[sdiy] mosfet/cmos opamp

[David G. Dixon]

> Do you find the CA3140 to be much better than the TL07x (or 08x) series
> in a sample hold application. I have used both but have not noticed that
> much difference.

I typically use LF44x (IBC 10pA typ, 100pA max) for S&H buffers, and have
been very pleased with the results.  These have similar input current specs
to CA3140 (IBC 10pA typ, 50pA max) and significantly better specs than TL07x
(IBC 65pA typ, 200pA max), but really any of them will do for S&H unless you
hold the sample for minutes.

Indeed, from my updated ASR circuit, with a 104 sampling cap (C = 10^5 pF)
and assuming the typical IBC specs for LF444 (I = 10 pA), then the voltage
rate of change is 0.1 mV/s, which means that it would take nearly 14 minutes
for the pitch to fall by one semitone if the sample were feeding a 1V/8ve
input.  Compare this to just over 2 minutes for TL07x.