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

[chris]

I'm not really sure about this. But my thought was to have the opamp
V- rail lower than the OTA rail, so the amp doesn't need to go down to
*its*own* V- rail.

Chris



On Wed, 30 Sep 2020 13:37:20 +0100 Tom Wiltshire <tom at electricdruid.net>
wrote:

> Isn’t that the wrong way around?
> 
> The OTA pin already has a couple of diode drops (or one for the ‘3080). I need to *raise* the driving amp.
> 
> I have considered putting a couple of diodes on the -V supply for the PIC, and running it from a floating ground a couple of drops higher than the lower rail. It might work, but it is pretty dirty, as you say.
> 
> 
> > On 30 Sep 2020, at 13:17, chris <chris at chrismusic.de> wrote:
> > 
> > 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
> >> 
> >> ==================
> >>       Electric Druid
> >> Synth & Stompbox DIY
> >> ==================