JFET Common Source DC Amplifier Temperature Compensation
D from BC
What are some ways to temperature compensate a common source JFET DC
amplifier.
Vdd=5V
|
10K
|
+------------Vout (biased for 50% Vdd)
|d
signal--->[ N JFET
|s
|
Gnd
'DC' amplifier.
Details:
1) The signal does contain a stable bias voltage.
2) Temp range: 20C to 50C
3) Vout thermal error of +/-10mVdc.
4) BW: DC to 2Mhz
I'm guessing compensation by:
1) PTC or NTC resistor
2) Use another JFET and feed in anti drift.
3) Find a special jfet (IC??) << not an op amp!
Possible ??
D from BC
Jim Thompson
What does "temperature compensate" mean? Bias point, gain, both?
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
America: Land of the Free, Because of the Brave
D from BC
Let's say the gate voltage is set for Vout = 2.5V...
Vout can vary with temperature..Especially with no JFET source
resistor.
For a DC amplifier like this, doesn't DC bias thermal drift at the
output also look like DC gain thermal drift?
IIRC the power supply drifting with temperature can cause bias drift
alone.
D from BC
Joerg
> My app uses a JFET..
>
> What are some ways to temperature compensate a common source JFET DC
> amplifier.
>
> Vdd=5V
> |
> 10K
> |
> +------------Vout (biased for 50% Vdd)
> |d
> signal--->[ N JFET
> |s
> |
> Gnd
>
> 'DC' amplifier.
>
> Details:
> 1) The signal does contain a stable bias voltage.
> 2) Temp range: 20C to 50C
> 3) Vout thermal error of +/-10mVdc.
> 4) BW: DC to 2Mhz
>
> I'm guessing compensation by:
> 1) PTC or NTC resistor
> 2) Use another JFET and feed in anti drift.
That would be my avenue. There are duals such as this one, pretty low noise:
http://www.semicon.toshiba.co.jp/docs/datasheet/en/Transistor/2SK3320_en_datasheet_071101.pdf
Buying those might be a challenge though.
> 3) Find a special jfet (IC??) << not an op amp!
>
> Possible ??
>
--
Regards, Joerg
Jim Thompson
<myreal...@comic.com> wrote:
Except for the "sweet spot" bias condition, gain (small signal) and
output Q-point probably vary independently.
Instead of your vague "specification" what is it you are trying to
accomplish?
Tim Williams
so source current (and thus Vgs) is always equal between the two?
Hmm...but that's for a source follower. Constant source current doesn't do
you much good. Well, it would work for setting DC bias nonetheless, with
the source bypassed to ground. Doesn't do anything for transconductance.
Tim
--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms
"D from BC" <myreal...@comic.com> wrote in message
news:pst3k39bb3k3k0q21...@4ax.com...
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Ahhh... I think you've caught on that I dumb down problems to the
point of making it the wrong problem :)
Gee...I dunno if I can devague it more...
I'll try...
I need a fast DC amplifier with an input range of 0 to
-200mV(negative) with a response up to 2Mhz.
The output of the amplifier has to reach 2.5V out for -200mV in.
There is no negative supply present.
I thought a JFET would be best..but spoiled by thermal drift.....
My alternatives were:
Use resistive network + op amp
Create a negative supply + op amp
Put one of those 'beyond the rails' op amps to the test.
Negative rail IC + op amp
D from BC
Fred Bartoli
Are you sure that they are independent?
For a given jfet model gm is strongly linked to the drain current (and
essentially free from IDSS).
> Instead of your vague "specification" what is it you are trying to
> accomplish?
>
> ...Jim Thompson
--
Thanks,
Fred.
Jim Thompson
<myreal...@comic.com> wrote:
If -200mV Input => +2.5V Output, what does 0V Input => ?? Output
Jim Thompson
>Jim Thompson a écrit :
>> On Mon, 19 Nov 2007 15:14:30 -0800, D from BC
>> <myreal...@comic.com> wrote:
>>
[snip]
>>
>> Except for the "sweet spot" bias condition, gain (small signal) and
>> output Q-point probably vary independently.
>>
>
>Are you sure that they are independent?
>For a given jfet model gm is strongly linked to the drain current (and
>essentially free from IDSS).
>
They're not "independent", but I don't think incremental gain and bias
Q-point will exactly track.
