[sdiy] Combining currents from different OTA outputs

[David G. Dixon]

Aaron,

A and B both work beautifully.  I recently did both (started with A but
decided ultimately on B) for adding the currents of two push-pull OTAs (both
halves of one LM13700) in a funky sort of balanced modulator.

Concerning C, if you use separate current sources (e.g., separate OTA
outputs, or separate combinations of OTA outputs) and feed them through the
A and B circuits separately, then this is no different than A or B alone.

However, if you try to feed a single current source (e.g., all OTA outputs
connected together) through the A and B circuits in parallel, then circuit B
will sink all of the current, since the - terminal of opamp B is at virtual
ground, and therefore, no significant voltage will develop at the + input of
circuit A (which is connected to it) to drive current across the
resistor-to-ground.

For this combined circuit to work, you must place a resistor between the
current source and circuit B.  Now, the ratios of the two resistors (circuit
B's resistor to ground and circuit A's input resistor to virtual ground)
will act as simple resistors in parallel to ground, and a voltage will
develop at the current source necessary to drive these parallel resistors.
Circuit A, being a simple follower after the resistor to ground, will
faithfully reproduce this voltage at its output.  Circuit B, being a simple
inverting amplifier, will produce some negative multiple of this voltage
depending on the ratio of its feedback and input resistors.

This scenario will work as long as circuit B's gain is small enough to avoid
its output from hitting the rails.  Presuming circuit B is at unity gain
(i.e., feedback and input resistors are equal), then this combined circuit
acts as buffered resistors in parallel.

Does that answer your question?  Is that what you had in mind?

> 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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