42. JFETs as Variable Resistors

JFETs can and are used as variable resistors in many circuits by keeping their operation in the ohmic region. This video shows how this can be accomplished using a Common Emitter amplifier. The same circuit is then tested on a breadboard.

[@t1d100]

Thank you for sharing and teaching. Just to provide some feedback to you, from the title "JFETs as Variable Resistors," I expected to learn about how to measure the JFET 's characteristics and set it up to run in the Ohmic region. By using an example of a JFET in an extended circuit, that basic information got lost and the subject became overly complicated. I did not finish the video, for that cause. You might want to update the title to "A Consideration of JFET Operations as Variable Resistors in an Extended Circuit." HTH. I still gave you a thumbs up, because of your effort and excellent teaching skills.

[@PelDaddy]

Thanks for this.

[@hubercats]

Excellent!

[@boonedockjourneyman7979]

Ty

[@alibehrouz9833]

so well

[@fieryfirevivin]

Is it possible to sweep the jfet resistance within a small range such as 348ohm to 350 ohm

[@aduedc]

I learned about interesting way you connected the JFET to the rest of the circuit using decoupling capacitance. We usually do that for varactors in oscillators. I guess we can use JFET in this topology for controlling the oscillation voltage, and reducing phase noise. I wonder if the JEFT capacitance changes with Vgate? I guess, it does after all Gate to channel capacitance is result of depletion region of pn junction, and by increasing the voltage you increase the size of depletion region, and as result reduce the capacitance. However, we can not use this configuration as Varactor, itself, because it has high resistance and as a result low Q.

[@cjvanstraeten1169]

I missed the Xc of the Capacitors in serie with the fet and in serie with the Rload these Ac-resistances 1/(jwC) are at a freq of 1000Hz so taking them not in account gives a fault in the calculation ..1/( 2x3.14x2.2u)= about 73ohm in serie with the jfet ..i liked the video thank you Sir 👍

[@kettelenegaspard685]

What would happen if you disconnect the gate from a scr diode

[@TheOffsetVolt]

Hello again, The device would appear as an open from cathode to anode. SCRs should only turn on when a gate current is present.

[@matthijshebly]

What's up with the 2.2uF connected to the FET?
That doesn't make sense at all.

[@TheOffsetVolt]

Hi Matthijs, You want the JFETs variable resistance to only adjust the AC portion of the signal, take out the cap and the DC current on the emitter will also vary changing the Q point.

[@matthijshebly]

Yes, that makes sense. Many thanks.

[@kettelenegaspard685]

what could cause v(out) to be less than normal on a jfet

[@TheOffsetVolt]

Hello, That would depend on the type of circuit so I can't give any specific answer to your question.

[@kettelenegaspard685]

The Offset Volt
Ok let's say if it's a common drain

[@TheOffsetVolt]

A common drain has no voltage gain, output should be close to the input voltage. If the output does go down, the something caused the curve change, gate bias or bad jfet.

[@kettelenegaspard685]

The Offset Volt
Does the same apply for a common source

[@TheOffsetVolt]

Then you would have to include the capacitor parallel o the source resistor. If it opens or changes gain will drop.

[@masbro1901]

2:39 forgive my newbie. what is pry me ?

[@TheOffsetVolt]

Hi, It is r'e, pronounced r prime e. It's the resistance between the emitter base junction of a bipolar junction transistor. I think I discussed it in ruclips.net/video/EoQiy--39sY/видео.html Hope that helps, let me know if you have any other questions.