JFET Common Source DC Amplifier Temperature Compensation

What does "temperature compensate" mean? Bias point, gain, both?

...Jim Thompson

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

That would be my avenue. There are duals such as this one, pretty low noise:

Buying those might be a challenge though.

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?

...Jim Thompson

Isn't the slick solution (given in AoE) to put another one in the source, 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 @

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

Jim Thompson a écrit :

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).

If -200mV Input => +2.5V Output, what does 0V Input => ?? Output

...Jim Thompson

They're not "independent", but I don't think incremental gain and bias Q-point will exactly track.

...Jim Thompson

Ah, a bit of french meaning slept in there.

Now you said they'd probably vary independently, which they don't (for ex.

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 :-)

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.

If you have a spare inverter somewhere you could make in inverting switcher.

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

The r-r opamps do work. Sounds like a lot less hassle than trying to work around a discrete jfet.

John

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 ;-)

...Jim Thompson

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

Looks like he doesn't even need output RR. Isn't there a LM324 on steroids somewhere?

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

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!

How about an ordinary single-supply op amp wired as an inverter? Ground the noninverting input, and bob's your uncle.

Cheers,

Phil Hobbs