[sdiy] Simple VCA?

[Roy J. Tellason]

On Sunday 13 April 2008 10:33, Eric Brombaugh wrote:
> Ingo Debus wrote:
> > Am 12.04.2008 um 19:18 schrieb Roy J. Tellason:
> >> It's a very simple circuit,  really.  An op amp inverting amplifier, 
> >> one resistor going from the non-inverting input to ground,  one input
> >> resistor, and one feedback resistor.  And,  a JFET connected between the
> >> two input terminals with the gate lead labeled as being the gain input.
> >>
> >> The caption for this reads "VOLTAGE-CONTROLLED GAIN -- 2N5457 FET acts
> >> as voltage-vaiable 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.
> >
> > I just don't understand how this would work. When the feedback loop is
> > working correctly there's virtually zero voltage between the two inputs
> > of the opamp. There would be no current through a resistor, variable or
> > not, between the two inputs, so this resistor doesn't do anything.
> > What am I missing?
>
> Remember that the ideal op-amp we use to understand how feedback
> circuits work is a fiction. In reality the open-loop gain of a real
> op-amp is finite, so there must be some small voltage difference between
> the + and - inputs in order for the output voltage to change. This
> circuit takes advantage of that by shorting out that small voltage to
> cut the output voltage of the amplifier to zero.
>
> As mentioned elsewhere in this thread, the actual results of this are
> going to depend to a large extent on the type of op-amp you're using,
> how it's biased and the topology of the feedback network you've built
> around it. Put more simply, this type of VCA is going to be hard to
> control. From a mass-production standpoint it may not even be
> manufacturable without hand-selected parts, and thus in high volume
> applications isn't even cheaper than other topologies.

The interesting thing about that is that although they specifiy the JFET used 
there they don't seem to specify any particular kind of opamp.  The feedback 
network is about as simple as you can get.  The reason I'm interested in this 
is primarily the simplicity of the circuit, and the lack of need for 
specialized chips that are all too often either unobtainium or way more 
expensive for casual fooling around.

As I'd said earlier,  maybe one of these days I'll get around to breadboarding 
it...

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