Can current flow in both directions through a JFET channel?

J

jalbers@bsu.edu

Guest
I am beginning to study JFETs . I want to limit this discussion to N
channel JFETs and when I talk about current flow I mean conventional
current flow.

Most of the sources that I have been studying use example where the
gate-source junction is reversed biased and the voltage at the drain
is positive with respect to the source and current flows from drain to
source.

What if the gate-source junction is still reversed biased and the
voltage at the drain is negative with respect to the source. Will
current now flow from the source to the drain?

I have also read somewhere that on "some" JFETs, the source and drain
connections are interchangeable while on others they are not. Is this
true? If so, how do you know which ones will do this?

Any help would be greatly appreciated. Thanks
 
jalbers@bsu.edu wrote:

I am beginning to study JFETs . I want to limit this discussion to N
channel JFETs and when I talk about current flow I mean conventional
current flow.

Most of the sources that I have been studying use example where the
gate-source junction is reversed biased and the voltage at the drain
is positive with respect to the source and current flows from drain to
source.

What if the gate-source junction is still reversed biased and the
voltage at the drain is negative with respect to the source. Will
current now flow from the source to the drain?

I have also read somewhere that on "some" JFETs, the source and drain
connections are interchangeable while on others they are not. Is this
true? If so, how do you know which ones will do this?

Any help would be greatly appreciated. Thanks
Your answers are there.
Click to expand...
Scroll down and you'll see "Analog Switch"

http://www.interfet.com/pdf/App_Notes.pdf

http://webpages.charter.net/jamie_5"
 
"jalbers@bsu.edu" wrote:

I am beginning to study JFETs . I want to limit this discussion to N
channel JFETs and when I talk about current flow I mean conventional
current flow.
Click to expand...
The answer is YES, at least up to a point since if you go too far it
starts distorting and I'm not interested past that point.

Think of it vaguely as a voltage controlled resistor.

Graham
 
"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:4gsLk.4899$UD6.2704@newsfe07.iad...
jalbers@bsu.edu wrote:

I am beginning to study JFETs . I want to limit this discussion to N
channel JFETs and when I talk about current flow I mean conventional
current flow.

Most of the sources that I have been studying use example where the
gate-source junction is reversed biased and the voltage at the drain
is positive with respect to the source and current flows from drain to
source.

What if the gate-source junction is still reversed biased and the
voltage at the drain is negative with respect to the source. Will
current now flow from the source to the drain?

I have also read somewhere that on "some" JFETs, the source and drain
connections are interchangeable while on others they are not. Is this
true? If so, how do you know which ones will do this?

Any help would be greatly appreciated. Thanks
Your answers are there.
Scroll down and you'll see "Analog Switch"

http://www.interfet.com/pdf/App_Notes.pdf
Click to expand...
That is a good app note. I have used JFETs (2N3819) in audio amplifiers I
made around 1970, and also as a voltage controlled resistance in a
programmable gain amplifier in a product I designed in the early 1980s.
This particular device is still available and can be very useful. I'm
fairly sure that the source and drain can be reversed, especially in low
signal level applications.

Another good app note, from Motorola, is:

http://rfwireless.rell.com/pdfs/AN_211A_D.pdf

Paul
 
"Paul E. Schoen" wrote:

I'm fairly sure that the source and drain can be reversed, especially in low
signal level applications.
Click to expand...
That's true of many jfet devices in fact.

Graham
 
Paul E. Schoen wrote:

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:4gsLk.4899$UD6.2704@newsfe07.iad...

jalbers@bsu.edu wrote:


I am beginning to study JFETs . I want to limit this discussion to N
channel JFETs and when I talk about current flow I mean conventional
current flow.

Most of the sources that I have been studying use example where the
gate-source junction is reversed biased and the voltage at the drain
is positive with respect to the source and current flows from drain to
source.

What if the gate-source junction is still reversed biased and the
voltage at the drain is negative with respect to the source. Will
current now flow from the source to the drain?

I have also read somewhere that on "some" JFETs, the source and drain
connections are interchangeable while on others they are not. Is this
true? If so, how do you know which ones will do this?

Any help would be greatly appreciated. Thanks

Your answers are there.
Scroll down and you'll see "Analog Switch"

http://www.interfet.com/pdf/App_Notes.pdf


That is a good app note. I have used JFETs (2N3819) in audio amplifiers I
made around 1970, and also as a voltage controlled resistance in a
programmable gain amplifier in a product I designed in the early 1980s.
This particular device is still available and can be very useful. I'm
fairly sure that the source and drain can be reversed, especially in low
signal level applications.

Another good app note, from Motorola, is:

http://rfwireless.rell.com/pdfs/AN_211A_D.pdf

Paul


That's a very interesting PDF, I just briefly looked it over and there
Click to expand...
is an assert on a subject that I had an argument once, with one of my
colleague's at work.. Not that it means anything now but I think it'll
be interesting to see the look on his face when he reads it because I
lost that battle with lack of documentation to back it up and since he's
the senior man, he didn't need any documentation :);.

Thanks.

http://webpages.charter.net/jamie_5"
 
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