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  • Author Author: tariq.ahmad
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How JFETs work -- DC to Daylight 15

In this episode we're exploring the JFET or junction field effect transistor as it applies to the common-source amplifier. We cover graphically approximating the transconductance curve, and how to set the DC bias point. We show how much this transconductance varies between devices within the same family. Then breadboard our circuit and verify that measurements approximate our calculations.

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  • Thank you Derek very interesting as always. I have a doubt, MOSFETs are FETs or unipolar transistors too. Today, what advantage can there be for a particular design to work with a FET transistor that is not a MOSFET for small signals and with a large input impedance need. What are the uses of FETs that are not MOSFETs?

  • in reply to javagoza

    Honestly, JFETs don't get much usage all that much any more. With the exception of low noise preamps and niche audio. In fact I have a few studio mics where, the first thing the audio cartridge is connected to is a JFET, then the signal travels through some bipolars, and op amps.. but, JFETs still survive in discrete audio, including analog switching circuits. This was/is still common in guitar pedal effects where the sound effects need to be able to switch in and out of the audio chain. Some prefer big clunky switches - personally I like the digital soft-switching that JFETS provide.

    Someone in the comments also pointed out that you can use them as ultra-low leakage diodes, which I've yet to try out. I can only think of a couple of applications - maybe an RF mixer or detector.

    Thanks for reaching out!

    Best regards,

    Derek

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  • in reply to javagoza

    Honestly, JFETs don't get much usage all that much any more. With the exception of low noise preamps and niche audio. In fact I have a few studio mics where, the first thing the audio cartridge is connected to is a JFET, then the signal travels through some bipolars, and op amps.. but, JFETs still survive in discrete audio, including analog switching circuits. This was/is still common in guitar pedal effects where the sound effects need to be able to switch in and out of the audio chain. Some prefer big clunky switches - personally I like the digital soft-switching that JFETS provide.

    Someone in the comments also pointed out that you can use them as ultra-low leakage diodes, which I've yet to try out. I can only think of a couple of applications - maybe an RF mixer or detector.

    Thanks for reaching out!

    Best regards,

    Derek

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