Importance of matching in expo convertors?!

[Rene Schmitz]

Hi Ben!

At 16:42 30.08.99 -0600, Ben Nise wrote:
>To create an exponential function, you only really need one transistor. I
>think
>the matched pair only comes into play if you want the exponential converter
>to be referenced
>to something (i.e, 0V in = some set current).

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. 

>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 ?

Bye
 René




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