This is a collection of various bits and pieces that I have found about OTA so I have to look in only one
place. It was also motivated by the fact that the datasheets for commonly available OTA IC contain way
too much handwaving and errors, some of them not very easy to spot. Of course I’ve probably added some
errors of my own in this document, corrections and ideas for improvement are always welcome.
1 Preface
The OTA is popular for implementing voltage controlled oscillators (VCO) and filters (VCF) for
analog music synthesizers, because it can act as a two-quadrant multiplier as we’ll see later. For
this application the control input has to have a wide dynamic range of at least 60 dB, while the
OTA should behave sensibly when overdriven from the signal input (in particular, it should not
lock up or phase reverse). Viewed from a slightly different angle an OTA can be used to implement
an electrically tunable resistor that is referenced to ground, with extra circuitry floating resistors
are possible as well.
The primary application for an OTA is however to drive low-impedance sinks such as coaxial cable
with low distortion at high bandwith. Hence, “improved” OTA such as the MAX436 or OPA660
have optimized these characteristics, but made it either impossible (MAX436) or considerably
harder (OPA660) to use them as two-quadrant multipliers. Four quadrant multipliers on the other
hand are hideously expensive, so that “obsolete” OTA like the CA3080 are still in widespread use.