Fred Bartoli
> On Tue, 20 Nov 2007 01:21:43 +0100, Fred Bartoli <" "> wrote:
>
>> Jim Thompson a écrit :
>>> On Mon, 19 Nov 2007 15:14:30 -0800, D from BC
>>> <myreal...@comic.com> wrote:
>>>
> [snip]
>>> Except for the "sweet spot" bias condition, gain (small signal) and
>>> output Q-point probably vary independently.
>>>
>> Are you sure that they are independent?
>> For a given jfet model gm is strongly linked to the drain current (and
>> essentially free from IDSS).
>>
>
> They're not "independent", but I don't think incremental gain and bias
> Q-point will exactly track.
>
Ah, a bit of french meaning slept in there.
Now you said they'd probably vary independently, which they don't (for
ex. http://www.alldatasheet.com/view.jsp?Searchword=2SK170 ).
But gain and bias point sure can't exactly track. For this to be so,
you'd have to have gm proportional to Id, which you can't ignore to be
the hallmark of exponential :-)
--
Thanks,
Fred.
Joerg
Many regular opamps can work down to about 300mV below the negative rail
but you'd have to find one that's fast enough and low enough in offset.
> Negative rail IC + op amp
>
If you have a spare inverter somewhere you could make in inverting switcher.
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
It's allowed to clip.. :)
After some thought ..and to answer your question earlier..
I'd say I'm looking for bias point thermal compensation.
Not small signal gain thermal compensation.
I believe it'll be ok for the thermal compensation to move the Q point
in order to maintain Vout at the bias level of 2.5V.
The Vout bias level is not to drift with temperature.
The new Q point due to thermal compensation and the resulting new
small signal Av.... is probably ok.
For my app..
The Avdc thermal stability (at only one Q point) is more important
than the small signal Avac thermal stability..
D from BC
John Larkin
<myreal...@comic.com> wrote:
>Put one of those 'beyond the rails' op amps to the test.
The r-r opamps do work. Sounds like a lot less hassle than trying to
work around a discrete jfet.
John
Jim Thompson
<myreal...@comic.com> wrote:
Is this part of your "alternating noisy edges" problem?
Seems you're requesting assistance at too small a scale... a larger
over-view might produce a more elegant solution when we can see loads
and continuing functions ;-)
D from BC
Bummer... :( No app note at the bottom.. :)
I get suspicious when parts are hard to get...It usually means I'm
doing something the wrong way..or old way.. :)
On occasion, it can be it being a very new way..but I don't think so
in this case..
D from BC
Joerg
Looks like he doesn't even need output RR. Isn't there a LM324 on
steroids somewhere?
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Good guess! :)
I know what your saying..
It's like Dr. Frankenstein putting bits and pieces of people together
to make Frankenstein..
IIRC Frankenstein wasn't an elegant solution.. :)
I'm just chicken to post my whole project. Also, there's the pride
factor too..
And...I'm trying to have a mostly original project..
So... it might wander around like a zombie and scare people...I made
it and it works...well kinda.. :)
D from BC
Winfield
> Jim Thompson a écrit :
>>> Jim Thompson a écrit :
>>>> D from BC wrote:
>
>> [snip]
>>>> Except for the "sweet spot" bias condition, gain (small signal)
>>>> and output Q-point probably vary independently.
>
>>> Are you sure that they are independent?
>>> For a given jfet model gm is strongly linked to the drain current
>>> (and essentially free from IDSS).
>
>> They're not "independent", but I don't think incremental gain
>> and bias Q-point will exactly track.
>
> Ah, a bit of french meaning slept in there.
>
> Now you said they'd probably vary independently, which they don't
> (for ex.http://www.alldatasheet.com/view.jsp?Searchword=2SK170).
>
> But gain and bias point sure can't exactly track. For this to be
> so, you'd have to have gm proportional to Id, which you can't
> ignore to be the hallmark of exponential :-)
Well, as I understand it, and based on my measurements as well,
gm does mostly track Id, more or less independent of Idss,
especially at currents well below Idss. But I don't get the Q
discussion y'all are having here. To my mind the Id operating
point is best set independently from the particular miserable
JFET's Vgs vs. Id, etc. I mean, part-to-part, sheesh! Bummer!
