[sdiy] Simple Stuff Again

[René Schmitz]

Hi Scott and all,

I've toyed with that approach. Although I don't think I'm the 
one you remember. 
Anyway, I think the FETs are ok-ish for use in VCAs, if one 
linearizes them. (such linearization is discussed in The Art of 
Electronics.) The circuit I played with was thumping a bit, and 
it has a remaining nonlinearity, that depends on the control voltage. 
My 3080 based VCAs thump also, so if thats ok, the FET one will be too.

I can imagine that noisewise they might even have an advantage 
compared with OTAs because you can use a FET as a linear 
resistor over say +/-500mV, which gets you farther away from the noise 
floor than the +/-25mV that an OTA will permit. 

The circuit I've used consisted of an nMOSFET (from a 4007) that 
worked into the -input of an opamp which was wired in the common 
I-to-V configuration (i.e. feedback resistor from out to -in and 
+in grounded). It ought to work similarly with a pJFET.
The linearization network consists of two 100k resistors one from drain 
to gate, one from gate to the control input. The source of the FET 
goes to the -input of the opamp. The drain is the input, which needs 
to be driven by a low impedance signal of about 1Vpp. 

Bye,
 René


At 11:42 15.09.01, you wrote:
>I think that I remember that someone on this list uses
>or has used a simple FET circuit as a VCA (voltage controlled
>attenuator).  If memory serves, the poster indicated that
>he liked the way it distorts.  Is that person here?
>I'd like to see schematic examples of this.  Also, under
>what conditions could this approach be appropriate.
>I know that quick changes in control voltage means thumping,
>So either you want the thump, or you don't use that technique.
>I'm wondering if it is useful.
>
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