[sdiy-interim] Trivial OTA summing quiz! (added 4th cirquit)

[Jeff Farr]

Hmm... please forgive a noob while I try to work this out..

Does having a current input to the opamp change the If = Iin rule?
And if not am I correct in assuming that the output voltage is then:
Vo = -(Iin - Itotal) * Rf ?  Itotal being the combined current from
the OTA's

I figure this like so:

Iin = (Vin/Rin) + It due to KCL

Shuffle that around and we get Vin = (Iin - It) * Rin

And then substitute that into the standard equation for an inverting amp:

Vo = -Vin * (Rf/Rin)

Do a bit of algebra and there it is... maybe.. (?)

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