MOTM

It a little hard to decipher your circuit, but in general terms, a JFET will
always conduct until you pinch it off. The part you mentioned is an
N-channel JFET and requires the gate to have a signal less than -8V to stop
conducting. If you never generate this negative voltage, you won't be able
to turn it off.

If it's pulling current from the base, you've got a forward bias on the gate
and that's not the normal mode for this device.


-----Original Message-----
From: motm@yahoogroups.com [mailto:motm@yahoogroups.com] On Behalf Of
Richard Arntzen
Sent: Friday, August 05, 2005 12:19 PM
To: motm@yahoogroups.com
Subject: [motm] Newbie JFET-switch problem

Hello all.

I have built myself a JFET-switch (2N3819) from a schematic kindly provided
by Ian Fritz, and I have some problems. Brief description of the circuit:

The JFET base is connected to the anode of a diode (cathode is grounded).
Base is also is connected to positive rail (+15V) through a 1.5M resistor,
and to a cap. The basic idea is that:

i) Static mode is that the anode of the diode should have a small positive
voltage, and the JFET conducts.

ii) when the cap is pulled down (by another part of the circuit), the base
will be pulled down and the JFET will close.

iii) The cap will then be slowly charged through the resistor, and
eventually the JFET will conduct again.

Now; my problem is with the input range of the switch. I see three modes:

A) When the source voltage VS is below 1.19V, it apparently pulls the gate
down. I see an <almost> linear relation between VG (gate voltage) and VS, up
to VS between 1.11 and 1.19V, after which VG flattens out at 0.37V
regardless of VS. In this region (VG < 0.37V), the drain voltage VD is
always 0.47V below VS.

B) When 1.19V < VS < 3.22V, the JFET is open (VD=VS).

C) When VS > 3.22V or thereabout, VD becomes <almost> constant at 3.26-3.3V
regardless of VS.

Having done some reading I see that C) happends because of saturation.
Question 1: is 3.3V a normal saturation limit for the 2N3819? Can't see it
from the datasheet (at least, I can't).

A) I don't understand - is this due to the threshold voltage? Why does the
JFET conduct anyway 
(with a constant resistance?), and why does it pull current from the base?

And finally; how do I fix this? Do I have to scale the source voltage to be
between 1.2 and 3.2V? Does these JFET parameters vary, and how can I get
this information from a datasheet?

Hoping for some help from the experts,

Richard




 
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