FET as Voltage Control Resistance

[prprog]

Please see the attach circuit. I try it using it as a substitute for the resistance/pot on my PC game port (Joystick) but cannot make it work. I apply voltage to the gate that are less than +5V (no more than 1 volt) but the resistance does not change. Do I need this gate voltages to be negative? Thanks a lot. I am using a MPF102 FET.

 

[dknguyen]

Maybe this will help

**broken link removed**

The graph on the webage looks strange though...I think the "Vgs=0V" and "Vgs=Vp labels" are reversed.

Maybe this will also help:
https://www.electro-tech-online.com/custompdfs/2008/04/Vishay-Silcnx_FET_VoltCntrlRES_AN105.pdf

I can't tell which is the source and drain on your JFET because of the component symbol, but if the drain is on top and source is on the bottom, the gate voltage you apply (if it is referenced to ground) will not be the same voltage that appears across the gate-source of the JFET because of the voltage drop across the 1K resistor. Maybe try moving the 1K resistor to the drain instead and connecting the source straight to ground. THis is just a guess- I've never done this before.

THe first link I gave you also says that the resistance-gate voltage curve is non-linear, so you need two resistors to linearize it.

 

[prprog]

Dknguyen thanks for the reply. What voltage is needed on the gate pin of the FET to make it voltage resistance? Thanks.

 

[dknguyen]

I can't give you advice. I've never done this before. I can only give you links that you can read and then mess around with your circuit.

 

[crutschow]

JFETs generally are depletion-mode, thus an N-Channel JFET is on with a zero gate bias and requires a negative voltage to turn off.

The MPF102 requires up to -8V gate-to-source to completely turn off.

 

[dknguyen]

Ah, so the graph's Vgs isn't reversed then. Vp is negative so it is less than 0V.

 

[audioguru]

Many JFETs are symmetrical and their drain and source can be swapped without changing the performance. Then their symbol is drawn with the gate at the exact center as is shown in this thread.

When is the pretty girl in your avatar going to entertain us?

 

[Ubergeek63]

Also in some the drain and source are interchangeable, but I do not know how many are anymore.

There are also depletion mode MOSFETs that could be used, they are actually better since the channel resistance is potentially very low by comparison.

**broken link removed**

For instance is a 200V depletion mode with an RDS on of 3.5 ohms.