Fri Jan 14 10:43:25 EST 2011

The Korg MS20 filter and reverse saturation transistors

In the paper[2] that can be found at Tim Stinchcombe's Korg[1] page it
is explained how variable resistors are implemented using BJTs in
reverse saturation mode.

What I don't fully understand is why it is reverse.  From what I can
find in the derivation it has something to do with the reverse current
gain being much lower than the forward gain.  As a result, it is only
the forward biased Base-Collector diode that significantly contributes
to the resistance, which is the deriviative of the V->I curve over the
Emittor-Collector terminals.

Picking a random 2N3904 PNP and measuring with a multimeter I find for
forward and reverse gain: 288 and 9.

[1] http://www.timstinchcombe.co.uk/synth/korg.html
[2] http://www.timstinchcombe.co.uk/synth/MS20_study.pdf