[sdiy] Operational Transconductance Amplifiers

[Tim Parkhurst]

Since we're on the subject of 3080s and OTAs, Magic Smoke Electronics
sells Thomas Henry books, including "Making Music With The 3080 OTA"
through our print to order service at   www.lulu.com/magsmoke   This
book is a very good introduction to the 3080, and includes quite a bit
of operating theory along with several schematics.

We also sell the CoolAudio 2164 Quad VCA chip (a clone of the SSM
part) for $3.50 (or less if you buy more than one). Email us at
magsmoke at gmail.com for more info.

Now back to our regular programming. ;)


Tim (it ain't easy bein' a synth pimp) Servo
-- 
"Sire, the church of God is an anvil that has worn out many hammers."
- H.L. Hastings



On Tue, Mar 9, 2010 at 10:42 AM, Walker Shurlds <walkershurlds at gmail.com> 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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