[sdiy] Simple VCA?

[Eric Brombaugh]

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?

Remember that the ideal op-amp we use to understand how feedback 
circuits work is a fiction. In reality the open-loop gain of a real 
op-amp is finite, so there must be some small voltage difference between 
the + and - inputs in order for the output voltage to change. This 
circuit takes advantage of that by shorting out that small voltage to 
cut the output voltage of the amplifier to zero.

As mentioned elsewhere in this thread, the actual results of this are 
going to depend to a large extent on the type of op-amp you're using, 
how it's biased and the topology of the feedback network you've built 
around it. Put more simply, this type of VCA is going to be hard to 
control. From a mass-production standpoint it may not even be 
manufacturable without hand-selected parts, and thus in high volume 
applications isn't even cheaper than other topologies.

Eric