According to electrostatics, for parallel plate capacitors, there is the following formula: C = ε s / 4 π KD ① That is, the capacitance is directly proportional to the dielectric constant of the medium, directly proportional to the area of the two plates, and inversely proportional to the distance between the two plates.
　　In addition, when a capacitor is charged with Q amount of charge, the two plates of the capacitor must form a certain voltage, which has the following relationship: C = q / V II.
　　For an electret microphone, there is a capacitor composed of diaphragm, gasket and plate. Because the diaphragm is charged and is a plastic film, when the diaphragm is subjected to acoustic pressure, the diaphragm will vibrate, thus changing the distance between the diaphragm and plate, thus changing the distance between the two plates of the capacitor, resulting in a change of Δ D Therefore, it can be seen from Formula 1 that there must be a change of Δ C, and it can be seen from formula 2 that because of the change of Δ C, the charging charge is fixed, so there must be a change of Δ v.
　　In this way, a conversion from acoustic signal to electrical signal is preliminarily completed.
　　Because the signal is very weak and the internal resistance is very high, it can't be used directly, so we have to carry out impedance transformation and amplification.
　　The FET is a voltage control element, and the output current of the drain is controlled by the source and gate voltages.
　　Because the two poles of the capacitor are connected to the S pole and G pole of the FET, it is equivalent to adding a Delta V change between the S pole and G pole of the FET, and the drain current I of the FET will produce a delta ID change. Therefore, the current change will produce a delta VD change on the resistance RL, and the voltage change can be output through the capacitor C0, and the voltage change is controlled by the sound Because of the pressure, the whole microphone completes an acousto-electric conversion process.