But anyway, whatever, I suggest that D just go ahead and use a
JFET opamp. I mean, I'm a "big" fan of JFETs and use them as I
can, for when they're best, but what an unholy pain they are!
Phil Hobbs
How about an ordinary single-supply op amp wired as an inverter? Ground
the noninverting input, and bob's your uncle.
Cheers,
Phil Hobbs
D from BC
Well..If I've posted anything goofy about JFET's, it's probably
because I've forgotten them.
I usually dodge discrete transistors and use op amps instead.
But..this time around, a JFET looked like a good fit.
It did the following:
Low noise
Impedance transformation
Level shifting
Below rail small signal amplification
Fast
small footprint
cheap
I have no idea how drify a JFET can be...
I just know that generally all transistors change with temperature.
I might go to an op amp solution just to avoid all the textbook
reading.. :)
D from BC
D from BC
<notthis...@removethispacbell.net> wrote:
Available from Digikey
Nearly 3000 in stock..
Two N JFETS in one package
http://www.nxp.com/acrobat_download/datasheets/PMBFJ620_1.pdf
D from BC
Fred Bartoli
I'm not arguing about "Q point". Just about gm dependency on Id.
I totally agree about the almost non dependency of gm on Idss and that
gm is mostly tied to Id for a given fet model (hence biasing is best
done by setting Id).
But gm doesn't track Id. I mean gm isn't proportionnal to Id, but rather
to sqrt(Id). Ok at very low current in the subthreshold region Id
becomes exponential, but this isn't the usual way of using jfets,
specially when we want low noise.
--
Thanks,
Fred.
RRogers
As I recall, you can stabilize the bias point by a source resistance
network providing a specific impedance at a specific voltage using a
Thevenin equivalent divider. I don't have time right now, but if you
need I will figure it out later. The match is accurate not needing
PTC or NTC; just matching one curve against another. Junction
threshold shift vs. mho's as I recall. It's been 40 years so the
memory has faded; actually 6 months is starting to be a stretch:)
RRogers
Vladimir Vassilevsky
D from BC wrote:
>>>>>What are some ways to temperature compensate a common source JFET DC
>>>>>amplifier.
>>>>>
>>>>> Vdd=5V
>>>>> |
>>>>> 10K
>>>>> |
>>>>> +------------Vout (biased for 50% Vdd)
>>>>> |d
>>>>>signal--->[ N JFET
>>>>> |s
>>>>> |
>>>>> Gnd
>>>>>
Here is an incredibly complex schematics:
http://www.abvolt.com/misc/jfet_.gif
Is it clear? Just watch for the dominant pole and don't forget the Ohm's
law.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Joerg
If his source can stomach the current, yes. But then he'd better make
sure it can operate properly with the input at the negative rail. Most
of those are ok a few hundred mV below, at least the older one.
Joerg
Nice! Not exactly a bargain and a bit noisy for my taste but those
should be really practical when you have to temperature-gang something.
Joerg
Be sure to check that close to DC. RF JFET can be rather noisy down there.
> Impedance transformation
> Level shifting
> Below rail small signal amplification
> Fast
> small footprint
Not many of them have been ported to SC75 though. That would be sweet.
> cheap
>
Yes!
> I have no idea how drify a JFET can be...
> I just know that generally all transistors change with temperature.
>
Check figure 10 here:
http://www.colorado.edu/physics/phys3330/phys3330_fa07/pdfdocs/AN102FETbiasing.pdf
The IMHO biggest issue with JFETs is the large production spread. That's
where arrays can come in handy.
> I might go to an op amp solution just to avoid all the textbook
> reading.. :)
>
Among mass product designers that would be considered taking the chicken
exit ;-)
John Larkin
GaAs fets can have nf's in the 0.5 dB range at rf, but be ghastly at
low frequencies. Even the Gm can go to hell below a couple of hundred
khz.
John
RST Engineering (jw)
gasfet and used it at lf so I can't argue from experience either.
I'd be interested in some explanation of what you observed to be true. I
can't think of any mechanism that honks up a gasfet at low freq ... it's not
like they internally matched it to optimize its rf response, or is that the
hiccup?
Jim
Winfield Hill
Yes, Fred, thanks, when I say track Id, I am thinking
of low currents, because I'm usually using large-die
parts, with 500mA Idss, etc., at "low" currents like
5mA, etc., where they act more like BJT transistors,
with Id vs Vgs exponential, as you say, and gm ~ Id.
