Semiconductor Devices: JFET Voltage Controlled Resistor

A voltage controlled resistor is a device whose resistance depends on a control voltage. In this video, we discuss how this can be achieved with a simple JFET using small drain-source potentials. This is known as the ohmic region.
References: Semiconductor Devices: Theory and Application/2E; Chapter 11, Section 7.
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[@leiferickson3183]

Super cool circuit!You can linearize this by putting another 1M from Gate to Drain as feedback. It reduces distortion quite a bit if I recall correctly. I feel like I may have added a DC blocking cap also between the added resistor and the drain but I'm a little fuzzy on this - It was 20 years ago.

[@ElectronicswithProfessorFiore]

Yep. You need the cap otherwise the added resistor will spoil the bias and the control won't be the same.

[@simonyoungglostog]

Thank you.

[@InflatableRobot]

If you were running this circuit from a 9v source, like a guitar pedal, would you need something like a charge pump to provide a negative voltage for the voltage control of the jfet?

[@ElectronicswithProfessorFiore]

You might consider using a P channel FET.

[@h7qvi]

Often used in audio compressor circuits

[@aerofart]

OK, so it’s a variable resistor. Why is it being used in parallel to the load? I’m assuming to maintain biasing in linear region. Can it be employed in series with the load? In parallel and in series to other jfet (resistors)?

[@ElectronicswithProfessorFiore]

Yes and yes. You'll want to stay tuned for an upcoming video covering an analog switch using JFETs.