[sdiy] Log/exp conversion

[Scott Bernardi]

The OTA's require that you push a current into the Iabc pin. That's the
the natural direction for a pnp collector current, so yes, pnp's are
used to drive OTA's.  I'm not sure what kind of circuit you're using
your transistor in, but bipolar transistors definitely do not have a
linear VBE vs Ic relationship - it's exponential, and we take advantage
of it in voltage to exponential current converters.

Here's Rene Schmitz's tutorial on exponential converters and temperature
compensation:
http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html

Another excellent resource for synth-diy stuff is the Musical Engineer's
Handbook by Bernie Hutchins of Electronotes fame.
http://electronotes.netfirms.com/

Here's an LFO circuit of mine built around an OTA with a simple
exponential converter
http://home.attbi.com/~sbernardi/elec/og2/og2_lfo.html

This next circuit is actual part of a filter circuit, but has a quality
exponential generator suitable for a long range VCO. I would use higher
quality matched transistors for the pnps such as a THAT-120 (available
from Jim Patchell at http://www.silcom.com/~patchell/parts/parts.html).
For playing around, you can use hand matched 2N3906's.



By the way, use "plain text" mode when sending messages to the list.
Yours came across with the the first few sentences writing over
themselves. It was unreadable until I hit "reply".

David Reichert wrote:

>
> Hi Everyone,
>
> I’m somewhat new to the whole synth/DIY thing, hope you don’t mind a
> relatively rookie question.
>
> Anyway, I’ve been working on a VCO design, but still need to work in
> the log conversion bit for the 1V/oct. Seems that everything I’ve
> tried so far doesn’t 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?
>
> The design is based around a CA3080 OTA. I read somewhere that PNP
> transistors are required for OTA designs. Is this true?
>
> Thanks and Merry Christmas,
>
> Dave
>
--
Scott Bernardi
sbernardi at attbi.com