[sdiy] Moog ladder & State-variable example lectures up

[René Schmitz]

Hi Aaron and all,

Aaron Lanterman wrote:
  > 3/15 - state variable VCF examples, Oberheim SEM, EML 101 (uses
> differential integrators, strange circuits), my rant about 
> Doepferization, and I also make an ass out of myself at the end of the 
> lecture where I try to analyze one of Rene Schmitz's VCAs on the fly and 
> get stuck...
> 
> Help! Here's where I got stuck - Check out Rene's VCA-2...
> 
> I see the diff pair ladder, which makes me want to make the collectors 
> be at opposite voltages, which I then want to sum up with an op amp 
> thingy - but looking at the op amp, I apply op amp rules, which makes 
> the collectors be at the same voltage.
> 
> So I must be reasoning about that circuit terribly wrong...

The collectors are at the same voltage since they are tied to the opamp 
inputs directly. (If you look at VCA1, there we have resistors and a 
voltage reasoning does make sense.)

So there is no voltage differential. You need to think about what 
happens to the currents at the opamp terminals. Its a differential 
transimpedance amplifier, a current differencing amplifier if you will.
The resistors to Ub "just" supply a common mode current, to the v+ and 
v- nodes, which then gets imbalanced according to the transistors 
transfer function. This common mode current has to be large enough so 
that the transistors stay within their linear operating range, otherwise 
they would get into saturation when the standing current is maxed out.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159