Importance of matching in expo convertors?!

[Rene Schmitz]

Hi all,


I've done some more research on the topic, and I remembered the rule of
thumb for the Is leakage current. Is ~ A exp(T/To)  (That is Is doubles for
10deg increase in temperature. A has todo with the number of free carriers,
which determine the intrinsic resistance. One can determine the bandgap
difference when measuring this. A similar law applies to NTC resistors,
this is the effect that is exploited in them.)  Putting this equation into
the equation from my previous mail, I get A1/A2. (when both Transistors are
on the same die, and Temperature)  
I think Is cannot depend on other parameters such as Ic, Ib, Vbe, Vce since
these are not the same for both transistors, and in that case it would not
cancel for the matched case either. This shows that A is a constant for the
Transistor. It seems that a mismatch (in a monolithic pair) results only in
a constant pitch offset, which we don't have to think about.


Bye
 René



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