idea is that you design
the circuit carefully so the distortion is minimized, usually by setting
it up so that it has just barely enough gain before the diodes kick in,
and just barely too little after they start to conduct. So at zero
amplitude there's enough gain to oscillate, and the effective gain gets
lower and lower the higher the amplitude gets (thanks to the diodes
conducting). At some amplitude the gain is stabilized and the thing
steadies out.
It looks like the problem in that circuit is that the gain with the
diodes out of the circuit is at least ten times the gain with them in
(look at the ratio of the 50K vs. the 50K in parallel with that 5K pot
that may be turned down).
The old traditional way of stabilizing a Wien bridge oscillator was with
a light bulb, with circuit impedances set so that it just barely start
getting warm from the audio. That would increase its resistance, which
would (if you put it in the right place in the circuit) decrease the
overall gain, and would stabilize the gain. As a method it worked
really really fine in a vacuum tube circuit, and sorta-OK in an op-amp
circuit back when you could get "grain of wheat" incandescents, and it
really sucks now that everyone uses LED's for illumination at that sort
of size/power scale.
I've always wanted to try building one of these with a variable shunt
element that's either a multiplying DAC, a FET, or a resistor and a
switch that brings it on-line at a really fast PWM rate. Then control
the shunt element from a small microprocessor to keep the amplitude dead
nuts on to what it's supposed to be, no muss, no fuss.
Of course, by the time you go that far down the mixed digital-analog
road, you're probably better off using a small DDS just so that
engineers fresh from college will be able to understand the circuit if
for no other reason.
Jim Thompson
wrote:
>On 03/29/2011 12:08 PM, Jim Thompson wrote:
>> On Tue, 29 Mar 2011 11:32:30 -0700 (PDT), jozamm<joz...@gmail.com>
>> wrote:
>>
>>> Hi all,
>>>
>>> I built a Wien bridge oscillator on a breadboard according
> >> to the guidelines in the LM324 datasheet (Fig. 11)
> >> http://tinyurl.com/6ejjvhz . The output is a distorted sine
> >> wave. I set to oscillate less than 1kHz. For diodes I used
> >> the 1n4148.
>>>
>>> Can someone give me some hints on what I am doing wrong?
>>>
>>> Regards,
>>> Jozamm
>>
>> Wien Bridge Oscillators are a PITA to amplitude stabilize.
>>
>> Try this....
>>
>> http://www.analog-innovations.com/SED/Sedra-Espinosa-Oscillator.pdf
>>
>> ...Jim Thompson
>
>Hey Jim:
>
>I'm sure I could figure that out if I weren't lazy. Could you save me
>some effort and give a run down on the theory? Obviously you're
>engaging in some trickery with that upper op-amp, and I assume that the
>diodes are limiting the output at least semi-softly, but that's all I
>get before the urge to grab some paper and a pencil battles with the
>urge to have another sip of tea.
A while back there was a thread here on Sedra-Espinosa filters. I'll
have to look to be sure, but I think the gimmick is that it has very
high Q, then the diodes roll the Q down softly. (It's been awhile ;-)
For the Shuttle program I did something like that, but used
comparators to set the amplitude accurately.
Don Klipstein
jozamm wrote:
One thing I can suggest adding: An added variable resistor in "rheostat
mode", in parallel with the diode pair. I would suggest a 100 one.
Variable resistors are usually sold as "potentiometers".
"Rheostat mode" means use only 2 leads - the "wiper" lead and one of
the two "ends".
First, set the 100K rheostat to maximum. Then, set the 5k one already
shown to get you a voltage level of AC that you like, or a little more.
After that, adjust the added 100K variable resistor to your tastes, for
sufficiently low distortion and sufficiently high stability of the
oscillation.
--
- Don Klipstein (d...@misty.com)
ChrisQ
>
> The old traditional way of stabilizing a Wien bridge oscillator was with
> a light bulb, with circuit impedances set so that it just barely start
> getting warm from the audio. That would increase its resistance, which
> would (if you put it in the right place in the circuit) decrease the
> overall gain, and would stabilize the gain. As a method it worked
> really really fine in a vacuum tube circuit, and sorta-OK in an op-amp
> circuit back when you could get "grain of wheat" incandescents, and it
> really sucks now that everyone uses LED's for illumination at that sort
> of size/power scale.