I'd better not go further out on the proverbial limb,
not having my measured data in front of me.
D from BC
That's like the circuit I thought up..
I just went above not below...if that makes sense..
http://www.members.shaw.ca/chainsaw/SED/JFETamp.jpg
613Kb LTSpice screen capture
Attempt at JFET anti-thermal drift.
Yeah...it keeps the bias level steady, but it cannot distinguish
between thermal drift and plain DC.
So..it's not a DC amp and the circuit acts like the electronic
equivalent of a coupling capacitor. :P
The nice part of my circuit is that J1 is 'untouched'. No parasitics
introduced. The JFET can be 'garnished' with reactive components as
used in RF techniques.
I think if I were to put a negative reference voltage on the J2 gate,
that would make J1 thermally stable only when Vin = VgJ2.
Which..in my app...would be ok..
But..I think it gets uglier.... I need a thermally stable negative
reference. :P
D from BC
D from BC
<notthis...@removethispacbell.net> wrote:
Thanks for the link...that's a good read...
On
http://www.colorado.edu/physics/phys3330/phys3330_fa07/pdfdocs/AN102FETbiasing.pdf
p. 4
"The self-bias scheme is a reasonable choice for single ended dc
amplifiers and for ac amplifiers. In unbypassed or dc circuits, some
compromise must be made between the gain loss due to current feedback
degeneration and the advantage of current stabilization achieved with
high Rs"
Translation:
For a DC JFET app, gain is traded off for thermal stability when using
Rs.
I'm proposing a 'no compromise' JFET DC amp. No Rs! Maximum DC gain
with thermal comp.. :)
If that's possible...
D from BC
Joerg
If you go to a company like Interfet and plop a million Dollars onto the
conference table they might be willing to run a few tightly controlled
wafers for you :-)
Jim Thompson
>My app uses a JFET..
>
>What are some ways to temperature compensate a common source JFET DC
>amplifier.
>
> Vdd=5V
> |
> 10K
> |
> +------------Vout (biased for 50% Vdd)
> |d
>signal--->[ N JFET
> |s
> |
> Gnd
>
>'DC' amplifier.
>
>Details:
>1) The signal does contain a stable bias voltage.
>2) Temp range: 20C to 50C
>3) Vout thermal error of +/-10mVdc.
>4) BW: DC to 2Mhz
>
>I'm guessing compensation by:
>1) PTC or NTC resistor
>2) Use another JFET and feed in anti drift.
>3) Find a special jfet (IC??) << not an op amp!
>
>Possible ??
>
>
>D from BC
Sort of like this...
http://analog-innovations.com/SED/DfromBC-Bias.pdf
Not dead-on, but close... exact solution is left as an exercise for
the student ;-)
Vladimir Vassilevsky
D from BC wrote:
> On Tue, 20 Nov 2007 14:22:23 GMT, Vladimir Vassilevsky
> <antispa...@hotmail.com> wrote:
>
>
>>
>>D from BC wrote:
>>
>>
>>>>>>>What are some ways to temperature compensate a common source JFET DC
>>>>>>>amplifier.
>>>>>>>
>>>>>>> Vdd=5V
>>>>>>> |
>>>>>>> 10K
>>>>>>> |
>>>>>>> +------------Vout (biased for 50% Vdd)
>>>>>>> |d
>>>>>>>signal--->[ N JFET
>>>>>>> |s
>>>>>>> |
>>>>>>> Gnd
>>>>>>>
>>
>>Here is an incredibly complex schematics:
>>
>>http://www.abvolt.com/misc/jfet_.gif
>>
>>Is it clear? Just watch for the dominant pole and don't forget the Ohm's
>>law.
>
> Yeah...it keeps the bias level steady, but it cannot distinguish
> between thermal drift and plain DC.
> So..it's not a DC amp and the circuit acts like the electronic
> equivalent of a coupling capacitor. :P
Here we go:
http://www.abvolt.com/misc/jfet2.gif
Paycheck time :-)
Fred Bartoli
Woah, 500mA IDss!
Are you speaking of jfets or depletion mosfets?
I've never seen jfets that high. Any favorite part#?
