[sdiy] Simple VCA?

[Roy J. Tellason]

On Sunday 13 April 2008 05:04, Bob Weigel wrote:
> I don't have time to look at the circuit right now...but I'd have to
> assume that this is perhaps in an inverting amplifier circuit with a
> resistance between the invert input and signal source?

No.  The stuff you quoted below describes it.  Three resistors,  input, 
feedback,  and non-inverting input to ground.  The FET goes between the two 
inputs of the chips.

> If so then this resistance becomes a pathway to ground other than the
> output's effect on the f/b resistance and thus proportionalizes with the
> input bias current requirement. Remember, usually when one considers an op
> amp design, they trivialize and make the assumption that the inputs have
> infinite resistance.  And for most conventional deisgnes we deal with, this
> works. Or..  ...here..let me explain it this way.
>
> Take it to the logical extreme.  What happens if the resistance is
> zero?  Will you still have an inverting amplifier then?   no of course
> not.  If the non-inv. is grounded and there is a resistance that is low
> between it and the invert, then it will have a large effect on the
> current that would be otherwise biasing the input of that amp.
>
> Hence the amount of resistance needed to provide a certain amount of
> gain will vary with the input characteristic of the op amp in such a
> design.  -bob

I'm gonna have to build this on a breadboard some time and try it out,  I 
think...

> 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?
> >
> > Ingo
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