PCB for MAT-04 VCA

[Joachim Verghese]

On Tue, 25 Feb 1997 gstopp at fibermux.com wrote:

>      It looks like this kind of VCA design gets around the problem of input 
>      limiting or pre-distortion by modulating the standing current in the 
>      differential pairs with the input signal, and then controlling the 
>      gain by modulating the bases with the control voltage.

>      Does this mean that the control voltage input is exponential?

Yes, CV response is exponential. I believe most CEM VCFs and VCAs
are based on this topology. There's a very detailed description
of this type of gain cell in Doug Curtis' patent docs from the
late '70s. I'm referring to the patent which describes the multiple
ganged (tracking), temperature compensated gain cell design.

It's been a year or so since I last reviewed the theory, so I don't
remember the details, but it's certainly very clever stuff - 
recommended reading. US patent #4,004,141 "Linear/logarithmic analog 
multiplier".

-joachim