[sdiy] Log/exp conversion

[René Schmitz]

Hi Dave!

At 09:40 22.12.02 -0500, David Reichert wrote:
>Anyway, Ive been working on a VCO design, but still need to work in the
>log conversion bit for the 1V/oct. Seems that everything Ive tried so
>far doesnt work. All of my transistor circuits seem to remain linear,
>even in the 0-700mV range. Does anyone know of a good source of
>information on the design of these converters?

I have written a short discussion about exp convertors. 
Its at my homepage. http://www.uni-bonn.de/~uzs159 
Follow the link "whats that exp and tempco stuff?".

>The design is based around a CA3080 OTA. I read somewhere that PNP
>transistors are required for OTA designs. Is this true?

Yes. 
The convertors in the discussion are of the type that sink current, 
for an OTA its required to source current. Hence you reverse all polarities, 
and use PNPs instead of NPNs. (Also you might look at my OTA based MS20 
filters, and how the expo conversion is done there, that should give you 
some reference.)

Merry Christmas,
 René 


-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159