--
Thanks,
Fred.
Jan Panteltje
Vassilevsky <antispa...@hotmail.com> wrote in
<JmJ0j.69509$YL5....@newssvr29.news.prodigy.net>:
>
>Here we go:
>
>http://www.abvolt.com/misc/jfet2.gif
>
>Paycheck time :-)
mm, that circuit simply stabilizes the output at 2.5 V.
Not at Vin + 2.5V.
No cigars.
To put it simply, any change in Vds from the lower FET, also
due to Vin, is regulated away.
Joerg
Oh, there used to be wonderful power JFETs. For example the P8000 and
P8002. All very short-lived and history now :-(
Fred Bartoli
This circuit copies the first jfet VGS to the second one (if they are
identical).
--
Thanks,
Fred.
Jan Panteltje
"> wrote in <474369be$0$23994$426a...@news.free.fr>:
>Jan Panteltje a écrit :
>> On a sunny day (Tue, 20 Nov 2007 22:39:37 GMT) it happened Vladimir
>> Vassilevsky <antispa...@hotmail.com> wrote in
>> <JmJ0j.69509$YL5....@newssvr29.news.prodigy.net>:
>>
>>> Here we go:
>>>
>>> http://www.abvolt.com/misc/jfet2.gif
>>>
>>> Paycheck time :-)
>>
>> mm, that circuit simply stabilizes the output at 2.5 V.
>> Not at Vin + 2.5V.
>> No cigars.
>> To put it simply, any change in Vds from the lower FET, also
>> due to Vin, is regulated away.
>>
>>
>Nope, that the first transistor vds that's stabilized at 2.5V
OK, my mistake, I thought he took the output from the drain of the lower FET.
Yes the opamp output will be Vin + 2.5V
>This circuit copies the first jfet VGS to the second one (if they are
>identical).
But then, if you have an opamp, why use the JFET end the extra 2.5 V?
OK, maybe it was not a FET opamp.
My apologies anyways to Vladimir, I just wanted to go to sleep
and realised that you ment the opamp output.
So I switched on the PC to correct it and found Fred's posting.
Winfield Hill
Yes, I'm sure you have, Fred.
The familiar J105 "switching" JFET can make
a great low-noise amplifier. Check it out,
Idss = 500mA. Awesome, but of course we'd
never think of using it in analog mode
anywhere near such high drain currents.
D from BC
Yup...I think that's where I'm headed..
Maybe call J2 a 'reference amp'. Where J2 is a reference circuit.
Relative to J2 drift, compensate to maintain J1's bias point.
Something like that... :P
Nice.. You did a temp sweep.
Heck...I don't even know if I can trust LT's JFET models and I haven't
spotted yet if I can do a temp sweep in LTSpice.
I'd say 90% of circuits I've seen use Rs in JFET linear circuits.
Without Rs...it's interesting to see the performance of the
alternatives. :)
D from BC
Jim Thompson
<myreal...@comic.com> wrote:
>On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
><To-Email-Use-Th...@My-Web-Site.com> wrote:
>
[snip]
>>
>>Sort of like this...
>>
>>http://analog-innovations.com/SED/DfromBC-Bias.pdf
>>
>>Not dead-on, but close... exact solution is left as an exercise for
>>the student ;-)
>>
>> ...Jim Thompson
>
>Yup...I think that's where I'm headed..
>
>Maybe call J2 a 'reference amp'. Where J2 is a reference circuit.
>Relative to J2 drift, compensate to maintain J1's bias point.
>Something like that... :P
>
>
>Nice.. You did a temp sweep.
>
>Heck...I don't even know if I can trust LT's JFET models and I haven't
>spotted yet if I can do a temp sweep in LTSpice.
Sure you can. Mikey did a good job with LTspice.
>
>I'd say 90% of circuits I've seen use Rs in JFET linear circuits.
>Without Rs...it's interesting to see the performance of the
>alternatives. :)
>
>
>D from BC
If you're really trying to make a comparator rather than a linear
amplifier, you might experiment with current source loads (hint, hint
Jim Thompson
<To-Email-Use-Th...@My-Web-Site.com> wrote:
[snip]
Load this file again...
http://analog-innovations.com/SED/DfromBC-Bias.pdf
It now contains another version... of course use a better OpAmp or
Comparator to get the speed ;-)
Jim Thompson
<To-Email-Use-Th...@My-Web-Site.com> wrote:
[snip]
Load this file yet again...
http://analog-innovations.com/SED/DfromBC-Bias.pdf
It now contains still another version... Comparator as Output.
