An electret microphone, also known as an electret microphone, consists of two parts: acoustic electric conversion and impedance conversion. The key component of acoustic electric conversion is the electret vibration film. It is an extremely thin plastic film, with a layer of pure gold film evaporated on one side. Then, after being polarized by a high-voltage electric field, opposite charges are placed on both sides. The vapor surface of the membrane is facing outward and connected to the metal shell. The other side of the membrane is separated from the metal electrode plate by a thin insulating ring. In this way, a capacitor is formed between the gold vapor film and the metal electrode plate. When the electret diaphragm encounters sound wave vibration, it causes a change in the electric field at both ends of the capacitor, resulting in an alternating voltage that varies with the sound wave. The capacitance between the electret membrane and the metal electrode plate is relatively small, usually several tens of pF. Therefore, its output impedance value is very high (Xc=1/2~tfc), about tens of megaohms or more. Such high impedance cannot be directly matched with an audio amplifier. So, a junction field-effect transistor is inserted into the microphone for impedance conversion. The characteristics of field-effect transistors are extremely high input impedance and low noise figure. A common field-effect transistor has three electrodes: active (S), gate (G), and drain (D). Here, a dedicated field-effect transistor is used to composite a diode between the internal source and gate. The purpose of connecting diodes is to provide protection for field-effect transistors when subjected to strong signal shocks. The gate of a field-effect transistor is connected to a metal electrode plate. In this way, the output lines of the electret microphone have three. The source electrode S is usually made of blue plastic wire, the drain electrode D is made of red plastic wire, and a woven shielding wire connecting the metal shell is generally used.
