Analog electronics, Electronic components, Electronics, Materials

Electret microphone: how does it work?

The electret microphone is very used in electronic projects, due to its low cost, size, good sensitivity, and mechanical resistance.

What is electret?

It’s a dielectric or insulating material, which has its electric dipoles organized in a way that the material has a static electric charge, even without the presence of an external electric field. Similar to a permanent magnet, which has a magnetic field on its own.

This is a cross-section of an electret with a metal electrode. Source: The Free Dictionary.

When this material is under pressure, mechanical deformation, or even infrared radiation, the electric charge concentration is changed, producing a small differential potential that needs to be amplified.

Electret microphone operation

Also known as capacitive microphone, because it works with the principle of capacitor. The electret membrane is a film made of mylar, with one side metalized.

The mylar forms a capacitor with the aluminum case and a pick-up plate, and a plastic spacer separates pick-up plate and the electret. The former is linked to a FET gate. Holes on pick-up plate serve to allow the air to escape. Source: Make:

Post about JFET operation.Click here

electret microphone
Electret microphone cross-section. The plastic amplifier housing serves for protection. Source: Cuidevices.

The black piece of cloth made of felt prevents dust entrance.

types of electret microphone
There are two types of electret microphones: two pins and three pins. Currently, the two pins type is much more common. saída=output. Source: Nova Eletrônica.

When sound enters on microphone, diaphragm moves, which in turn moves the electret film with a metallic side. This vibration changes capacitance and charge concentration on the electret, that generates a variation in the electric signal, which is amplified by FET.

Polarizing electret microphone

Circuit to polarize the microphone.

According to many datasheets, the maximum current consumption on electret microphones is 0.5mA. With that, you can calculate the value of R resistor, which depends on supply voltage VCC. Usually, VCC voltage range is between 1.5V and 12V. The capacitor serves to filter direct current from the source, allowing only audio signal’s passage. C’s value can be between 100nF and 10μF. Choosing arbitrarily VCC, can calculate R.

R=\frac{VCC}{I}=\frac{VCC}{0,5m}

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