[sdiy] Coping with diode drop(s) on Iabc input with LM13700/CA3080

Roman Sowa modular at go2.pl
Wed Sep 30 17:03:37 CEST 2020

you have ruled out the best way right from the start. Why so much 
objections against opamp working near the rails? There are more rail to 
rail opamps available than the ones without that feature. You could even 
consider older than us both the legendary LM324 and all its variants - 
it can work close to negative rail, both in and out.

Or if you want to avoid opamp at all, and use just 1 resistor - add 
compensation curve to your control output in firmware. First 
aproximation will be just 0.7V offset and some gain. This should work 
much more linear with LM13700 than just a resistor straight from 0..5V 
And if you add exponential law and even add temperature compensation 
(some PICs have temperature sensor), then it's stable as a rock!


W dniu 2020-09-30 o 12:53, Tom Wiltshire pisze:
> Hi All,
> I’d like a bit of advice, please.
> I’ve got a 0-5V control voltage from one of my PIC LFO chips. I’d like to use that to control a OTA-based VCA running on a 0-9V single supply.
> The typical current source circuits I know are no good for this as they would require the op-amp to operate close to the rails with the 0V CV. Just using a resistor to the Iabc input isn’t an option since the lowest CV values will do nothing (the won’t overcome the diode-drop, or two on the LM13700).
> I spent a day running LTspice sims of various other methods I culled from various stompbox schematics and synths, but didn’t come up with anything that seemed to work effectively.
> How should I drive the Iabc input for best linearity in this situation?
> Any guidance would be much appreciated.
> Thanks,
> Tom
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