[sdiy] Three-ball OTA

[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?

Regards,

Rutger