The electret microphone consists of two parts: acoustic and electrical conversion and impedance transformation. The key component of acoustic-electric conversion is the electret diaphragm. It is a very thin plastic film with a pure gold film on one side. Then, after passing through the high-voltage electric field, the opposite sides are respectively charged with opposite charges. The vaporized gold of the diaphragm faces outward and communicates with the metal casing. The other side of the diaphragm is separated from the metal plate by a thin insulating collar. Thus, a capacitor is formed between the vaporized gold film and the metal plate. When the electret diaphragm encounters acoustic vibration, the electric field across the capacitor changes, result in an alternating voltage that varies with the change in sound waves. The capacitance between the electret film and the metal plate is relatively small, generally several tens of pF. Therefore, its output impedance value is very high (Xc = 1/2 ~ tfc), about tens of mega ohms or more. This high impedance cannot be directly matched to the audio amplifier. Therefore, a junction field effect transistor is connected to the microphone for impedance transformation. The FET is characterized by extremely high input impedance and low noise figure. The common field effect transistor has three poles: the source (S), the gate (G) and the drain (D). Here is a dedicated FET that recombines a diode between the internal source and the gate. The purpose of the diode is to protect the FET when it is subjected to a strong signal. The gate of the FET is connected to the metal plate. Thus, there are three output lines for the electret microphone. That is, the source S, generally with a blue plastic wire, a drain D, generally with a red plastic wire and a braided shield wire connecting the metal casing.
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