[sdiy] Operational Transconductance Amplifiers

[Tom Wiltshire]

I learned what I know about them from one of the R.M.Marston books.  
It might have been the  "Op-Amps circuit manual".

They're pretty straightforward really - the gain you see out of the  
end of it is a function of the current into the control input (Iabc  
or whatever its called) as well as the usual gain resistors. So  
they're a VCA, with a control-current (that'd be a 'CCA', right?).  
There's loads of example circuits with the 13600/13700 in the  
R.A.Penfold books too - "Electronic Synthesizer Construction" and  
"More Advanced Electronic Music Projects" are full of 'em.

HTH,
T.

On 9 Mar 2010, at 18:42, Walker Shurlds wrote:

> 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?
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