[sdiy] Simple VCA?

[ASSI]

On Samstag 03 Mai 2008, Ingo Debus wrote:
> I still think the circuit that was posted by Roy here a while ago is
> based on a drawing error.

So much is given, the question is "where and when?"  The fully 
differential circuit has so many advantageous aspects when compared 
with the other stuff floating around that I believe it does have it's 
roots in a real application.  Someone somehow probably had decided to 
draw it single-ended for the FET databook and didn't really understand 
how it was working, I guess.

> Look at NatSemi's AN-32 (Aaron posted a 
> link). There's a "Variable Attenuator", but with the FET in series
> with the input resistor. The circuit description uses almost the same
> words as the description Roy quoted. Perhaps this is the corrected
> version?

To quote the quote again:

> The caption for this reads "VOLTAGE-CONTROLLED GAIN -- 2N5457 FET
> acts as voltage-variable resistor between differential input terminals
> of opamp. Resistance variation is linear with voltage over several
> decades of resistance, to give excellent electronic gain control.
> Values of resistors depend on opamp used."  --"FET Databook",
> National Semiconductor, 1977, p. 6-26 -- 6-36.

AN-32 predates the FET databook by seven years and that circuit clearly 
does not provide a "voltage-variable resistor between differential 
input terminals of [the] opamp."  So I don't think these two are 
related.


Achim.
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