[sdiy] Temperature stable lin-exp converter

[Tony Allgood]

>If one were to take a third transistor and run a constant current
through it, the Vbe would vary with temperature. You could then amplify
the Vbe to a useful level and polarity, and inject it into the summing
node of the expo converter input.

But your expo convertor is already stabilised for Vbe effects across the
temperature range. The classic two NPNs and op-amp servo do this nicely.
The problem therefore is V/oct scaling... but you could use the Vbe of
this third NPN to control an OTA based scaling circuit. Isn't this how
the 3340 works?

Before I knew about music synths, I built my own waveform shaper for a
YIG oscillator controller. This used one NPN in a 3046 in the feedback
of an op-amp invertor to perform a simple log function with another
array transistor compensating for the drift by adding a current into the
summing node. I didn't have to worry to much about scaling in this case,
since this was done by a DAC controled by some software which checked
the temp occasionally.

Regards,

Tony Allgood                       Penrith, Cumbria, England

Oakley Sound Systems                     www.oakleysound.com
Modular projects            www.oakleysound.com/projects.htm