According to electrostatics, for parallel plate capacitors, the following formula is obtained: C= e S/4 PI KD... (1) The capacitance is proportional to the dielectric constant of the dielectric, 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 a charge of Q, then the two plates of the capacitor will form a certain voltage, there is the following relationship: C = Q / V... (2)
For an electret probe, there is a capacitor consisting of a diaphragm, a gasket and a plate. Because the diaphragm is charged and is a plastic film, the diaphragm vibrates when the diaphragm is subjected to sound pressure, thus changing the distance between the diaphragm and the plate, thus changing the two capacitors. The distance between the plates produces a change of d, so from Formula 1 we can see that a change of C will occur, and from formula 2 we can see that the charge is fixed because of the change of C, so a change of V will occur.
In this way, a conversion from acoustic signal to electrical signal is initially completed.
Because the signal is weak and has high internal resistance, it can not be used directly, so the impedance transformation and amplification must be carried out.
FET FET is a voltage control element. The output current of drain is controlled by the voltage of source and gate.
Since the two poles of the capacitor are connected to the S and G poles of the FET, a variation of V is added between the S and G poles of the FET, and the drain current I of the FET produces a variation of ID, so the variation of the current produces a variation of VD on the resistance RL, and the variation of the voltage can pass through. Capacitor C0 output, this voltage change is caused by the sound pressure, so the whole microphone completed a sound-to-electricity conversion process.