Exp converters (silly idea?)

[Harry Bissell]

Use an Inductor ???     :^)  Harry

Don Tillman wrote:

>    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