The working principle of electret microphone head

From electrostatics, we can see that for parallel plate capacitors, there is the following relationship: c=. s/l.

(1) That is to say, the capacitance is proportional to the dielectric constant of the medium, 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 into Q, the two plates of the capacitor must form a certain voltage, and the relationship is as follows: C=Q/V.

(2) For electret microphones, there is a capacitor made up of diaphragms, spacers and plates inside. Because the diaphragm is charged and is a plastic film, when the diaphragm is subjected to sound pressure, the diaphragm will vibrate, thereby changing the distance between the diaphragm and the plates, thereby changing the distance between the two plates of the capacitor, causing d to change. Therefore, the formula

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(1) A change in C is inevitably produced by the formula

(2) As we all know, due to the change of C, the charge is fixed, so the change of V is inevitable.

In this way, the conversion from sound signals to electrical signals has been preliminarily completed. Because the signal is very weak and the internal resistance is very high, it cannot be used directly, so impedance conversion and amplification are required.

Field effect transistor is a voltage control element. The output current of the drain is affected by the source and gate voltages. Since the two electrodes of the capacitor are connected to the source and drain of the field effect transistor, this is equivalent to adding a voltage difference between the source and drain of the field effect transistor. The drain current of the field effect transistor produces an incremental change. Therefore, the current change produces a Vd change on the resistor R1, and the voltage change can be output through the capacitor C0. The voltage change is caused by sound pressure, so the entire microphone completes an acoustic-electric conversion process.