[sdiy] Simple VCA?

[ASSI]

On Samstag 12 April 2008, Roy J. Tellason wrote:
> 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.

What you have there is a "condensed" version of an adder/subtractor 
circuit.  When the JFET is off, you get just the normal inverting 
amplifier, when it is conducting, you mix the inverted with some 
portion of the non-inverted signal.  In this particular configuration 
the input resistor for the non-inverting input is shared with the 
non-inverting input.  Besides saving a resistor it keeps the voltage 
across the JFET near zero, which presumably helps getting a control law 
that is independent from the input signal (you'd suffer from that if 
the JFET was in the feedback path).



Achim.
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