Exp converters (silly idea?)

[Don Tillman]

   From: "Thierry Rochebois" <tiar at canamcomputers.com>
   Date: Fri, 20 Aug 1999 13:11:02 +0200

   Just a (maybe) silly idea.

   As the discharge curve of a capacitor is an A*exp(-t/tau),
   maybe, it is possible to calculate an exp thanks to it with the
   help of a linearly discharging capacitor (constant current) figuring -t
   and comparing -t with the input. The comparator would trigger a
   sample and hold and output the A*exp(-t/tau).

Sounds too good to be true, right?  The capacitor discharge equation
looks remarkably close to what we want in a VCO.  And the curve starts
out fast and slows down, it really seems like it could work.

It's just backwards.  The V and the T are the wrong sides of the
exponential function.  ("Damn!")  Now if you were looking for a log
VCO... 

  -- Don