[sdiy] fun with 2164

[David G. Dixon]

Hey Team!

I'm working on a 2164-based 4P lowpass filter and I'm thinking I might want
to use the 2164 as a reverse-exponential amplifier.

Basically, the idea is to use an opamp inverter/summer with a certain fixed
gain to amplify a given signal to the maximum degree I require, and then use
another inverter and the 2164 to add various amounts of the inverse of the
signal back into the summer, thus "cancelling out" the original input with
increasing 2164 gain.  Presuming equal input resistors on the summer, at 0V
on the 2164 gain pin, the summer output should disappear, and as the 2164's
gain pin voltage is increased (thus causing an exponential decrease in 2164
gain), the output signal should increase to full strength, abruptly at
first, but then very gradually near the end.  I'm thinking of using this
technique to apply resonance gain to the filter, such that there is a lot of
control over the approach to self-oscillation.  I'm not sure it will really
work, as it seems that it will require very careful trimming of the maximum
opamp gain setting.

So, has anyone ever used an exponential VCA in this "reverse" manner before?