[sdiy] Operational Transconductance Amplifiers

[Walker Shurlds]

Recent post called Source for 3080's reminded me, so I thought I'd ask.

As a noob, albeit one with circuit analysis skills, I've decided to
analyze every part of the ASM-1 until I grok it before building
anything serious. There are a few minor things I don't quite get yet
(the capacitor between the negative input and the output of the expo
converter op-amp, for instance) but I'll figure it out in time.

But when I flipped the page from the VCO to the VCF I said "WHAT THE
HELL IS THAT!?".  Eventually figured out that it's one of these OTA
things.

Anyway, I'm having difficulty figuring out how they work.  Wikipedia
gave me an equation for the open-loop gain (Iout = Vdiff*gm), but that
doesn't tell me at all what Iabc or Ibias are for at all, and doesn't
help much with solving a circuit. Based on the ASM-1 VCF schematic, Iabc
is the part that actually does the "VC" in "VCF"!

So, yeah.  How exactly do these things work and/or does anyone know of
a good website/book that explains them thoroughly?