[sdiy] Cataloging and understanding expo V-to-I converters

[Aaron Lanterman]

Between Chamberlin's book and Rene's page, I have a relatively quasi 
decent understanding of typical expo converter circuits used in modern 
VCOs, and I can flip my head out and figure out, say the expo source (vs. 
sink) circuit used in the polyfusion VCF:

http://www.synthfool.com/schematics/polyvcf.jpg

or, for instance, Ray Wilson's VCA (one I twiggle the diagram around in my 
head a bit,) I can see how it looks like Rene's circuit.

Quick question, before going onto the big question - there's a cap in both 
designs around the op amp that's used to hold one end of the reference 
resistor at ground that's not shown in Rene's diagram. What is the purpose 
of that cap?

OK now on to the heavier question.

I lectured on that last semester, and I'm going to be lecturing on it 
again tomorrow (alas, my lectures are not being videoed this time around).

Anyway, I'd like to try to give my students a better understanding of a 
wider variety of expo V-to-I circuits they may encounter, but of course, 
to do that, I need to understand them myself first, and hence as usual, I 
turn to this list (and try to in lecture give credit to where it's due.)

For instance... the expo converter used in the Polyfusion VCA

http://www.synthfool.com/schematics/polyvca.jpg

Is that a good circuit to explain? And if so, how do I explain it?

Are there other common expo conversion schemes I should make sure they 
understand?

Should I try to explain how PNP/NPN style converters work - or should I 
just tell them that they should stick with dual NPN or dual PNP (depending 
if they need a source or a sink) in their designs since the matched pairs 
are (relatively) easier to come by nowadays?

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
School of Electrical and Comp. Eng.    Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: Centergy 5212