>
The later method for solid state wein bridge stabilisation was to use a
small glass encapsulated thermistor, which provides very low distortion. The
thermal time constant of the thermistor keeps the resistance pretty
constant at stabilisation point, but this starts to degrade at very low
frequencies. The Mullard "Manual of Transistor Circuits" from 1962 used
this method, fwir.
There's also the dual integrator op amp sine / cosine oscillator which
can give good results. Don't have a circuit to hand but it was one of
the examples in the old nat semi linear circuits data book. Most likely
on the web as well...
Regards,
Chris
ChrisQ
>
> In addition to other comments, LM324s have a lot of distortion all on
> their own. You can even see it at 60 Hz. Horrible opamp.
>
> John
>
Right, mainly crossover distortion. You can make them much better by a
drain resistor from the output to one of the rails. A mA or two is
usually enough...
Regards,
Chris
Don Klipstein
>On Tue, 29 Mar 2011 11:32:30 -0700 (PDT), jozamm <joz...@gmail.com> wrote:
>
>>I built a Wien bridge oscillator on a breadboard according to the
>>guidelines in the LM324 datasheet (Fig. 11) http://tinyurl.com/6ejjvhz .
>>The output is a distorted sine wave. I set to oscillate less than 1kHz.
>>For diodes I used the 1n4148.
>>
>>Can someone give me some hints on what I am doing wrong?
>
>their own. You can even see it at 60 Hz. Horrible opamp.
I did forget to add in my previous response, that LM324 is prone to
"crossover distortion". It appears to me that LM324 is some approximation
of a Class B amplifier, while its main competitors for audio are Class AB.
An LM324 can often be "cleaned up" by adding a 2.2K or a 1.0K resistor
from output to either supply rail - preferably the lower one.
Then again, my favorite "audio workhorse" op-amp is TLO84 / TL084,
notably Texas Instruments TL084CN.
Andrew Holme
"Tim Wescott" <t...@seemywebsite.com> wrote in message
news:cNudnWiFooxFow_Q...@web-ster.com...
> On 03/29/2011 12:08 PM, Jim Thompson wrote:
>> On Tue, 29 Mar 2011 11:32:30 -0700 (PDT), jozamm<joz...@gmail.com>
>> wrote:
>>
>>> Hi all,
>>>
>>> I built a Wien bridge oscillator on a breadboard according
> >> to the guidelines in the LM324 datasheet (Fig. 11)
> >> http://tinyurl.com/6ejjvhz . The output is a distorted sine
> >> wave. I set to oscillate less than 1kHz. For diodes I used
> >> the 1n4148.
>>>
>>> Can someone give me some hints on what I am doing wrong?
>>>
>>> Regards,
>>> Jozamm
>>
>> Wien Bridge Oscillators are a PITA to amplitude stabilize.
>>
>> Try this....
>>
>> http://www.analog-innovations.com/SED/Sedra-Espinosa-Oscillator.pdf
>>
>> ...Jim Thompson
>
> Hey Jim:
>
> I'm sure I could figure that out if I weren't lazy. Could you save me
> some effort and give a run down on the theory? Obviously you're engaging
> in some trickery with that upper op-amp, and I assume that the diodes are
> limiting the output at least semi-softly, but that's all I get before the
> urge to grab some paper and a pencil battles with the urge to have another
> sip of tea.
Since V(U2.IN+) = V(U2.IN-) you could just as well connect U1.IN- to U2.IN+.
Then you could replicate the R1, R2 divider so both opamps have their own.
Loop gain from U1.OUT to U1.IN+ is ~ -6dB and 0 degrees @ 20 KHz. U1 is a
voltage amp with 6 dB gain.
You get +45 degrees phase shift from U2 and -45 degrees from R4,C2.
