[sdiy] Interesting VCA circuit on MAT04 schematic

[jhaible at debitel.net]

No Blackmer derivate here.

It's close to the MC1496 4-quadrant multiplier.

The 1496 and similar structures have a linear input
(the current sources ...), and a tanh input
(the differential pairs). The MC1494 (and AFAIK,
modern chips like the AD633, too), add linearising
diodes to the nonlinear input. I think the wole thing
is called Gilbert multiplier then.

Now you can just leave the nonlinear input as it is,
and take advantage of its nonlinearity. 
That's what the MAT-04 fig. 9 circuit does.
Feed the signal into the linear input(s), and
use the nonlinear input for expo gain control.

Oh, and there's a difference to the 1496, too:
The way the collectors of the "upper" transitors
are connected, or not connected.
Haven't analysed that part - may make the whole
thing 2-quadrant.

There have been VCA chips that use this principle:
I remember some "3340" (*not* the CEM part; MC maybe.)

BTW, I have used a 1496 in my first frequency shifter,
also taking advantage of the nonlinear input.
I feed the audio signal into the linear (lower)
input, and the modulator into the nonlinear
(upper) input. The latter makes a crude triangle
to sine conversion, so I can use triangle waveforms
for the modulator.

JH.

> 
> http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/mat04.pdf
> 
> Check out Figure 9.
> 
> The MAT04 is used sort of like... wait, this is weird. It looks like the 
> "lower half of an OTA," as in the VCAs on Rene's site - but everything is 
> switched around. The control is going in what I usually think of as the 
> audio input; and the audio input goes in what I think of as the control 
> input, but not before it goes into... oh wait, are those OP-41 and 
> 2N2222's facilitating logarithmic conversion to cancel out the expo?
> 
> And there's a copy of that structure. Interesting!
> 
> Is this some kind of souped up Blackmer cell? The datasheet says it's 
> "widely used in professional audio circles."
> 
> - Aaron
> 
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