[sdiy] Temperature stable lin-exp converter

[Tim Ressel]

Yo,

A while back someone posted an EDN article showing a
temperature-compensated expo converter. The design
used a third matched tranny to generate a reference
voltage. The circuit as shown wasn't readily adaptable
to a VCO, but it got me to thinking. If one were to
take a third transistor and run a constant current
through it, the Vbe would vary with temperature. You
could then amplify the Vbe to a useful level and
polarity, and inject it into the summing node of the
expo converter input. I think this would give the
needed temp compensation. What do you think folks?

--TR


--- Ian Fritz <ijfritz at earthlink.net> wrote:
> At 04:19 AM 5/28/2002, jhaible at debitel.net wrote:
> 
> >Very interesting - I didn't know that!
> >Which raises the question about the tempco of these
> metals approximately 
> >compensating the
> >tempco of a silicon pn junction. Is this just
> coincidence, or is there a 
> >system in it?
> 
> No special connection between the two phenomena.
> 
> >(To think of it, I don't even know if other
> semiconductors, Ge, GaAs, 
> >etc., have the same
> >tempco or not.)
> 
> Yes, if they make ideal junctions. If you go back
> and recheck the Ebers 
> Moll derivation you will se that there is no special
> use of Si parameters. 
> That stuff mostly goes into the prefactor that gets
> cancelled when you use 
> a differential pair.
> 
>    Ian
> 
> 
> 
> 
> 
> 
> >JH.
> 


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