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

Aaron B. aaron at zadzmo.org
Wed Sep 30 18:00:28 CEST 2020

This will invert the polarity of the CV, but if that isn't a big deal -
drive the OTA through a current mirror. The PIC's output will sink
current through a programming resistor at the mirror's input;
the mirror's output then sources Iabc to the OTA.

If the classic two or three transistor mirrors aren't linear enough,
and/or getting transitors matched is too much of a hassle, an opamp and
a FET can make a pretty good mirror.

I can put together a diagram later today if interested.

On Wed, 30 Sep 2020 11:53:00 +0100
Tom Wiltshire <tom at electricdruid.net> wrote:

> 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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Aaron B. <aaron at zadzmo.org>

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