Importance of matching in expo convertors?!

[Ben Nise]

Rene --

>>There are other mismatch mechanisms that affect effect BJT's besides Is,
>>namely
>>Beta mismatch and tempco mismatch.  

>Sorry for the stupid question, but what do you mean by tempco mismatch?! 
>That the transistors still need to be on the same die or at least be
>thermally close coupled ?

What I really meant by "tempco" mismatch is that the parametric behavior
of two BJT's (Rb, Beta!!, Is, etc...) may not behave the same way over
temperature
for two devices that are not on the same die.  This would most likely show
up as
a temperature dependant offset -- (even with thermal coupling).

Regards,

Ben Nise
bnise at sonici.com