[sdiy] Three-ball OTA

[ASSI]

On Friday 23 September 2011, Rutger Vlek wrote:
> Inspired by this (http://www.uni-bonn.de/~uzs159/ota3080.html)
> comprehensive explanation of an OTA, I have been playing around with an
> OTA modelled in Spice. Towards the end of the article a "three-ball" OTA
> is proposed, effectively leaving out one current mirror (the one that
> deals with the control current Iabc). Instead the article proposes to
> "suck out" current from the NPN differential pair directly with a NPN
> transistor.

Good idea in theory, but this really comes from discrete circuits.  A better 
idea for an integrated circuit would have been to add another current 
mirror.

> This idea of a three-ball OTA seemed to make sense at first and performed
> well in simulation when I "sucked" current out of the differential pair
> with a simulated current source. However, as soon as I replaced the
> simulated current source with a more reallistic NPN expo convertor,
> distortion characteristics went up (in the wrong direction)
> tremendously. I'm having a hard time understanding this behaviour.
> Anyone willing to explain it to me?

Look at what happens to the biasing of both the expo and the input 
differential pair.  I'd say you would have to specifically design an expo 
converter that references to a negative voltage rather than ground for this 
to work.  Alternatively you could put a common mode offset voltage to the 
input differential pair. 


Achim.
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