Fri Jan 14 10:43:25 EST 2011
The Korg MS20 filter and reverse saturation transistors
In the paper that can be found at Tim Stinchcombe's Korg 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
Picking a random 2N3904 PNP and measuring with a multimeter I find for
forward and reverse gain: 288 and 9.