expo-converter

[Jim Patchell]

jhaible wrote:

> > (all kidding aside)  Expo converter design is still black art after all
> > these years... Temperature is a problem. There are Temperature
> > Compensated designs (like Q81 resistor), Compensation schemes (like Rene
> > Schmitz design) that "cancel" the effect... and Temperature Stabilized
> > (heated) designs...
>
> I've recently done some simulations. I think I've found a circuit for a
> fully
> compensated expo current source (with pnp's, for OTAs !) that will only
> use 1/2 of a LM13700, a few opamps and a pnp pair. The temperature
> sensing is done directly in the pair, so you don't need an array, or thermal
> coupling from one chip to another. It looks very good in simulation, but
> I will only post it when I've done some breadboarding.
>
> JH.

    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