According to electrostatics, for parallel plate capacitors, there is the following relationship: C=εS/4πkd...①That is, the capacity of the capacitor is proportional to the dielectric constant of the medium, proportional to the area of ​​the two plates, and The distance between the two plates is inversely proportional.

In addition, when a capacitor is charged with the amount of charge of Q, then the two plates of the capacitor must form a certain voltage. The relationship is as follows: C=Q/V ……②

For an electret microphone, there is a capacitor composed of a diaphragm, a gasket and a plate inside, because the diaphragm is charged with electric charge and is a plastic film, so when the diaphragm is subjected to sound pressure , The diaphragm needs to vibrate, which changes the distance between the diaphragm and the plates, and thus changes the distance between the two plates of the capacitor, resulting in a change of Δd. Therefore, from the formula ①, it is necessary to produce a The change of ΔC is also known from the formula ②. Because of the change of ΔC, the charging charge is fixed and unchanging, so a change of ΔV will inevitably occur.

This preliminarily completes a conversion from acoustic signals to electrical signals.

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Because this signal is very weak and the internal resistance is very high, it cannot be used directly, so impedance transformation and amplification are required.

The FET field effect tube is a voltage control element, and the output current of the drain is controlled by the voltage of the source and the gate.

Since the two poles of the capacitor are connected to the S pole and the G pole of the FET, it is equivalent to adding a change of Δv between the S pole and the G pole of the FET, and the drain current I of the FET generates a The change of ΔID, therefore, the change of this current produces a change of ΔVD on the resistance RL. This change of voltage can be output through the capacitor C0. This change of voltage is caused by sound pressure, so the entire microphone The head completes a sound-to-electric conversion process.