[sdiy] Simple VCA?

[Scott Gravenhorst]

I don't have any of these publications.

Does it look like this ASCIImatic:


                                FB ?

                       +------\/\/\/\---+
            510R       |                |
                       |  |\            |
    IN *--/\/\/\/---+--+--|-\           |
                    |     |  \          |
                    |     |   \         |   out
                  |-+     |    >--------+----->
                  |       |   /
cont *-/\/\/\/--->|-+     |  /
                    |  +--|+/
          ???       |  |  |/ 
                    +--+
                    |
                    |
                    /
                    \ 510R
                    /
                    \
                    /
                    |
                    |
                  GND




tim102 at tstinchcombe.freeserve.co.uk wrote:
>> The interesting thing about that is that although they 
>> specifiy the JFET used 
>> there they don't seem to specify any particular kind of 
>> opamp.  The feedback 
>> network is about as simple as you can get.
>
>Just found the circuit in the back of an old Teledyne Semiconductor databook
>('JFET Design and Application Manual', 1980), which has a small section of
>application notes at the end - unfortunately it is very sparse too: the op
>amp is shown as a 741; the JFET either 2N4302 or 2N4220; input resistor (on
>-ve in) and resistor to ground (on +ve in) are both 510 ohms; feedback
>resistor 'RF' not given (useful huh?). No equations or anything, but the
>figure caption says: "Electronic gain control utilizing the low offset
>voltage of the 741, the JFET will have linear resistance over several
>decades".
>
>I'm sure I have seen this somewhere else, but a quick skim of my FET books
>didn't uncover it - maybe I'll have another more leisurely peruse.
>
>I agree with Achim that it must be acting as some sort of subtractor, but my
>feeble attempt so far at analyzing it hasn't made much sense!
>
>Tim
>__________________________________________________________
>Tim Stinchcombe 
>
>Cheltenham, Glos, UK
>email: tim102 at tstinchcombe.freeserve.co.uk
>
>
>
>
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>

-- ScottG

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-- Scott Gravenhorst
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