It is known from electrostatics that for parallel plate capacitors, there is the following relationship: C = ε. S/L ......① is the capacitance of the capacitor 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 with a Q amount of charge, then the two plates of the capacitor must form a certain voltage, which has the following relationship: C=Q/V ......②
For an electret microphone, there is a capacitor consisting of a diaphragm, a gasket and a plate. Because the diaphragm is filled with electric charge and is a plastic film, when the diaphragm is subjected to sound pressure, the diaphragm To generate vibration, which changes the distance between the diaphragm and the plate, thereby changing the distance between the two plates of the capacitor, resulting in a change in Δd, so from equation ①, it is necessary to produce a change in ΔC. It is also known from equation ② that the charge charge is fixed again due to the change of ΔC, so a change of ΔV is inevitably generated.
This initially completes a conversion from an acoustic signal to an electrical signal.
Since this signal is very weak, the internal resistance is very high and cannot be used directly, so impedance transformation and amplification are also performed.
The FET is a voltage control element whose output current is controlled by the source and gate voltages.
Since the two poles of the capacitor are connected to the S and G poles of the FET, a change of ΔV is added between the S pole and the G pole of the FET, and the drain current I of the FET generates a change quantity of ΔID, so the amount of change in this current produces a change in ΔVD on the resistor RL. The amount of change in this voltage can be output through the capacitor C0. The amount of change in this voltage is caused by the sound pressure, so a conversion process of sound and power of the entire microphone head is completed. .
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