On Thu, 19 May 2011 18:14:09 -0500, "Dave" wrote:
"Dave" wrote in message
news
Am trying to bias the JFET in the attached schematic to feed signal to the
following BJTs while keeping Vgs very near Pinch-Off. Works fine as long
as the batteries are fresh, but as they begin to wane Vgs surpasses
Pinch-Off (which is 4.0 volts for the JFET I am using, while I try to hold
it at 3.95 VDC.) It has been suggested that I use a BJT (configured for
Common Emitter) to do this, but I am in the dark as to how this might be
accomplished. ANY help would be greatly appreciated...
Thanks,
Dave
Okay, Murphy's Law rules. Was effecting a minor change in the biasing of
the JFET and when I hooked it all back up I let +12VDC hit the varactor D4
out of utter carelessness and too much wire where it wasn't needed. Now all
my varactors are blown and I may have damaged one or more transistors. Will
be back later after I have sorted all this out.
JT: Thanks for the input on the source resistor, but the NTE451 apparently
limits Id to 3mA by itself (much to my frustration). I don't need to do
that for it. What I do have to do is work with the 3mA it allows.
Anyway, I appreciate your input and may be back later if what I was trying
doesn't work to my satisfaction.
Thanks.
Dave
With the wild-spec on pinch-off the current mirror approach tightens
things up, provided no more than ~4V of source-lift is required to
reach equilibrium.
Have fun!
...Jim Thompson
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