[sdiy] JFETs (arrggh!)

[David G. Dixon]

> You can never go wrong if you see what parts Ian Fritz chooses: 2N4391
> or PN4391. Hmmm, 2N4391 doesn't seem to show up on findchips but
> PN4391 does.

Yes, Ian always has the right idea.  I'm sitting at his feet on several
fronts at the moment.

> I believe Ray Wilson lists PN4391 as the FET of choice in his latest
> iteration of his VCO (which I think rolls up a lot of advice from Ian
> and perhaps other folks):

Yes, I hadn't actually seen that yet, although I troll Ray's site fairly
regularly.  Note that he recommends J210 or 2N5457 as possible substitutes
for 2N4391.  Uh-uh!  I put 2N5457 in my VCO, and the integrator won't reset
to ground unless I put a huge resistor (100k plus) between the integrator
and the comparator, or a huge capacitor (47p plus) on the comparator
feedback loop, or both.  Of course, this buggers up HF tracking.  According
to simulations, J210 would barely work in this application (I don't have a
SPICE model for 2N5457, but it seems to be about as bad in real life), while
2N4391 will work swimmingly with a small-enough comparator RC combination to
give tracking to within a fraction of a Hz in the kHz range without HF trim.