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

[chris]

What about putting a diode drop in the OTA's V-, so that the driving amp
doesn't need to go down to the *PSU* V- ?

Or would that be too filthy? 
I remember seeing this to raise a fixed voltage regulator a bit.

Chris



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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