[sdiy] Combining currents from different OTA outputs

[Oren Leavitt]

Aaron Lanterman wrote:
> Howdy gang,
> 
> Suppose I have several OTAs with current outputs, and I would like to  
> add those currents and turn that sum into a voltage.
> 
> A) Could I tie all the OTA outputs together, and run that output down  a 
> resistor to ground, and then buffer it with an op amp?
> 
> B) Could I tie all the OTA outputs together, run that into the  negative 
> terminal of an op amp with a a resistor in a feedback loop,  with the + 
> tied to ground (this would be an inverting current-to- voltage conversion.
> 
> Assuming A and B are valid...
> 
> C) Could I add some of the currents with a plus sign and some of the  
> currents with a minus sign by combining A and B above using the  
> principle of linear superposition?
> 
> If I can, I think I can do something rather cool.
> 
> In theory, if the OTA outputs act as ideal current sources, the above  
> should work... but I don't think I've ever seen OTA outputs ganged up  
> like that in practice.
> 
> I'm worried that there may be some "in practice" reason why OTAs might  
> not like having their output currents summed in such a direct fashion.
> 
> - Aaron
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Hi Aaron,

Definite Yes to the above.
I have done this years ago to make a 3 channel voltage controlled 
mixer/panner.
I don't have the schematic handy, but basically it's 3 CA3080 based 
linear VCAs sharing one output op-amp to convert the sum of currents to 
a voltage.

- Oren