Jim Thompson
wrote:
Why? Run the math... caution... your foot is rapidly approaching your
mouth ;-)
>
>Loop gain from U1.OUT to U1.IN+ is ~ -6dB and 0 degrees @ 20 KHz. U1 is a
>voltage amp with 6 dB gain.
>
>You get +45 degrees phase shift from U2 and -45 degrees from R4,C2.
>
George Herold
> On 03/29/2011 11:32 AM, jozamm wrote:> Hi all,
>
> > I built a Wien bridge oscillator on a breadboard according to the
>
> > guidelines in the LM324 datasheet (Fig. 11)
Newark still carries them, I can give you a part number.
I think my first post on SED was about motorboating in 'my' light bulb
stabilized Wein bridge oscillator. If I didn't post the solution
before. The motorboating problem was cured by replacing mylar film
caps with lower distortion
polypropylene(sp). I still have no idea what caused the problem.
But as Tim points out to get nice low distortion you run the circuit
down where it just turns on.... I retro-spec it's a high Q circuit.
>
> I've always wanted to try building one of these with a variable shunt
> element that's either a multiplying DAC, a FET, or a resistor and a
> switch that brings it on-line at a really fast PWM rate. Then control
> the shunt element from a small microprocessor to keep the amplitude dead
> nuts on to what it's supposed to be, no muss, no fuss.
>
> Of course, by the time you go that far down the mixed digital-analog
> road, you're probably better off using a small DDS just so that
> engineers fresh from college will be able to understand the circuit if
> for no other reason.
No one will duplicate the 'nice' bouncy amplitude stabilization you
get with a light bulb. I still remember an old hp200 used in some
student lab
(circa 1980's).
George H.
>
> --
>
> Tim Wescott
> Wescott Design Serviceshttp://www.wescottdesign.com
George Herold
Web-Site.com> wrote:
> On Tue, 29 Mar 2011 13:20:33 -0700, Tim Wescott <t...@seemywebsite.com>
> wrote:
>
>
>
>
>
> >On 03/29/2011 12:08 PM, Jim Thompson wrote:
> >> On Tue, 29 Mar 2011 11:32:30 -0700 (PDT), jozamm<joz...@gmail.com>
> >> wrote:
>
> >>> Hi all,
>
> >>> I built a Wien bridge oscillator on a breadboard according
> > >> to the guidelines in the LM324 datasheet (Fig. 11)
>
> Remember: Once you go over the hill, you pick up speed- Hide quoted text -
>
> - Show quoted text -
Hic, I swear I didn't read tha before posting... makes sense!
George H.
Fred Bloggs
>
> > I built a Wien bridge oscillator on a breadboard according to the
>
> > guidelines in the LM324 datasheet (Fig. 11)
> > I set to oscillate less than 1kHz. For diodes I used the 1n4148.
>
>
>
> > Can someone give me some hints on what I am doing wrong?
>
> Using a circuit out of a data sheet? Designed by an applications
> engineer who either really loves his job or couldn't get a 'real job' as
> a circuit designer? And whose job is to puff off chips, not make things
> work?
It's probably a Widlar circuit!
> Wescott Design Serviceshttp://www.wescottdesign.com
Phil Hobbs
C'mon, Tim, I learned half of my electronics by assuming that all data
sheet circuits were junk and then finding out why. ;)
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
Fred Bloggs
>
> I built a Wien bridge oscillator on a breadboard according to the guidelines in the LM324 datasheet (Fig. 11)http://tinyurl.com/6ejjvhz. The output is a distorted sine wave. I set to oscillate less than 1kHz. For diodes I used the 1n4148.
>
> Can someone give me some hints on what I am doing wrong?
>
> Regards,
> Jozamm
What did you use for a circuit at the common reference voltage node????
Bitrex
I don't think one do what Mr. Holme is suggesting - U1's inverting
input connection to the other opamp's inverting input is how the system
obtains very close to precisely unity gain. If you attempted to give U1
its own voltage divider to set the gain to +6 dB to compensate for the
-6 dB drop across the voltage divider, small variations in component
tolerance would probably cause U1 to either hit the rails or fail to
oscillate.
