[sdiy] Three-ball OTA

[René Schmitz]

Am 23.09.2011 13:35, schrieb Rutger Vlek:
> Dear all,
>
> 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.
>
> 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?

The trouble here seems to be that the emitters of the differential pair 
are sitting near GND, if the voltage compliance of the current sink 
(i.e. standard expo current sink) is also GND. This will restrict your 
common mode voltage range on the diff pair. (all is well if you move the 
voltages up, e.g. centered around half of the positive supply.)
Compare this to the two-ball OTAs as used in the ssm2040 clones by JH 
and myself.

Cheers,
  René