[sdiy] 2164 Expo correction

[David G. Dixon]

> In addition to JFETs, you might want to check out MOSFETs too.

Yes, I thought about MOSFETs.  Of course, they would be more of a "natural"
choice, since a JFET cannot actually be wired up like a diode-connected BJT;
the polarities are wrong (the gate needs to be more negative than the
source, but the drain needs to be more positive to give current flow in the
right direction, so obviously gate and drain cannot be connected).  However,
if I only drive the gate with the opamp, connect the source to the CV
summing junction, and put a quick-n-dirty current source on the drain (+15V
through a 1M resistor, say) then it should work well.  I'm looking at
something like the 2N4117, as I don't need very much current at all (about
4uA max -- that unit will only supply 20 to 90uA, fully on, so it would be
perfect).

Any other thoughts anyone might have would be most welcome!