Bitrex
Correction - I meant the -6 dB drop through the RC network.
Tim Wescott
> Tim Wescott wrote:
>> On 03/29/2011 11:32 AM, jozamm wrote:
>>> Hi all,
>>>
>>> I built a Wien bridge oscillator on a breadboard according to the
>> > guidelines in the LM324 datasheet (Fig. 11)
>> > http://tinyurl.com/6ejjvhz . The output is a distorted sine wave.
>> > I set to oscillate less than 1kHz. For diodes I used the 1n4148.
>>>
>>> Can someone give me some hints on what I am doing wrong?
>>
>> Using a circuit out of a data sheet? Designed by an applications
>> engineer who either really loves his job or couldn't get a 'real job' as
>> a circuit designer? And whose job is to puff off chips, not make things
>> work?
>>
>>
>> Of course, by the time you go that far down the mixed digital-analog
>> road, you're probably better off using a small DDS just so that
>> engineers fresh from college will be able to understand the circuit if
>> for no other reason.
>>
>
> C'mon, Tim, I learned half of my electronics by assuming that all data
> sheet circuits were junk and then finding out why. ;)
Data sheet circuits are great for setting you on the path to a good
circuit that works. But they often aren't good circuits that work, at
least not for your particular application.
My 2nd job out of college was working for a guy who would copy circuits
out of data sheets and into his schematics. When he was stumped he'd
get applications engineers to fax him circuits. He wouldn't change them
_a bit_. For some strange reason he had all sorts of problems with
noise, drift, and impedance matching. And that was before you took into
account the fact that he'd let the autorouter run digital lines
underneath sensitive analog circuits on mixed-signal boards.
Jim Thompson
wrote:
>On 03/29/2011 04:16 PM, Phil Hobbs wrote:
When I was at GenRad I'd mark out a section of PCB and designate it as
a no man's land... any attempt at digital violation would garner great
grief... very rarely would anyone try :-)
...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 |
Phil Hobbs
Some folks like debugging, I suppose. When my children were young, and
were going on about how nice it would be to have some ridiculously huge
house, I used to ask them if their hobby was vacuuming. ;)
Bill Bowden
> On 03/29/2011 11:32 AM, jozamm wrote:> Hi all,
>
> > I built a Wien bridge oscillator on a breadboard according to the
>
> > guidelines in the LM324 datasheet (Fig. 11)
> > I set to oscillate less than 1kHz. For diodes I used the 1n4148.
>
>
>
> > Can someone give me some hints on what I am doing wrong?
>
> Using a circuit out of a data sheet? Designed by an applications
> engineer who either really loves his job or couldn't get a 'real job' as
> a circuit designer? And whose job is to puff off chips, not make things
> work?
>
So, what's your opinion of the wien bridge circuit in the LTspice
examples (wien.asc)? It uses a jfet in the feedback loop and a LT1001
op-amp. Output is about 3 volts p-p from +/- 15 volt supplies.
-Bill
> Wescott Design Serviceshttp://www.wescottdesign.com
Jamie
> On Mar 29, 12:33 pm, Tim Wescott <t...@seemywebsite.com> wrote:
>
>>On 03/29/2011 11:32 AM, jozamm wrote:> Hi all,
>>
>>
>>>I built a Wien bridge oscillator on a breadboard according to the
>>
>> > guidelines in the LM324 datasheet (Fig. 11)
>> >http://tinyurl.com/6ejjvhz. The output is a distorted sine wave.
>> > I set to oscillate less than 1kHz. For diodes I used the 1n4148.
>>
>>
>>
>>
>>>Can someone give me some hints on what I am doing wrong?
>>
>>Using a circuit out of a data sheet? Designed by an applications
>>engineer who either really loves his job or couldn't get a 'real job' as
>>a circuit designer? And whose job is to puff off chips, not make things
>>work?
>>
>
>
> So, what's your opinion of the wien bridge circuit in the LTspice
> examples (wien.asc)? It uses a jfet in the feedback loop and a LT1001
> op-amp. Output is about 3 volts p-p from +/- 15 volt supplies.
