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JFET Common Source DC Amplifier Temperature Compensation

J

Jan Panteltje

Jan Panteltje a écrit :
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.
 
D

D from BC

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 ;-)

...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.

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
 
W

Winfield Hill

Winfield Hill a écrit :
Winfield a écrit :
Fred Bartoli wrote:
Jim Thompson a écrit :
Fred Bartoli >
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!
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.
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.

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.
 
J

Jim Thompson

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson
[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
 
J

Jim Thompson

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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
 
J

Joerg

Winfield said:
Winfield Hill a écrit :
On Nov 20, 7:59 am, Fred Bartoli <" "> wrote:
Winfield a écrit :
Fred Bartoli wrote:
Jim Thompson a écrit :
Fred Bartoli >
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!
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.
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.
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.

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.
 
D

D from BC

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 :)

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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

D from BC

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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
 
W

Winfield

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?
 
J

Jim Thompson

Jim said:
On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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
 
J

Jim Thompson

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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

The tracking is only exact for Vgs1 == Vgs2 and Vds1 == Vds2

...Jim Thompson
 
J

Jim Thompson

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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

I replaced the LM339 with a 10ns comparator... overall delay is 60ns.
Does that fit with your needs?

...Jim Thompson
 
J

Joerg

Winfield said:
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 the P8000 and P8002 JFETs, who made them
and what kind of package did they come in?


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.
 
F

Fred Bartoli

Joerg a écrit :
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.

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.
 
J

Joerg

Jim said:
On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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

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 :)
 
J

Jim Thompson

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.

...Jim Thompson
 
D

D from BC

On Tue, 20 Nov 2007 15:30:40 -0700, Jim Thompson

[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

The tracking is only exact for Vgs1 == Vgs2 and Vds1 == Vds2

...Jim Thompson

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
 
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