I can now walk, swaying a bit, without aid of a walker or cane...
guess it's time to try the water ;-)
Joerg
I don't remember whether the P8000/8002 were classified as switching
JFETs but we used them up to where the heat sink had that new wood stove
smell.
Joerg
> On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
> <To-Email-Use-Th...@My-Web-Site.com> wrote:
>
> [snip]
>
> Load this file yet again...
>
> http://analog-innovations.com/SED/DfromBC-Bias.pdf
>
> It now contains still another version... Comparator as Output.
>
> I can now walk, swaying a bit, without aid of a walker or cane...
Great!
> guess it's time to try the water ;-)
>
Try Shiraz, tastes better. But it won't exactly help with that swaying.
D from BC
<antispa...@hotmail.com> wrote:
Neato!
DC amp: check
Thermal compensation: check
BW to 2Mhz: check (Looks fast! (1) (2))
Amplifies below rail voltage: check
Inverting output: n/a check
Uses matched dual JFET pack: check
Low noise: op amp noise introduced (eh...might be ok).. check
Parts count: great
Prone to oscillation: dunno yet
(1) No gain!
(2) I suspect thermal compensation can be traded off for more gain by
including negative feedback about the op amp.
In some ways this circuit is like the inclusion of Rs... Gain is
traded off for thermal stability.
Impressive :)
D from BC
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
>On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
><To-Email-Use-Th...@My-Web-Site.com> wrote:
>
>[snip]
>
>Load this file yet again...
>
>http://analog-innovations.com/SED/DfromBC-Bias.pdf
>
>It now contains still another version... Comparator as Output.
>
>I can now walk, swaying a bit, without aid of a walker or cane...
>guess it's time to try the water ;-)
>
> ...Jim Thompson
Hmmm.. :)
Constant current combining with thermal compensation current.
Wow .. Thermally stable for Vgs = -200mV.
That's only point that really needs to be temp. stable in my app.
The comparator version might seem out of place here... But I suspect
some eerie precognition. :)
The circuit following the JFET amp in my app is ..a comparator :P
I guess the idea here is to have Vgs J2 = Vgs J1..
When that happens, the output shouldn't drift much.
It's like a voltage divider.
For R1 = R2, if R1 drifts just like R2 then it doesn't show up in the
output.
D from BC
Winfield
> Winfield Hill wrote:
>> Fred Bartoli wrote:
>>> Winfield Hill a écrit :
>
>>>> of low currents, because I'm usually using large-die
>>>> parts, with 500mA Idss, etc., at "low" currents like
>>>> 5mA, etc., where they act more like BJT transistors,
>>>> with Id vs Vgs exponential, as you say, and gm ~ Id.
>>>> I'd better not go further out on the proverbial limb,
>>>> not having my measured data in front of me.
>
>>> Woah, 500mA IDss!
>>> Are you speaking of jfets or depletion mosfets?
>>> I've never seen jfets that high. Any favorite part#?
>
>> Yes, I'm sure you have, Fred.
>
>> The familiar J105 "switching" JFET can make
>> a great low-noise amplifier. Check it out,
>> Idss = 500mA. Awesome, but of course we'd
>> never think of using it in analog mode
>> anywhere near such high drain currents.
May I also add, the J105 in its original TO-92 package had
many manufacturers in its day, even tho now there are only
a few. 10 years ago Fairchild introduced the smt JFTJ105,
in a hefty sot-223 package. NSC's NDSJ105 sot-23 package
was discontinued, being far too wimpy to cage the beast!
> I don't remember whether the P8000/8002 were classified as
> switching JFETs but we used them up to where the heat sink
> had that new wood stove smell.
I don't remember the P8000 and P8002 JFETs, who made them
and what kind of package did they come in?
Jim Thompson
<notthis...@removethispacbell.net> wrote:
>Jim Thompson wrote:
>> On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
>> <To-Email-Use-Th...@My-Web-Site.com> wrote:
>>
>> [snip]
>>
>> Load this file yet again...