>
> -Bill
>
>
>
I remember I had a piece or gear years ago (Tube) that had a Wien
bridge in it that used incandescent lamps to stabilized the bias.
Jamie
Phil Hobbs
master's thesis at Stanford, and then became HP's first product, the
HP200A audio oscillator. See e.g.
http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/0002history.html
jozamm
Thanks very much for the good feedback I received. I will try to change the op-amp and put the pots as suggested.
Given that there was some criticism about taking circuits from application notes can some suggest a good book on oscillators since I am lecturing on that subject. I would appreciate if it has some practical circuits so that they can be done as experiments for my students.
Thanks very much,
Regards,
RST Engineering
idea of the theory and application of the material.
Your university ought to be more careful on who they hire as faculty.
Jim
On Tue, 29 Mar 2011 23:37:13 -0700 (PDT), jozamm <joz...@gmail.com>
wrote:
>Hi,
George Herold
Wow, you are teaching this?
Try this. Get rid of the diode and 5k pot. (Yeah and get a good opamp
and run it bipolar not single supply.) Set the oscillator frequency
at some low value... 10 Hz or so. Now with the pot on the feedback
resistor slowly increase the gain... while watching the output. At
some point you'll see the circuit just start to oscillate... the
amplitude will grow slowly. (The low frequency will let you to watch
this on the 'scope in real time. (I guess I'm assuming you've got a
digital 'scope.))
Reduce the gain a bit and the amplitude should slowly go down... you
may need a small 'tweaker' pot to get the fine control you will need.
If you let the amplitude grow it eventually hits the power supply
rails and clips.
George H.
whit3rd
> Hi,
>
> Thanks very much for the good feedback I received. I will try to change the op-amp and put the pots as suggested.
>
> Given that there was some criticism about taking circuits from application notes can some suggest a good book on oscillators since I am lecturing on that subject. I would appreciate if it has some practical circuits so that they can be done as experiments for my students.
Well, the Wien bridge is a classic, but should come relatively late in the course,
after filter networks. This particular circuit would benefit greatly from a
pullup resistor on the LM324 op amp output (to force it into class A operation)
or a different op amp (LT1013 was suggested). LM324 is power-stingy because
of its output impedance dead-band, but that's a bad quality in a low distortion
application. It would also be improved by a 'correct' setting procedure for the
adjustable resistor. Actually, I'd lobby for a few precision resistors and a resistor
in parallel with the diodes. Forcing most of feedback current through the
diodes seems ill-advised, IMHO. Your goal with that adjustment is exactly a voltage
gain of 3.000000000... (because higher gain makes the oscillator clip, lower makes it stop
oscillating).
Pay no attention to critics of 'taking circuits from application notes', it's VERY MUCH
a part of our technology to do so. I was able for years to rebuild electronics
with no access to the circuit diagrams, because nearly every user of IC jungle
chips followed slavishly the manufacturer's application examples. Make sure
the students see, too, that some of the notes are ... incomplete.
Tim Wescott
I know how strongly I worded things, but perhaps a better way of putting
what I said would be that it is foolish to _unthinkingly_ use a circuit
from an app note. App note circuits are often very good, but sometimes
they're bad -- and the app note doesn't say. App notes also rarely say
_when_ the circuit is good for one thing or another.
So app note circuits can be great starting points, but should never be
used without question.
Tim Wescott
> Hi,
>
> Thanks very much for the good feedback I received. I will try to change the
> op-amp and put the pots as suggested.
>
> Given that there was some criticism about taking circuits from application
> notes can some suggest a good book on oscillators since I am lecturing on
> that subject. I would appreciate if it has some practical circuits so
that
> they can be done as experiments for my students.
I like "Oscillator Design & Computer Simulation" by Randal W. Rhea,
Prentice-Hall, 1990. The "computer" part of the simulation is
excruciatingly dated, but you can take what he says and apply it to
SPICE quite easily. Similarly, when he says "Oscillator" he's just
assuming that you know he means "RF oscillator with resonant elements".
But the principles still apply.
