"John Larkin" wrote in message
...
On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote:

Okay, like I said once before, I'm ignorant. But I am trying to fix that.

Attempting to bias 2N5486 JFET as shown in the diagram below. According
to
what I read, Vg should be 2.65 VDC, which it does measure to be with my
Fluke. But Vs should be 5.92 or some such, and only measures 2.97,
indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC
which, to me, doesnt jive at all. Somebody please tell me, what am I
missing here?

Thanks,

Dave




Jfets have absurd unit-to-unit variations.

Pull the fet out and measure Idss. I bet it's about 4 mA. The spec
range is 8 to 20, but with NTE one never knows...

I assume everything is connected OK. Does it amplify?

John


Hey John, thanks for the reply. Sorry it took me so long to get back onto
this. Believe it or not I have been trying to eliminate variables and
possibilities since I posted previously.

You are right on the money, Idss is roughly 4mA. So I tried to work with
that limitation and bias the device accordingly. Please see attached
file...

On my workbench, input to the device is is a 500mV P-P 10kHz square wave,
but the device output is like 50mV. I have checked and double checked the
configuration and all connections, replacing the device when I ran out of
things to check. No change. I am posting the entire schematic, just for
complete measure, cause I am obviously missing something but have no idea
what it is. Output from the entire circuit is 500mV P-P 10kHz, so I figure
that the rest of the circuit is at least trying to overcome the deficiencies
of the first stage of pseudo-amplification. In case anyone is wondering,
the 5M trimpot is intended to eliminate the possibility of poorly chosen
values for R2, in the potential divider biasing setup. This way I can
adjust it for maximum output, if it can be called that...

Could my problem be the large values I am using for the potential divider on
the NTE451? I read that they must be quite large to avoid other problems
(slips my mind at the moment) but am now wondering. *Any* ideas are eagerly
sought. Vg measures 3.4V, and Vgs measures approx .2V, but changing these
values has no positive effect. Beginning to suspect that Ian was right, and
there just isn't enough "space" for using a JFET. Wondering if I need to
make the shift to 12VDC.

Out of other ideas...

Dave