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