[sdiy] Matched transistors and EFM circuits?

[Paul Schreiber]

> Further, although I'm
> aware that matching transistors in an expo converter is important for
> temperature stability, 

Errr....nope.

It *is* important to have the 2 transistors on the same silicon die,
so that the 2 have the same junction temperature, with makes the Vbe
the same (voltage from base-to-emitter), and so the current ratios in
the collectors are temperature stable.

A tempco is used to *alter* the 'zero temperature coefficient' Vbe
so that as the ambient temperature goes up/down, the Vbe changes
a small amount. This Vbe is divided down from the 1V/Oct summing amp.

Paul S.