[sdiy] CA3046 VCA

[Ryan]

hi,

Hope this isn't too much, electronics and not enough synth for you... 
I'm trying to learn about some different VCA designs. I'm looking at the 
CA3046 VCA that was in Modulus E-Zine 5. here:
http://www.modulus.synth.net/modular/modulus5.pdf

It consists of a CA3046, and 2 Opamps, 1 diode, and some resistors. This 
circuit was also used in the Oakley ADSR/VCA.  I'm trying to figure out 
how it works but am having some trouble. I wonder if anyone can give me 
a little hint as to how to go about analyzing it.  They keep teaching us 
in school about small signal analysis, but I want to keep track of the 
nonlinear effects of each transistor so thats no good.

my idea was to break the circuit apart, and first just try to find out 
how the transistors which are not part of the differential pair affect 
the base voltages of the differential pair.  I tried solving for the 
voltages at the bases of these 3 transistors (which are all connected), 
and ended up with equations that look like this:

Ic=(15V-Vb)/100K, emitter is grounded, base & collector are shorted for 
this one.
and
Ic=Is*(e^(Vb*q/kT)-1)
(I know it's not completely accurate, but I'll start simple)

There's my problem, the only way I know to solve these for 'Vb' are 
graphically or with newton's method, and I'm trying to leave, 'Is', and 
'q/kT' in the equations as letters so that I can check how well the 
circuit works with temperature change (I think it probably cancels out, 
atleast 'Is' in the pairs?).  I'm thinking there must be a better way to 
go about doing this?

I've spent a few days trying to figure it out. I don't think I can use 
this VCA for my synth, unless I can understand how it works completely.

also, If anyone knows of any books that deal with this type of circuit 
in detail. Please let me know.

Thanks,
Ryan