[sdiy] Log/exp conversion

[patchell]

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
>
>

    Hi Dave-

    You remind me of my situation 25 years ago when I was trying to
figure these things out...

    Exponential conversion, fortunately, is very simple, as long as you
know the secrets...although, it is not all that secret.

    Yes for the CA3080 OTA, you must use PNP transistors for the
exponential converters.

    If you look at
http://www.silcom.com/~patchell/synthmodules/vocalfilter.pdf, you will
see what I did for exponential converters that are used with the CA3280
(just a dual version of the CA3080, same thing, basically).  Look on
page 7, you will see the expo converters that I used to control the
mixer.  While this is not the simplest expo converter, it is probably
the most predictable.  The circuit is fairly simple.

    Now, if you look at page 9, you will see a somewhat more complicated
expo converter that also compensates for the temperature dependent scale
drift.  If you are just starting, I would advise you not to look at this
page... :-)

    Also, take a look at
http://www.oldcrows.net/~patchell/archives/archives.html, there is a
whole lot of good stuff here as well about exponential converters.  Hope
this help.


--
 -Jim
------------------------------------------------
* Visit:http://www.silcom.com/~patchell/
*-----------------------------------------------
*I'm sure glad Merry Christmas comes just once a year
* -Yogi Yorgensen
------------------------------------------------