[sdiy] Couple of OTA questions

[ASSI]

On Donnerstag 01 November 2007, Cornutt, David K wrote:
> 1.  The OTA tries to source/sink a current that is proportional to
> the difference of the inputs, and the amplifier bias current.  What
> if the output is presented with an open circuit?  Will it maintain a
> voltage that is proportional to the inputs?  Or will it try to rise
> to the supply rail?

It tries to go against the rails, but the voltage output range is 
limited by the internal biasing network anyway.  If you want any 
sensible output response from an OTA, it must be presented with a 
suitably low impedance.

> 2.  Given the scarcity of in-production OTAs, I've been looking at
> the Burr-Brown one (OPA630 IIRC).  It has a pinout that's quite
> different from what I'm used to seeing.

You are talking about that "diamond transistor" thing, right?  It is 
sort of an OTA in that it has a high-impedance input and a 
low-impedance output.  It is not an OTA in that it has a second 
low-impedance input thanks to how it is connected internally.  It can 
not (easily) be used in the same way that you'd use a CA3280 for 
instance.


Achim.
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