[sdiy] Simple VCA?

[Bob Weigel]

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?  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

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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