[sdiy] Simple VCA?

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at verizon.net> wrote:
>On Sunday 13 April 2008 17:12, Scott Gravenhorst wrote:
>(Snip)
>> >I'm still unclear as to just how this would work.  What control voltage
>> > would you be applying to the gate of that JFET for zero output?
>>
>> Zero is probably not achievable, but probably grounding the gate will give
>> minimum VCA output.  The JFET must be reverse biased, so a negative voltage
>> would control it, the more negative the gate, the higher ther resistance
>> the JFET presents.
>
>I suppose I could use positive voltages with a P-channel JFET,  then...
>
>And for zero?  There seem to be a number of transistors out there spec'd 
>for "mute" applications -- I've not seen any application of those but I'd 
>guess that they would simply have the collector in the signal path and the 
>emitter tied to ground,  that'd be one way to fix it.
>
>> >For maximum?
>>
>> The max negative voltage the FET can take should cause very high resistance
>> across D & S. The higher the resistance across S & D the more it's like the
>> FET isn't there.  If the FET resistance is millions of ohms, it effectively
>> isn't there at all.
>
>Makes sense.
>
>> >What sort of a characteristic would varying this control voltage have?
>>
>> I'd not expect this to be linear, but wasn't there a post that said linear
>> control through several decades?  Maybe the first post?
>
>Mine would've been the first post,  and I was quoting what was written in that 
>book I have here.  I guess I'm gonna have to breadboard it one of these 
>days...

I wish I could get near my breadboard...  I'm one of those messy guys...  I'd love to try it
with a transistor from a 4007  (c:  They have both P and N types.  

-- ScottG

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-- Scott Gravenhorst
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