help in N-jfet and P-jfet

Papabravo

Joined Feb 24, 2006
14,701
An n-channel JFET is on when the gate and the source are at the same potential. The drain current is:
\(I_{dss}\).
As the gate voltage goes negative with respect to the source, the channel is "pincehed-off" and the drain current drops to zero.

I'm not familiar with a p-channel JFET. I'll have to get back to you on that one.
EDIT:
For a p-channel JFET, with no voltage on the gate and the source more positive than the drain, current will flow. To turn the device off the gate is driven more positive than the drain. At some gate voltage, the channel will be "pinched off" and no current will flow.

NOTE: Since p-channel JFET devices use "holes" for conduction they are slower and thus seldom used for most purposes. That is why I couldn't remember precisely how they worked -- I never needed to know.
 
Last edited:

Thread Starter

andrew132

Joined Feb 2, 2017
96
An n-channel JFET is on when the gate and the source are at the same potential. The drain current is:
\(I_{dss}\).
As the gate voltage goes negative with respect to the source, the channel is "pincehed-off" and the drain current drops to zero.

I'm not familiar with a p-channel JFET. I'll have to get back to you on that one.
EDIT:
For a p-channel JFET, with no voltage on the gate and the source more positive than the drain, current will flow. To turn the device off the gate is driven more positive than the drain. At some gate voltage, the channel will be "pinched off" and no current will flow.

NOTE: Since p-channel JFET devices use "holes" for conduction they are slower and thus seldom used for most purposes. That is why I couldn't remember precisely how they worked -- I never needed to know.
thank you
 

OBW0549

Joined Mar 2, 2015
3,566
For a p-channel JFET, with no voltage on the gate and the source more positive than the drain, current will flow. To turn the device off the gate is driven more positive than the drain. At some gate voltage, the channel will be "pinched off" and no current will flow.
More positive than the source, not the drain.
 

OBW0549

Joined Mar 2, 2015
3,566
I may have been misled by my internet source, but is it not the case that you can ground the drain, and supply voltage through a resistor to the source. I thought the devices worked in either direction.

http://www.learningaboutelectronics.com/Articles/P-channel-JFET
I read that article and found its explanation of P-JFET biasing a bit strange, though not exactly incorrect.

A far more straightforward way to look at a P-JFET is that it is simply, with respect to voltage polarities, the mirror of an N-JFET: take any N-JFET application circuit, replace the FET with a P-JFET, change the polarity of all supply voltages, and you're good to go (with the possible exception of having to make appropriate adjustments to bias voltages to establish the desired operating point).

As for the devices working in either direction, JFETs (both N-channel and P-channel) possess one characteristic not found in MOSFETs: drain and source can be interchanged. (You can't do that with MOSFETs because of the body diode.) Either the drain or the source terminal of the JFET can be used as the source, and either can be used as the drain. As far as I know, this applies to all JFETs in all applications-- at least, I've never encountered any exceptions to this.

As you noted, P-channel JFETs are almost never used, although I did find one circuit using a 2N5462 as an audio switch.
 

cmartinez

Joined Jan 17, 2007
7,175
I read that article and found its explanation of P-JFET biasing a bit strange, though not exactly incorrect.

A far more straightforward way to look at a P-JFET is that it is simply, with respect to voltage polarities, the mirror of an N-JFET: take any N-JFET application circuit, replace the FET with a P-JFET, change the polarity of all supply voltages, and you're good to go (with the possible exception of having to make appropriate adjustments to bias voltages to establish the desired operating point).

As for the devices working in either direction, JFETs (both N-channel and P-channel) possess one characteristic not found in MOSFETs: drain and source can be interchanged. (You can't do that with MOSFETs because of the body diode.) Either the drain or the source terminal of the JFET can be used as the source, and either can be used as the drain. As far as I know, this applies to all JFETs in all applications-- at least, I've never encountered any exceptions to this.

As you noted, P-channel JFETs are almost never used, although I did find one circuit using a 2N5462 as an audio switch.
I don't mean to hijack the thread, but I think my question is relevant. Do you know an example circuit in which only JFETs could be used for a particular purpose, and not mosfets, bipolars or any other type of transitor? My question goes along the line of: "what's so special about JFET's?"
 

atferrari

Joined Jan 6, 2004
4,122
I don't mean to hijack the thread, but I think my question is relevant. Do you know an example circuit in which only JFETs could be used for a particular purpose, and not mosfets, bipolars or any other type of transitor? My question goes along the line of: "what's so special about JFET's?"
Just being the first FETs built?
 

OBW0549

Joined Mar 2, 2015
3,566
Do you know an example circuit in which only JFETs could be used for a particular purpose, and not mosfets, bipolars or any other type of transitor?
Not really. The closest I could come would be to note that some JFETs-- but very few-- have lower noise than the best BJTs, so could be more suitable in low-noise amplifiers.

My question goes along the line of: "what's so special about JFET's?"
They can be used, under certain circumstances, as voltage-controlled resistors. Other than that, I can't think of anything.
 

Papabravo

Joined Feb 24, 2006
14,701
Interesting... so what you're saying, first came bipolars, then the jFets, then the Mosfets and lastly the IGBT's? So jFets could be thought of as more primitive versions of the latter?
Well the physics is a bit different. The JFET is a depletion mode device. That means with no biasing the channel is open and conducts. As bias is applied to the gate the channel is pinched off and stops conducting. The MOSFET is an enhancement mode device. That means that with no biasing on the gate the channel does not conduct. As you add positive bias the conduction of the channel is enhanced.

According to the Wikipedia article, the inventors of the transistor were trying to make FETs at the same time and failed. Technology did not allow them to succeed until almost a decade later.
 

GopherT

Joined Nov 23, 2012
8,012
I don't mean to hijack the thread, but I think my question is relevant. Do you know an example circuit in which only JFETs could be used for a particular purpose, and not mosfets, bipolars or any other type of transitor? My question goes along the line of: "what's so special about JFET's?"
The good part of JFETs is
- high input impedance (practically infinite) for very weak sensors.
- high speed (low input capacitance)
- no need for biasing the input

The bad...
- no need for biasing the input (would allow shoot-through on class-B or -AB amplifier)
- limited to low current (0.1 to 100 mA typically).
 
Top