[sdiy] Two transistor expo converter

[Rutger Vlek]

Ha, it's expo week!

On 3 feb 2015, at 21:20, Richie Burnett wrote:

> 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, 
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