[sdiy] Voltage controlled resistor, sort of

[David G. Dixon]

> To that end, I decided the charging circuit was really dependent on the
> magnitude of current, so I built a voltage controlled current source
> (VCIS) from an OpAmp and the simulations work fine, but I was wondering if
> someone could point me to a "standard" way to replace a pot like this with
> a voltage control? I like the OpAmp solution, but in order to get the
> thing to work, I need to run the VCIS off +/-15 VDC with the UJT circuit
> running on + 12VDC.  I'd rather not have to re-invent the wheel especially
> when I'm just re-learning all this.

Isn't this exactly what an OTA is intended for?  Half of an LM13700 should
work fine.  A (small) differential voltage input is converted to a current
proportional to a control current, which can simply be a voltage across a
resistor (since the control current pin is pinned to V-).