[sdiy] Newbie JFET-switch problem

[Tony Clark]

> I have uploaded the schematic at
> http://home.chello.no/~richarnt/ewicv-v1.pdf and the measurements at
> http://home.chello.no/~richarnt/JFET-meas.jpg - now if I add let's say an
> opamp with strong positive amplification between R24/D3/C17 and the base of
> Q2, will that move the JFET operation to the extremes of the transfer
> function?

   I really don't know what that would do, but having looked at the 
circuit, I think I can understand your problem.  The diode should be 
turned around.  What it is doing is clamping the voltage at the base of 
the JFET to 0.7V or so.  This isn't enough to turn the JFET on.

   Instead, here's what I would do:

   First remove D3.  Second, lower R24 down to 100k or so.  Then remove 
C17 and connect the collector of Q1 straight to the base of the JFET.  
Then remove R23.
   Now what you should have is a transistor that, when it conducts, 
clamps the base of the JFET straight to ground.  Actually, if you are 
trying to use the JFET as a sample-hold, you probably want to pull the 
base negative, since as I recall, their maximum non-conductance (highest 
off resistivity) is at a fairly negative voltage.
   Anyway, you can then adjust the timing of the 555 timer to adjust the 
on/off time period of the transistor/JFET.

   Hope that helps!

   Tony

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