[sdiy] Simple VCA?

[Scott Gravenhorst]

"Roy J. Tellason" <rtellason at verizon.net> wrote:
>On Sunday 13 April 2008 17:53, Scott Gravenhorst wrote:
>> >> >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've been a little disorganized lately,  myself.  I don't even know where mine 
>are at this point in time.
>
>> I'd love to try it with a transistor from a 4007  (c:  They have both P and
>> N types.
>
>Good idea!  Except that those parts are kind of limited in terms of what 
>voltages they can deal with,  by comparison with discrete JFETs.  OTOH,  I 
>bet I could lay hands on one of those a whole lot easier than I could find a 
>JFET...

And you get 3 P-MOSFETs in each package.

As far as the voltages go, 4007 can handle up to 15 volts.  The voltage accross D and S
would be small anyway, and the gate needs a range from the + rail to ground, so I think it
could work.

I think both JFETs and MOSFETs are worth a try.  What's so nice about 4007 is that you have
P and N right there.  If you want/need it to work inversely proportional, use the N, if you
want/need it proportional, then use the P.

>For that matter,  if it's pssible to bias say an inverter to linear operation 
>I wonder why something of the sort couldn't be done with a transmission gate?

The inverters you refer to are unbuffered, as in 4069UB (same can be done with 4007).  They
can work as linear amps because each inverter is just 2 transistors with the gates connected
together.  The 4066 buffers the control line so it can't apply anything but rail voltages to
the gates of the pass transistors.  The 4016 is quite a bit simpler, but still allows access
to only one of the pass transistor's gates.
 
>Use a pair of those transistors.  One of those pins at least can be sitting 
>at virtual ground,  anyhow...
>
>-- 
>Member of the toughest, meanest, deadliest, most unrelenting -- and
>ablest -- form of life in this section of space,  a critter that can
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>-
>Information is more dangerous than cannon to a society ruled by lies. --James 
>M Dakin
>

-- ScottG

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-- Scott Gravenhorst
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