Importance of matching in expo convertors?!

[Grant Richter]

For a really low parts count expo converter, check John Simontons
design for the Q control on the new state variable filter module.

http://www.paia.com/syntfsch.gif

If you stare at it long enough the temperature cancellation of the
second transistor starts to make sense. This circuit works too.

> 
> Yes and No. It *is* important when we want the reference current to
appear
> at the output for Vin=0V. But we don't need that our usual application.
> As I see it the second transistors main purpose here
> is to eliminate the Is's temperature dependance, the two Is cancel out to
> unity or - what I'm hoping - to at least a constant without too good
> matching. Temperature dependancy would drive us mad in a VCO with a
single
> transistor, while this may be usable in VCAs and maybe Filters. 
> 
>