[sdiy] Combining currents from different OTA outputs

[Scott Nordlund]

You should be able to do all of them, though I'm not entirely sure what you mean by C.  In fact I have seen this in VCA datasheets as the recommended way to mix multiple voices together, etc.

----------------------------------------
> From: lanterma at ece.gatech.edu
> To: synth-diy at dropmix.xs4all.nl
> Date: Sat, 23 May 2009 22:14:10 -0400
> Subject: [sdiy] Combining currents from different OTA outputs
>
> 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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