N-JFET

[zachtheterrible]

Quick question. How do I know what the minimum voltage at the gate is in order to obtain the most resistance between the drain and source?

Ive looked on datasheets and cant seem to come up with it. I know it is Vgs that I am looking for, but i only see the maximum value.

Is the reason for this because minimum Vgs varies with the supply voltage? If so, how can I calculate what minimum Vgs will be given my supply voltage?

Thank you :lol:

 

[eblc1388]

Ive looked on datasheets and cant seem to come up with it.

Does the data sheet mentioned anything about "gate-source cutoff voltage" or "pinch-off voltage Vp"?

Does it shows a graph of Id vs Vgs? If it does, then the most resistance point is where Id=0.

 

[audioguru]

Hi Zach,
The datasheet for the 2N5457 Jfet shows it conducting only 10nA with a Vgs of -0.5V to -6.0V. With 5V between drain and source it calculates to be 500M ohms. If you need a higher resistance just give it more negative gate voltage, up to its max of 25V.

What are you doing with a Jfet? :lol:

 

[zachtheterrible]

Oh, ok i see how it works now.

This is a N-channel jfet, so why is it saying negative voltage?? An N-channel Jfet's resistance goes up when it has a positive voltage, correct?

Im using it for my alarm system :lol:

 

[audioguru]

Hi Zach,
Jfets conduct as hard as they can with the lowest resistance from drain to source when the gate voltage equals the source voltage. Since the source is made from N material in an N-channel Jfet you don't want its P material gate to be positive with respect to the N material source or the diode would conduct, ruining the very high impedance at the gate.
So you apply a negative voltage to the gate of an N-channel Jfet to make it a higher resistance, just like a vacuum tube.

For a Jfet amplifier, you don't need to have a negative supply for the gate voltage. You can use a source resistor to ground which raises the source voltage and connect the gate to ground with a high resistance. :lol:

 

[zachtheterrible]

For a Jfet amplifier, you don't need to have a negative supply for the gate voltage. You can use a source resistor to ground which raises the source voltage and connect the gate to ground with a high resistance.

Can i do the same thing to make the jfet a switch?

I figured out how to find the resistor value, its just a simple voltage divider pretty much. The thing that i cant figure out though is: Which resistance do i use for the jfet, the open or closed resistnace?

 

[audioguru]

Hi Zach,
What are you switching?
A Jfet as an audio switch makes horrible distortion if it has more than about 100mVp-p across it when it is turned on. You can add some of the signal to the gate to reduce the distortion and extend the limit to about 200mV across it.
There are 4 ways to switch audio:
1) Series. The Jfet passes the audio to the output when it is on and blocks it when it is off. To avoid loss when the Jfet is on, the output needs a fairly high impedance load which causes poor attenuation at high frequencies when the Jfet is off due to the capacitance across the Jfet.
2) Shunt. A resistor passes the audio to the output when the Jfet is off, and the Jfet shunts the audio to ground at the output when it is on. To have good attenuation when the Jfet is on, the resistor must have a high value that causes high frequency attenuation due to load capacitance. To avoid loss, the load impedance must be high.
3) Use both Series and Shunt methods at the same time for excellent switching.
4) Cmos transmission gate. A CD4016, 4066, 4051, 4052 and 4053 make excellent low-cost audio switches when they are biased correctly and their signal swing doesn't exceed their supply voltage. :lol:

 

[zachtheterrible]

im not using it for audio or rf or anything, it is just a simple switch to pass DC :lol:

It is also going to be used to detect power loss to make a UPS.

EDIT: heres a picture of basically wut i want to do.

 

[audioguru]

Hi Zach,
Since the gate cutoff voltage for a Jfet part number varies so much (the one above from 0.5V to 6.0V), maybe an ordinary transistor or darlington might be more predictable as a detector/switch.

 

[audioguru]

In your pic, if your Jfet has a gate cutoff voltage of -6.0V, an input voltage more positive than -6V will make its source output follow that input voltage as it rises.
If the Jfet has a gate cutoff voltage of -0.5V, it will act almost like a piece of wire.

It has no gain, if the input rises slowly then the output also rises slowly.

 

[zachtheterrible]

Thanx audio, im gonna have to redraw some of my plans for my security system now. I didnt realize that jfets are rather high voltage devices. Its kind of hard to use them in my 5 volt security system :lol:

 

[williB]

zach you might be better off if you put your fet below and your resistor on top ..

 

[zachtheterrible]

zach you might be better off if you put your fet below and your resistor on top ..

How is that? I thought that the resistor there isolates the source from ground a little bit so that the voltage at the source relative to ground is a bit higher.

for example if the FET has a Vgs of .6v, then you would find the right resistance so that you measure 1 volt or so at the source, relative to ground. When you connect the gate to ground, the gate is at a lower potential than the source. And in this case, it would cause the FET to stop conducting.

 

[audioguru]

Hi Zach,
With the gate grounded, your Jfet will never turn off. If it tries to turn off, the current in the source resistor drops and therefore so does the voltage across it, keeping the Jfet conducting somewhat. The only way to turn it off is to apply a negative voltage to the gate.

 

[williB]

zach , what transistor are you using??

 

[williB]

because it is very possible that the fairchild data sheet is incorrect..

 

[zachtheterrible]

i am using an MPF102.

For a Jfet amplifier, you don't need to have a negative supply for the gate voltage. You can use a source resistor to ground which raises the source voltage and connect the gate to ground with a high resistance.

You said yourself audio that a negative voltage isnt needed. I meant to say that i would be using a high resistance from gate to ground.

willi, could you go a little bit more indepth about what you are saying?

 

[williB]

i am saying that an N channel FET should turn on with a positive Vgs..
and the fairchild data sheet may be incorrect..

 

[williB]

i just checked a motorola sheet for the same part..
it says the same thing..i dont get it?
an N channel Fet should turn on with a positive Vgs.

 

[Roff]

i just checked a motorola sheet for the same part..
it says the same thing..i dont get it?
an N channel Fet should turn on with a positive Vgs.

This is true of an N channel MOSFET, but positive Vgs on an N channel JFET will cause gate current to flow, which is generally not desirable.