Importance of matching in expo convertors?!

[Joachim Verghese]

Rene Schmitz wrote:

> One gets the usual form of this equation when both transistors are
> matched, then Is1=Is2. Now, wouldn't this mean that we needn't use
> matched pairs in expo convertors?!

This is what I reckon, yes. In theory, at least, one only needs
to match for thermal behaviour. Any difference in Is1 and Is2
will result in a tuning offset only (assuming you're using the
expo converter to drive a VCO or VCF).

Some manufacturers (notably ARP) used NPN/PNP pairs for their
expo converters, and I don't think they matched for Is1 = Is2.

What would be interesting to examine is the relationship (if any)
between thermal behaviour and Is1/Is2 of transistor pairs.

-joachim