Squarerooting transistors.

Don Tillman don at till.com
Fri Apr 5 18:35:36 CEST 1996

   From: Christopher List <Christopher_List at sonymusic.com>
   Date:  4 Apr 96 14:55:09 

   I'm building this Serge-like mixer panel using hot-off the press
   SSM2164 quad VCA's. One 2164 per mixer channel. One VCA for the gain,
   two for the pan, and one for the gain CV. The problem is that they
   have an exponential voltage control input and I want the control of
   the gain CV to be linear.

   What I'm wondering is this: You can use two transistors to create an
   exponential current source from a linear input voltage. Is there a way
   to wire them in reverse so that you have a square-root (reverse log?)
   voltage source from a linear input current?  Just a point in the
   proper direction would be helpful.

Why would you want "square-root"?  And "reverse log" is exponential,
which you've already got.

What I think you're really asking for is a log amp, the circuit isn't
too difficult, and can be found in most published collections of opamp
circuits.  Basically a transistor feedback around an opamp.

However... there's likely to be a far simpler solution.  I don't have
the data sheets for the SSM2164 on me, but most devices like this have
a diode junction at their control input, which means if you drive the
device with a low-impedance voltage you get exponential and if you
drive it with a high-impedance current you get linear control.  The
data sheet will tell you for sure.

  -- Don

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