AW: expo-converter

[patchell]

jhaible wrote:

> >     Sounds something like I might be doing.  I am trying to use the Vbe of
> the
> > reference transistor as the thermometor (since the collector current is
> > constant, Vbe should be linear with temperature) and using that voltage to
> > control an OTA that is in a feedback loop of opamp to cancel out the scale
> > drift.
> >
> >     -Jim
>
> This is more or less what I was proposing some time ago. Only that the
> differential
> pair at the input of the OTA (in opamp feedback loop) would do the
> compensation
> all by itself. Rene then went on one step further and used a differential
> pair (no more
> OTA) to to the same trick - the advantage was that the transistors could be
> all on
> one array.
> Disadvantage of both methods is the nonlinearity of the OTA (or noise, if
> you're
> in the linear region).
>
> A linearized OTA would do the trick. No more opamp tempco now, so back to
> temperature sensing the expo pair and multiplying. OTA still in the feedback
> loop
> of an opamp.
>
> Now, why not use the OTA straight forward (no feedback). Multiplication done
> with I_diode rather than I_abc. The circuit I'm proposing is at
> http://home.debitel.net/user/jhaible/jh_temp_expo.html ,
> but mind you, it's *still untested*, so don't blame me if it doesn't work.
> If someone is faster than me on going to the breadboard (which is likely),
> feedback is higly welcome !
>
> JH.

    I like that.  I was going to use either the CA3280 or LM13700, depending on
which one had the best diode linearzation, and I did not like the idea of the
OTA in a feed back loop but that was the only way I could see doing it.  I like
the diode current idea.  Don't think I would have thought of that in a million
years.  I am just too rusty in the brain.

    -Jim


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