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

Tom Wiltshire tom at electricdruid.net
Wed Sep 30 14:37:20 CEST 2020


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
>> ==================
> 
> 
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