>>
>> http://analog-innovations.com/SED/DfromBC-Bias.pdf
>>
>> It now contains still another version... Comparator as Output.
>>
>> I can now walk, swaying a bit, without aid of a walker or cane...
>
>
>Great!
>
>
>> guess it's time to try the water ;-)
>>
>
>Try Shiraz, tastes better. But it won't exactly help with that swaying.
I meant WALK on the water ;-)
Jim Thompson
The tracking is only exact for Vgs1 == Vgs2 and Vds1 == Vds2
Jim Thompson
I replaced the LM339 with a 10ns comparator... overall delay is 60ns.
Does that fit with your needs?
Joerg
This is how we used them in high-IP3 RF amps back in the old days:
http://www.mydarc.de/dc4ku/KW_Transceiver.pdf
Of course me being in high school and then university the budget often
only allowed for the BF245 where you had to glue half a dozen together
to achieve that kind of performance. Later I did some projects with
P8002 transistors after I landed a nice side job at our RF institute.
Search for the text string "In deze schakeling heeft mijn P8002" in this
page and underneath you find a P8002 in TO220 style housing:
http://www.xs4all.nl/~pa0fri/Mods/Atlas215/atlas215.htm
BTW the radio described in there was probably one of the best and most
cost efficient designs ever. I've got one here and learned a lot from it
when young. It uses JFETs bidirectionally so stages could be operated
transmit and receive without switch gear.
IIRC TI and Siemens produced them. Marvelous FETs. But all good things
come to an end some day.
Fred Bartoli
I too found the Atlas page, and another one leading to the 8002 datasheet:
http://www.dibbert.org/p8002.htm
Nice fet but idss is spec'ed at 140mA max.
--
Thanks,
Fred.
Joerg
Yes, but you could easily have it dissipate several watts. Try that with
those SOT23 JFETs :-)
Joerg
Got a fast comparator in mind that isn't a lot more expensive than the
LM339? Always looking for a bargain here ...
And yes, I know your standard answer that you just roll your own :-)
Jim Thompson
<notthis...@removethispacbell.net> wrote:
>Jim Thompson wrote:
[snip]
>>
>> I replaced the LM339 with a 10ns comparator... overall delay is 60ns.
>> Does that fit with your needs?
>>
>
>Got a fast comparator in mind that isn't a lot more expensive than the
>LM339? Always looking for a bargain here ...
>
>And yes, I know your standard answer that you just roll your own :-)
Yep ;-)
Larkin can probably give you a part number... he's fond of using LVDS
and PECL parts as comparators.
D from BC
<To-Email-Use-Th...@My-Web-Site.com> wrote:
Ahhh.
From another point of view...
I think I see common mode and differential mode.
The DC drift is set up to be common (common mode).
The DC signal is from a differential.
D from BC
Vladimir Vassilevsky
D from BC wrote:
>>>>>>>>>What are some ways to temperature compensate a common source JFET DC
>>>>>>>>>amplifier.
>>>>>>>>>
>>>>>>>>> Vdd=5V
>>>>>>>>> |
>>>>>>>>> 10K
>>>>>>>>> |
>>>>>>>>> +------------Vout (biased for 50% Vdd)
>>>>>>>>> |d
>>>>>>>>>signal--->[ N JFET
>>>>>>>>> |s
>>>>>>>>> |
>>>>>>>>> Gnd
>>>>>>>>>
>>>>
>>
>>Here we go:
>>
>>http://www.abvolt.com/misc/jfet2.gif
>>
>>Paycheck time :-)
>>
>
> Neato!
>
> DC amp: check
> Thermal compensation: check
> BW to 2Mhz: check (Looks fast! (1) (2))
> Amplifies below rail voltage: check
> Inverting output: n/a check
> Uses matched dual JFET pack: check
> Low noise: op amp noise introduced (eh...might be ok).. check
> Parts count: great
> Prone to oscillation: dunno yet
>
> (1) No gain!
> (2) I suspect thermal compensation can be traded off for more gain by
> including negative feedback about the op amp.
Added two resistors. Now there is gain.
http://www.abvolt.com/misc/jfet_3.gif
> In some ways this circuit is like the inclusion of Rs... Gain is
> traded off for thermal stability.
This schematics is a sort of the differential amplifier.
> Impressive :)
>
Sure, as I always do.