[sdiy] Combining currents from different OTA outputs

[JH.]

Sure, this works.

And if you build your OTAs from discrete transistors, you can even tie the 
collectors of the differential input pairs together, and share the current 
mirror stuff among all channels.

JH.

----- Original Message ----- 
From: "Oren Leavitt" <obl64 at ix.netcom.com>
To: "Aaron Lanterman" <lanterma at ece.gatech.edu>
Cc: "synth diy" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, May 24, 2009 4:46 AM
Subject: Re: [sdiy] Combining currents from different OTA outputs




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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> Synth-diy at dropmix.xs4all.nl
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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
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