[sdiy] Simple VCA?

[Roy J. Tellason]

I've got this "Modern Electronic Circuits Reference Manual" I acquired 
recently,  not all that modern since it was published in 1980,  but I started 
browsing it anyhow...

There's one circuit I ran across that I'm wondering about,  never saw it 
actually used anywhere and I don't know how well this would work,  so I 
figured I'd toss it out for you guys to pick apart.

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.

Comments on this?  Could that be done that simply?  Or what am I missing here?

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