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

[Tom Wiltshire]

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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       Electric Druid
Synth & Stompbox DIY
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