It is very instructive to analyze that circuit _on paper_, both for the
circuit behavior with the diodes removed (simulating the low-amplitude
case) and the circuit behavior with the diodes shorted (simulating the
high-amplitude case). You can do both by getting the system
characteristic equation in the Laplace domain with the feedback
resistance left as a variable, then substituting in various values.
Then do a root-locus of the system as the feedback resistance (or
conductance) varies.
Find the critical value of feedback resistance for which the system is
metastable, and the sensitivity of that value on the other component
values in the system. At that point, you have about 98% of the story,
and you only need to ask yourself what happens when you throw diodes
into the mix, and how you might select different values of the feedback
resistances to get lower distortion.
John - KD5YI
>
> I know how strongly I worded things, but perhaps a better way of putting
> what I said would be that it is foolish to _unthinkingly_ use a circuit
> from an app note. App note circuits are often very good, but sometimes
> they're bad -- and the app note doesn't say. App notes also rarely say
> _when_ the circuit is good for one thing or another.
>
> So app note circuits can be great starting points, but should never be
> used without question.
>
Remembering the usual precautions of depending on simulators, do you
think that passing a simulation would be sufficient to warrant
proceeding with a particular app note circuit?
John
Phil Hobbs
A lot of the problems with datasheet circuits have to do with:
1. Unsuitable parts for the job. The mfg wants you to use their parts
exclusively, regardless of whether they're cost-effective in the
application.
2. Lack of protection networks. You can blow up a 7805 by shorting its
input to ground, for instance.
3. Lack of attention to startup and transient situations.
4. Oversimplifying. An LM317 plus a pot makes an okay current source,
unless the wiper of the pot bounces momentarily (which it's going to,
eventually). When that happens, the 317 will very likely blow up
whatever it's driving.
5. Other things that I've forgotten.
Cheers
Phil Hobbs
Joel Koltner
> Similarly, when he says "Oscillator" he's just assuming that you know he
> means "RF oscillator with resonant elements".
Rhea's other book, "HF Filter Design and Simulation," means "design and
simulation for filtered typically at UHF and up..." :-)
But hey, he came from the cable TV industry, they talk funny there, it would
seem!
Andrew Holme
"Jim Thompson" <To-Email-Use-Th...@On-My-Web-Site.com> wrote in
message news:ccn4p6tlmp2nd5921...@4ax.com...
I just found the above transformation helped me understand and analyse it.
Replicating the divider was unnecessary.
Here are three (steady-state) equivalent re-arrangements in one sim:
Version 4
SHEET 1 1684 692
WIRE -2160 -768 -2304 -768
WIRE -1568 -768 -2080 -768
WIRE -1184 -768 -1328 -768
WIRE -592 -768 -1104 -768
WIRE -208 -768 -400 -768
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WIRE -208 -624 -288 -624
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WIRE -48 -624 -144 -624
WIRE 96 -624 32 -624
WIRE 144 -624 96 -624
WIRE 272 -624 208 -624
WIRE 352 -624 272 -624
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WIRE 656 -624 512 -624
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WIRE 736 -624 656 -624
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WIRE -144 -592 -144 -624
WIRE 512 -576 512 -624
WIRE 656 -576 656 -624
WIRE 736 -576 736 -624
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WIRE -2096 -528 -2096 -576
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WIRE -1776 -528 -1776 -656
WIRE -1776 -528 -1808 -528
WIRE -1712 -528 -1776 -528
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WIRE -960 -528 -1040 -528
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WIRE -592 -528 -656 -528
WIRE -1568 -496 -1568 -528
WIRE -592 -496 -592 -528
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WIRE 160 -496 96 -496
WIRE -400 -480 -400 -608
WIRE -144 -480 -144 -512
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WIRE 272 -480 224 -480
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WIRE 512 -464 512 -512
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WIRE -592 -400 -800 -400
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WIRE -592 -384 -592 -400
