JFET Common Source DC Amplifier Temperature Compensation

Available from Digikey Nearly 3000 in stock.. Two N JFETS in one package

D from BC

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.

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

Here is an incredibly complex schematics:

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

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.

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.

Be sure to check that close to DC. RF JFET can be rather noisy down there.

Not many of them have been ported to SC75 though. That would be sweet.

Yes!

Check figure 10 here:

The IMHO biggest issue with JFETs is the large production spread. That's where arrays can come in handy.

Among mass product designers that would be considered taking the chicken exit ;-)

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

I'm sure you are correct by experience, and I've never wasted money on an RF 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

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.

That's like the circuit I thought up.. I just went above not below...if that makes sense..

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

Thanks for the link...that's a good read... On

"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

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 :-)

Sort of like this...

Not dead-on, but close... exact solution is left as an exercise for the student ;-)

...Jim Thompson

Here we go:

Paycheck time :-)

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Woah, 500mA IDss! Are you speaking of jfets or depletion mosfets? I've never seen jfets that high. Any favorite part#?

On a sunny day (Tue, 20 Nov 2007 22:39:37 GMT) it happened Vladimir Vassilevsky wrote in :

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.

Oh, there used to be wonderful power JFETs. For example the P8000 and P8002. All very short-lived and history now :-(

Jan Panteltje a écrit :

Nope, that the first transistor vds that's stabilized at 2.5V

This circuit copies the first jfet VGS to the second one (if they are identical).