Hello,
I'm designing a low-noise, low-frequency current-to-voltage converter (TIA). Since the really nice JFET-input, low-noise OpAmps such as the OPA627 are scarce and expensive I decided to build the input stage with a discrete dual JFET followed with a bipolar OpAmp. The idea is to keep the gain of the input stage just high enough to "hide" the voltage noise of the OpAmp (if the JFET has, say, 4nV/rtHz and the OpAmp has 16, a gain of 5 should be enough).
Out of curisoity I opened up a commercially available TIA to see how they did it and if I could learn from their design. I found a dual JFET running with Ids=3.5mA and Ugs=-1.7 V. It's probably something like a 2N5912 (it's in a 7-leg TO78 can, part # scraped off). The odd thing is: the drain resistors are only 220 ohms off a 9V rail. The parts (at least the ones I can find) that fit this Vgs/Ids combo have a few mS transconductance which would get the input stage's gain barely over unity. Parts with significantly higher gm run at tens of mA in this gate voltage range.
What I don't understand: If unity gain is enough, why not just use the JFETs as source followers?
Disclaimer: I'm not ripping off anybody's product (and besides a JFET diff amp ain't rocket science). It's just instructive to look at other people's ideas, but in this case I can't make much sense of it.
robert