JFET voltage dependent resistor

[ag]

Just a question:
How do you use a JFET as voltage dependent resistor? I know you use it in the triode region but i don't have any data on what voltages will be at the gate in order for the drain-source resistance to vary in the manner i want.
I know these values vary from device to device and therefore this is a rather subjective question, so can anyone tell me values for a popular or well-used JFET?
thanks

 

[Optikon]

Just a question:
How do you use a JFET as voltage dependent resistor? I know you use it in the triode region but i don't have any data on what voltages will be at the gate in order for the drain-source resistance to vary in the manner i want.
I know these values vary from device to device and therefore this is a rather subjective question, so can anyone tell me values for a popular or well-used JFET?
thanks

Well, you can make a voltage dependent resistor by placing a real resistor in the source lead and tying that back to the gate.. This is the principal behind current limiting diodes or current regulator diodes.. it can behave as a diode when given a forward bias and the current will grow linearly proportional to voltage for smal fwd voltages (behaing just like a linear resistor) but as the voltage increases, the effective resistance will increase thus providing a current limit (in this sense it is a non-linear voltage controlled resistor)..

for actual values you will need to work out the math based on the JFET datasheet and the source resistor you have chosen. A common JFET is a 2n4392 or 2n4391 N channel JFET.

You might want to consider something called a GIC general impedance converter made with opamps... google for it, you'll find lots.. I suppose one of these can be made to take a voltage control input and provide a proportional impedance output..

More details of your application would be helpful..

 

[ag]

Thanks Optikon, I was planning to construct a tremolo effect stage in a guitar pedal circuit with this. A triangle waveform voltage would be applied to some sort of VDR (hopefully a JFET) which would be on of the feedback resistors in an inverting amp, together forming an amp with oscillating gain. Sorry i don't have a diagram, but i'm sure you get what i'm saying.