[sdiy] Combining currents from different OTA outputs

[ASSI]

On Sunday 24 May 2009, Aaron Lanterman wrote:
> 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?

Yes.

> 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.

Yes.

> 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?

Yes.  Or use a current mirror and pull the minus currents away from 
the summing point.

> 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.

Keep in mind that they need to be biased to work as an approximation 
of a current source.  If you're tying together very dissimilar OTA 
configurations or if you provide bias with a feedback network then 
things may become more difficult if the outputs can not agree on a 
common bias voltage.  In practise you should not see a problem when 
all OTA are the same type and run from the same rails with the same 
style of Iabc generation.

> 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.

They're pretty good at faking an ideal current source in most 
circuits. :-)


Achim.
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