Composition and working principle of electret condenser microphone

Tue Dec 21 21:38:37 CST 2021

Electret microphone consists of acoustic electric conversion and impedance conversion. The key element of acoustoelectric conversion is electret diaphragm. It is a very thin plastic film, in which a layer of pure gold film is evaporated on one side. Then, after the electret of high voltage electric field, there are anisotropic charges on both sides. The evaporated gold surface of the diaphragm is outward and connected with the metal shell. The other side of the diaphragm is separated from the metal plate by a thin insulating lining ring. In this way, a capacitance is formed between the evaporated gold film and the metal plate. When the electret diaphragm encounters acoustic vibration, the electric field at both ends of the capacitor changes, resulting in an alternating voltage varying with the change of acoustic wave. The capacitance between the electret diaphragm and the metal plate is relatively small, generally tens of PF. Therefore, its output impedance value is very high (XC = 1 / 2 ~ TFC), about tens of megaohms or more. Such a high impedance cannot be directly matched with the audio amplifier. Therefore, a junction field effect transistor is connected into the microphone for impedance conversion. FET is characterized by high input impedance and low noise figure. The common FET has three electrodes: active electrode (s), grid electrode (g) and drain electrode (d). Here, a special FET with another diode between the internal source and grid is used. The purpose of the diode is to protect the FET from strong signal impact. The gate of the FET is connected to a metal plate. In this way, there are three output lines of electret microphone. That is, the source s is generally blue plastic wire, the drain D is generally red plastic wire and the braided shielding wire connecting the metal shell.

Post time: Aug-28-2023