Why Electret Microphones Use JFET

Thread Starter

Jack Lew

Joined Jul 2, 2018
1
As we know, there is a JFET inside electret microphones to amplified signal.
But why JFET, instead of MOSFET? Both JFET and MOSFET have a high input impedance.
 

AlbertHall

Joined Jun 4, 2014
12,345
MOSFETs usually have a large gate capacitance which would reduce the signal available from the electret.
I don't know whether it would be possible to make a MOSFET with a gate capacitance comparable to a JFET (e.g. MPF102 7pF).
 

MisterBill2

Joined Jan 23, 2018
18,176
A.H got it right! The capacitance of the electret part is quite small, and the charge delivered is really tiny. So the device to connect to it must therefore have a very small amount of charge required to produce a useful output. A Jfet is the easiest solution. All of the other options are far more expensive and far less convenient.
 

OBW0549

Joined Mar 2, 2015
3,566
Another factor in favor of JFETs over MOSFETs is biasing: JFETs are depletion-mode devices, and conduct drain-source current when the Vgs is zero. Most MOSFETs are enhancement-mode devices which need a non-zero gate voltage to conduct any drain current, so using a MOSFET would require some sort of bias circuit-- which complicates the design.

With a JFET, all that need be done is to use a JFET with an appropriate Idss and the microphone is ready to use with a drain pullup resistor.
 
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