[sdiy] Temperature stable lin-exp converter

[patchell]

    No, you can't sum the voltage back in, you must multiply.  The temp
drift in an expo converter is a scale drift, so you need to adjust the
scale.  At the moment, I am using both halves of an LM13700 to do this
job.  It seems to work pretty good.  This weekend, I am hoping to do
some more experiments with this...I just got a new piece of equipment
that should make my experiments a little more meaningful....a
thermometer!  Should have done this a while ago...got a RATSHACK digital
for about $10.

Tim Ressel wrote:

> 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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--
 -Jim
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