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FLAG -2304 -528 0
FLAG -2224 -528 0
FLAG -2096 -528 0
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FLAG -592 -272 0
FLAG -1328 -528 0
FLAG -1248 -528 0
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SYMATTR InstName U1
SYMBOL Opamps\\opamp -1888 -480 M0
SYMATTR InstName U2
SYMBOL res -1584 -400 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res -1584 -512 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res -2032 -656 R0
SYMATTR InstName R4
SYMATTR Value 8k06
SYMBOL res -1616 -544 R90
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SYMBOL res -2064 -784 R90
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SYMATTR Value 1n
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WINDOW 3 24 8 Left 0
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SYMATTR Value 1n
SYMBOL diode -2288 -576 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL diode -2240 -640 R0
SYMATTR InstName D2
SYMATTR Value 1N4148
SYMBOL Opamps\\opamp -720 -608 M180
SYMATTR InstName U1b
SYMBOL Opamps\\opamp -912 -480 M0
SYMATTR InstName U2b
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SYMBOL res -608 -512 R0
SYMATTR InstName R2b
SYMATTR Value 10k
SYMBOL res -1056 -656 R0
SYMATTR InstName R4b
SYMATTR Value 8k06
SYMBOL res -640 -544 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5b
SYMATTR Value 7k87
SYMBOL res -1088 -784 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R6b
SYMATTR Value 100k
SYMBOL cap -896 -544 R90
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WINDOW 3 32 32 VTop 0
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SYMATTR Value 1n
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SYMATTR Value 1n
SYMBOL diode -1312 -576 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D1b
SYMATTR Value 1N4148
SYMBOL diode -1264 -640 R0
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SYMATTR Value 1N4148
SYMBOL Opamps\\opamp -320 -560 M180
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SYMBOL res 448 -640 R90
WINDOW 0 0 56 VBottom 0
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SYMATTR Value 8k06
SYMBOL cap 528 -512 R180
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SYMATTR Value 1n
SYMBOL diode 672 -512 R180
WINDOW 0 24 72 Left 0
WINDOW 3 24 0 Left 0
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SYMATTR Value 1N4148
SYMBOL diode 720 -576 R0
SYMATTR InstName D2c
SYMATTR Value 1N4148
SYMBOL Opamps\\opamp 192 -544 R0
SYMATTR InstName U2c
SYMBOL res 48 -640 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
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SYMBOL res -192 -640 R180
WINDOW 0 36 76 Left 0
WINDOW 3 36 40 Left 0
SYMATTR InstName R20
SYMATTR Value 100k
SYMBOL cap 208 -640 R90
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SYMBOL res -160 -464 R0
SYMATTR InstName R1c
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SYMBOL res -160 -608 R0
SYMATTR InstName R2c
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TEXT -2304 -432 Left 0 !.lib opamp.sub
TEXT -2312 -384 Left 0 !.tran 1m
TEXT -2304 -336 Left 0 !.ic V(V1a)=1u
TEXT -1328 -336 Left 0 !.ic V(V1b)=1u
TEXT 240 -352 Left 0 !.ic V(V1c)=1u
John - KD5YI
I am now beginning to agree completely. I just simulated my second data
sheet circuit and I have found (barring mistakes made by me) that it is
trash. This one comes from AN-43, page AN47-24, from Linear as suggested
by George Herold in the original post.
I was interested in Fig 27, the lock-in amplifier. It didn't look right
to me so I simulated the front-end of it. I found that the bridge would
not null.
If I am wrong, I would appreciate someone pointing out my error.
Thanks,
John
Jim Thompson
wrote:
I do remember a subtle difference in GBW depending on which of those
nodes were merged. It was discussed here sometime in the past year.
I'll see if I can find the message-ID.
Tim Wescott
I don't think that it's a good idea to proceed with any circuit that
simulates OK but that you don't understand. _If_ it simulates OK, _and_
you understand the circuit well enough to know that it'll probably work,
_or_ if you have the time to spend on a board spin, then yes -- go
ahead. OTOH, if you have no clue whatsoever of why it works or how,
then having it simulate OK means nothing, whether the circuit came from
a data sheet or your own fertile mind.
