[sdiy] Log/exp conversion

[Scott Bernardi]

Oops. I forgot to add the link for the "higher quality exponential circuit":

http://home.attbi.com/~sbernardi/elec/og2/og3_filterbank_control.gif

Something I found amusing (maybe I'm easily amused), is that the first two
other people that replied to your question were the two that I referenced -
Rene Schmitz and Jim Patchell.

Scott Bernardi wrote:

> 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

--
Scott Bernardi
sbernardi at attbi.com