Fri Jan 7 20:12:36 EST 2011

BJT current controlled resistor

Some early Korg synths and also the recent Monotron[1] (follow links
for Monotron schematic) have a Sallen-Key filter with current
controlled resistors implemented by BJTs.  Might be a good alternative
to FETs, which have large V_T spread so might need more calibration.

It seems that the problem is that these are all used as AC / small
signal resistors, and rely on the large base current to flow.

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