[sdiy] Two transistor expo converter

[Richie Burnett]

While we're on the subject of exponential converters, can anyone direct me 
to a good explanation of how the two transistor (NPN/PNP) expo converter 
works?

The one comprised of Q10 and Q11 in the TB-303's VCF current sink for 
example:

http://www.timstinchcombe.co.uk/synth/diode_18_24/tb303_vcf.gif

I think it is Q11 that is actually performing the exponentiation here, and 
Q10 just looks to be operating as an emitter follower to buffer the summed 
CVs through R63 and R72.  Does the temperature dependent Vbe of Q10 
compensate for the temperature dependent Vbe of Q11?  Or is it the 
temperature dependent Vbe of Q9 that is supposed to compensate for that of 
Q11?

I'm a little bit confused about the maths, and particularly how the input 
from the "cutoff" pot influences the operation of the expo circuit via R73. 
I know that this is a linear weighted input and that's why the cutoff pot in 
this synth is a "log" to achieve an approximately normal octave scaling 
across it's span.  A website or book covering the maths would help settle my 
mind i think!

-Richie,