Wikipedia explains that an electret microphone is composed of an electret material and a FET for pre-amplifying the audio signal:

enter image description here

What I don't understand is how it is working, as the FET must certainly have a high threshold gate voltage compared to the (supposedly) low-voltage emitted by the electret material? Also the FET acting like a switch (?) how is it possible to obtain analog output signal?



1 Answer 1


It's a JFET and a JFET in this configuration self-biases its gate to produce some current between drain and source (although the simplistic circuit representation of an electret doesn't imply that).

Here's a graph that shows the JFET's drain-source characteristic. Note how the gate has to become negative with respect to the source in order to turn off the drain current: -

enter image description here

  • \$\begingroup\$ Thanks! Although I still miss a thing: what is the value of Vds in an elecret microphone general configuration? Is it okay to say Vgs is stationary, as electret material voltage is very small? I still don't understand how signal is amplified! \$\endgroup\$
    – Vincz777
    Commented Oct 3, 2016 at 7:51
  • \$\begingroup\$ There is no simple answer. Vds could be 0.1V all the way up to 48V for some studio phantom powered devices. The JFET is an amplifier. \$\endgroup\$
    – Andy aka
    Commented Oct 3, 2016 at 8:14
  • \$\begingroup\$ Studio phantom powered mics won't normally be electret mics though, or single ended output. \$\endgroup\$
    – user16324
    Commented Oct 3, 2016 at 9:42

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