[sdiy] JFET & FET-opto as voltage controlled resistor - H11F1 etc.

[Robin Whittle]

Thanks Aaron (in "JFET as VCR")for this link:

> http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm

Another way of looking at this circuit is that the applied voltage
(input to R1), which used to be relative only to the source and not at
all to the drain, is now made relative to the average of the drain and
source voltages.  This means it is relative to the centre of the
channel, which explains why the resistance now becomes largely
independent of the source-drain voltage.

The trouble with this arrangement is that R2 affects the signal at the
drain, as a resistance to ground and/or as a resistor to the control
voltage, which will therefore feed through to whatever is being
controlled.  Placing an op-amp buffer from the drain, driving the right
end of R2, would fix this problem.

You wrote:

> It looks like one limitation though is that, as described by the
> author, it seems to want to keep one of the ends of the "VCR" at a
> fixed voltage (like grounded.)

Another approach would be to use a bunch of resistors and op amps to
buffer the source and drain voltages, take the average of them, and then
sum this with the control voltage, to create the gate voltage.  Then,
neither the source nor drain would need to be fixed at ground.

I used some General Electric H11F3s,
http://www.fairchildsemi.com/pf/H1/H11F3.html (lower breakdown voltage
version of H11F1) in the late 70s, to make a stereo volume control.
Audio came from a Korg 700 via a reverb unit and then into the two
optos.  LED current was controlled by blowing into or sucking on a
Honeywell gas pressure sensor - so I could blow sound to the left and
suck to make sound come through on the right.  I used this with a dual
Teac 4-track 1/4" reel-to-reel two channel delay loop - the tape went
from one machine to the other a few metres away.  I made the audio
recirculate with about 80% gain, so it faded away slowly after multiple
repetitions, in approximate sync with a Z-80 computer generated digital
percussion rhythm which was also in stereo and fed to this delay system.
 Then I could spray reverbed Korg 700 sound on the left or right just
with my breath, which was fun!  It was a very intuitive, proportional,
way to make sound.

I am not sure that the system was extremely low distortion, or that its
frequency response, or distortion, was the same for the low LED currents
which just allowed some sound through as for high currents.  Any such
distortion and frequency response changes sounded good to me - I recall
them having a good feel as if made by a physical mechanism.

 - Robin        http://www.firstpr.com.au