How does an electret microphone work?

Several microphones are frequently used in our electrical devices. Among these is the electret microphone. Its mode of operation differs from that of conventional microphones and one wonders how it works. Find out here how an electret microphone works.

Presentation of an electret microphone

An electret microphone is a microphone that has a material called electret whose role is comparable to that of a capacitor. Its polarization is made permanently during its conception. The electrostatic capacitor it has takes care of eliminating the need for a polarizing power supply thanks to a material that has a permanent charge.

These electret condenser microphones are often found in laptops, intercoms and smartphones. These microphones have a magnetic layer that is located at the rear membrane, which justifies their lack of power requirements from the outside. In addition, the electret microphones can be recharged with a battery, or input from a computer, smartphone or camera.

Principle and mode of operation of an electret microphone

Its principle consists first of all in supplying the microphone with a DC voltage. Then, this recovers its alternating voltage belonging to the audio signal . Whether it’s a 2-wire capsule or a 3-wire capsule, there is a pin that is electrically connected to the metal housing of the microphone. The latter corresponds to the mass of the microphone.

The micro-electret essentially has three elements. It’s about :

  • of the diaphragm,
  • of 2 electrodes
  • and an integrated JFET (Junction Field Effect Transistor).

The diaphragm is made with a thin Teflon material better known as electret. The latter has a fixed charge located in the center of the two electrodes. This assembly of electret and electrode acts as a variable capacitor whose outer part responds to sound waves .

It is this response which causes the capacitance variation of the two electrodes. Then, one of the electrodes is moved in the form of air pressure by the sound vibrations on the visible part of the electret causing several variations. It is these variations which propagate through the capacitive plates and the operation of the electret microphone is observed.

Electret Microphone Wiring

The power supply of the electret capsule depends on the number of wires installed on its case. For the electret microphone the number of wires can be 2 or 3. Compared to two-wire capsules, the power pin and the output pin are common and the wiring in this case is unique. For three-wire capsules, however, one of the wires is power -specific , so the wiring can be done so that it looks like a two-wire capsule.

Despite the fact that there is no specific pin for powering 2-wire capsules, powering them is very easy. It may happen that a 2-wire capsule is used as a 3-wire electret capsule.

Operation of the 2 and 3 wire capsule power supply

At the level of the capsules with two wires, one of the wires of the micro-electret is connected to the mass and the second makes it possible to feed the microphone and the case. Its resistor serves as a polarizer for the transistor inside the microphone. Similarly, the capacitor stops the DC voltage supplied by the resistor. It only allows the passage of the alternating current of the audio signal.

Regarding the 3-wire capsules, their power supplies take place in two phases. During the first phase, the first two wires connect successively to ground, one at the level of the metal casing of the microphone and the other on the lower terminal of the FET (Field-effect transistor). The last wire receives the DC current supply and provides current to the BF signal.

During the 2nd phase, no change at the level of the 1st wire is noticed. On the other hand, the 2nd receives the BF signal at the level of the lower terminal of the FET and the 3rd receives the continuous power supply at the upper terminal of the FET.

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