Tim Wescott
I'd never thought of (mostly power oscillators, and oscillators that
were intentionally made with a minimum of components. On top of that, I
read the book, then I went and sat down and designed an oscillator in
SPICE using his methods*, then I waltzed over to my bench, built the
SPICE circuit, and it worked as predicted. I thought that was a pretty
good indication of how well the book explained things.
* Meaning both that I never needed to get it oscillating in SPICE, nor
did I ever have to do anything other than linearized analysis.
George Herold
>
> - Show quoted text -
Hi John, It looks like that circuit is just using a bridge to measure
a thermistor resistance. The bridge is always out of balance...
except for one temperature.
George H.
Jamie
Nice Wheatstone bridge there. The bases for strain gauge and Kelvin
bridge measurements.
Jamie
John - KD5YI
Have you simulated it? If so, please give me the thermistor's resistance
that results in balance.
John
John - KD5YI
> On 03/30/2011 12:38 PM, John - KD5YI wrote:
>> On 3/30/2011 2:25 PM, Tim Wescott wrote:
>>>
>>> I know how strongly I worded things, but perhaps a better way of putting
>>> what I said would be that it is foolish to _unthinkingly_ use a circuit
>>> from an app note. App note circuits are often very good, but sometimes
>>> they're bad -- and the app note doesn't say. App notes also rarely say
>>> _when_ the circuit is good for one thing or another.
>>>
>>> So app note circuits can be great starting points, but should never be
>>> used without question.
>>>
>>
>> Remembering the usual precautions of depending on simulators, do you
>> think that passing a simulation would be sufficient to warrant
>> proceeding with a particular app note circuit?
>
> I don't think that it's a good idea to proceed with any circuit that
> simulates OK but that you don't understand. _If_ it simulates OK, _and_
> you understand the circuit well enough to know that it'll probably work,
> _or_ if you have the time to spend on a board spin, then yes -- go
> ahead. OTOH, if you have no clue whatsoever of why it works or how, then
> having it simulate OK means nothing, whether the circuit came from a
> data sheet or your own fertile mind.
>
That was one of many the things I meant by "Remembering the usual
precautions of depending on simulators".
I think you agreed with my philosophy on that. Thanks.
John
George Herold
Dang, I hardly ever simulate circuits. It's a lot more fun to build
them!
You want me to give you the balance condition of a bridge? ??
It's a resistor ratio on each 'side' of the bridge. If you put 'say'
10k resistors every where then it's balanced when the thermistor R is
10k ohm... whatever temperature that is.
Bridges are really nice circuit elements. Have you ever used one?
George H.
John - KD5YI
I understand and have used bridges. I think you have not looked deeply
into the schematic. I believe the bridge is permanently unbalanced *with
respect to the sensing op-amp* by the zero crossing circuit.
If you do not simulate, I will accept a mathematical analysis. But you
must show all components attached to the bridge in the analysis.
I am not trying to be argumentative. It will be educational which ever
way this goes.
John
John - KD5YI
BTW, George, do you have LTSpice?
John
John - KD5YI
I think the capacitor in the zero-crossing circuit puts a permanent
imbalance on the bridge. Just as in an SWR bridge, there is no way to
balance without a balancing reactance somewhere. Note that the
zero-cross circuti forces current flow through the upper left hand
bridge resistor.
Does this make sense?
John
George Herold
Hi John, Sorry.
I looked at the 'stuff' below the bridge and then just ignored it.
Does it change the balance? (That sucks.) I'd do synchronous
detection differerntly, but I got the idea. I do have LTspice. I've
not tried any new circuits lately, too much ...else... to do. What's
an SWR bridge? Standing wave ratio? Ahh.. don't get me started on
nights spent keeping a low temp ESR spectrometer in balance... (the
liquid helium level would slowly move down the waveguide)
George H.
John - KD5YI
I was going to offer you an LTSpice ascii file for you to play with and
see the problem with the circuit. However, it sounds as if you are very
busy and I don't want to impose on that. It isn't that important.
Cheers,
John
George Herold
Thanks for the offer John. Aren't you always busy? But still it's
nice to take a little time to do something fun and educational. That
bridge circuit just didn't do it for me.
George H.