[sdiy] Simple VCA?

[Roy J. Tellason]

On Sunday 13 April 2008 04:37, 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?

That's what I'm wondering,  too...    :-)

Still,  in troubleshooting gear over the years,  one handy test that I've 
found when you have an op amp that has an output sitting on one of the rails 
is to short the two inputs together. If it settles out in the middle then the 
problem is with some earlier stage,  if not then probably a bad chip (most of 
the ones I've seen fail do so with the ouput at one rail or the other).

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