[sdiy] Simple VCA?

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at verizon.net> wrote:
>On Sunday 13 April 2008 13:11, ASSI wrote:
>> 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.
>
>There wasn't any non-inverted signal,  just that resistor going to ground...
>
>> 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).
>
>I'm still unclear as to just how this would work.  What control voltage would 
>you be applying to the gate of that JFET for zero output?  

Zero is probably not achievable, but probably grounding the gate will give minimum VCA
output.  The JFET must be reverse biased, so a negative voltage would control it, the more
negative the gate, the higher ther resistance the JFET presents.

>For maximum?  

The max negative voltage the FET can take should cause very high resistance across D & S. 
The higher the resistance across S & D the more it's like the FET isn't there.  If the FET
resistance is millions of ohms, it effectively isn't there at all.

>What sort of a characteristic would varying this control voltage have?

I'd not expect this to be linear, but wasn't there a post that said linear control through
several decades?  Maybe the first post?


-- ScottG

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-- Scott Gravenhorst
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