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I saw JFET in an electret mic circuit, and it is quite recent, so I'm wondering why.

Since we are looking for the highest gate impedance, MOSFET would seem to be a better choice.

I've also read that JFET have got a flatter response because of a higher drain resistance. I don't even understand what the drain resistance is. Is it the source drain resistance? And what does it have to do with flattness?

And more generally, what are the advantages of JFET over MOSFET?

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2 Answers 2

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The JFET has several advantages over the MOSFET. The most important are:

  • higher gain
  • lower noise

These are the overriding factors when building preamplifiers for low-level signals, such as those from microphones.

Also, since there's no thin gate oxide that can be punctured by ESD, they're a little more "rugged" in that sense.

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    \$\begingroup\$ I have used them to control start-up behaviour of amplifier outputs as they are an 'ON' switch with no power applied with gate and source tied together. \$\endgroup\$
    – Peter Smith
    Commented Apr 14, 2019 at 14:44
  • \$\begingroup\$ @Dave Tweed Thank you for your response. And what about the comment about flatter response and "drain resistance"? Does it mean anything? And why would a lower gate capacitance be better than a higher gate impedance? \$\endgroup\$
    – Jonas Daverio
    Commented Apr 18, 2019 at 12:58
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JFETS have a useful biasing, like a vacuum tube biasing. Simply place a 100 ohm resistor in the Source pin to Ground to control the current, and you can then connect Gate to a DC_conducting sensor such as Moving Coil vinyl-record cartridges and enjoy the JFET response down to DC with no need for DC_blocking capacitors.

schematic

simulate this circuit – Schematic created using CircuitLab

This works for N-JFETs because they are internally built to be conducting at ZERO volts gate-source whereas N-MOSFETS are internally built/doped to be non-conducting at ZERO volts gate-source. Same physics, but different doping intensities.

Read diyAudio.com/simplistic_NJFET_RIAA thread for excellent use of Hitachi low noise JFET.

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  • \$\begingroup\$ Wouldn't it be possible with depletion-mode MOSFETs? \$\endgroup\$
    – Jonas Daverio
    Commented Apr 15, 2019 at 19:43
  • \$\begingroup\$ Remind me about depletion-mode MOSFET behaviors. I've read/heard of silicon process engineers "fine tuning" the gate thresholds, by tweaking the implants. But I don't recall the nuances of enhancement versus depletion mode devices. \$\endgroup\$
    – analogsystemsrf
    Commented Apr 16, 2019 at 1:43
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    \$\begingroup\$ I think the depletion-mode one decreases its conductance with a voltage, while enhancement mode increases it. JFET are only depletion-mode and normally-on devices. MOSFET can also be depletion-mode, so I don't think JFET are chosen because they are depletion-mode. \$\endgroup\$
    – Jonas Daverio
    Commented Apr 16, 2019 at 9:17
  • \$\begingroup\$ @JonasDaverio You can technically make enhancement-mode JFETs, they just don't work very well (and I'm not aware of any commercially available ones). The now-obsolete device called a unijunction transistor (or UJT) is similar to an enhancement-mode JFET. \$\endgroup\$
    – Hearth
    Commented Apr 17, 2019 at 13:03

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