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Electret microphone

Author:admin   AddTime:2021-03-16

The electret microphone is composed of two parts: acoustic-electric conversion and impedance conversion. The key element of the acoustic-electric conversion is the electret diaphragm. It is a very thin plastic film with a thin metal film evaporated on one side. Then after passing through the high-voltage electric field electret, there are charges of opposite sex on the two sides respectively. The vaporized gold surface of the diaphragm is outside and communicates with the metal shell. In the electret microphone, there is a field effect tube for pre-amplification, so when the electret microphone works normally, a certain bias voltage is required, and this bias voltage is generally not greater than 10v.

working principle

An electret microphone, also known as an electret microphone, consists of two parts: acoustic-electric conversion and impedance conversion. The key element of the acoustic-electric conversion is the electret diaphragm. It is a very thin plastic film with a pure gold film evaporated on one side. Then after passing through the high-voltage electric field electret, there are charges of opposite sex on the two sides respectively. The vaporized gold surface of the diaphragm is outside and communicates with the metal shell. The other side of the diaphragm is separated from the metal pole plate by a thin insulating liner. In this way, a capacitor is formed between the vaporized gold film and the metal plate. When the electret diaphragm encounters sound wave vibration, it causes the electric field at both ends of the capacitor to change, thereby generating an alternating voltage that changes with the sound wave. The capacitance between the electret diaphragm and the metal plate is relatively small, generally tens of pF. Therefore, its output impedance is very high (Xc=1/2~tfc), about tens of megaohms or more. Such a high impedance cannot be directly matched with an audio amplifier. Therefore, a junction field effect transistor is inserted into the microphone to perform impedance conversion. Field effect tubes are characterized by extremely high input impedance and low noise figure. Ordinary field effect transistors have three electrodes: the source (S), the gate (G) and the drain (D). What is used here is a special field effect tube that combines a diode between the internal source and the gate. The purpose of connecting the diode is to protect the FET when it is impacted by a strong signal. The grid of the field effect tube is connected to the metal plate. In this way, there are three output lines for electret microphones. That is, the source S, generally blue plastic wire, drain D, generally red plastic wire and braided shield wire connected to the metal shell.



Connection
Connection of electret microphone and circuit
There are two ways to connect the electret microphone to the circuit: source output and drain output. The source output is similar to the emitter output of a transistor. Three lead wires are required. Drain D is connected to the positive pole of the power supply. A resistor Rs is connected between the source S and the ground to provide the source voltage, and the signal is output from the source through the capacitor C. The braided wire is grounded for shielding. The output impedance of the source output is less than 2k, the circuit is relatively stable, and the dynamic range is large. But the output signal is smaller than the drain output. The drain output is similar to the common emitter of a transistor. Only two lead wires are required. The drain D is connected to the positive pole of the power supply by a drain resistance RD, and the signal is output from the drain D through the capacitor C. The source S is grounded together with the braided wire. The drain output has a voltage gain, so the microphone sensitivity is higher than the source output, but the dynamic range of the circuit is slightly smaller.
The size of Rs and RD should be determined according to the size of the power supply voltage. Generally, it can be selected between 2.2 and 5.1k. For example, when the power supply voltage is 6V, Rs is 4.7k and RD is 2.2k. In the output circuit, if the power supply is positive and grounded, you only need to swap D and S, and it can still be the source and drain output. There are two different connections for the source output and drain output of the electret microphone in the pre-amplification stage of a voice control circuit. Finally, it is necessary to explain that no matter it is the source output or the drain output, the electret microphone must provide DC Voltage can work because it has a field effect tube inside

Polarity discrimination
The electret microphone is small in size, simple in structure, good in electro-acoustic performance, low in price, and widely used. The internal structure of the electret microphone is shown in Figure 1. It is composed of two parts: an acoustic-electric conversion system and a field effect tube. There are two ways to connect its circuit: source output and drain output. The source output has three lead wires, the drain D is connected to the positive pole of the power supply, the source S is grounded through a resistor, and then a capacitor is used for signal output; the drain output has two lead wires, and the drain D is connected to the positive pole of the power supply through a resistor, and then through A capacitor is used for signal output, and the source S is directly grounded. Therefore, before using the electret microphone, the polarity must be determined first.
A diode is connected between the gate and the source of the field effect tube, so the forward and reverse resistance characteristics of the diode can be used to distinguish the drain D and the source S of the electret microphone.
Set the multimeter to R×1kΩ, connect the black test lead to either pole, and the red test lead to the other pole. Reverse the two test leads and compare the two measurement results. When the resistance value is low, the black test lead is connected to the source S, and the red test lead is connected to the drain D.

Condenser Microphone (Condenser Microphone) does not have a coil or magnet, and relies on the change of the distance between the two separators of the capacitor to produce voltage changes. When sound waves enter the microphone, the diaphragm vibrates, because the substrate is fixed, so that the distance between the diaphragm and the substrate will change with the vibration, according to the characteristics of the capacitor
(Is the partition area, is the partition distance). When the distance between the two spacers changes, the capacitance value will change. Pass again
(For the power, the capacitor plate voltage in a condenser microphone will maintain a fixed value) It can be seen that when it changes, it will cause a change in the power. Because the condenser microphone needs to maintain a fixed plate voltage, this type of microphone needs an additional power source to operate. The common power source is a battery or powered by a phantom power (Phantom Power). Condenser microphones are often used for high-quality recording due to their high sensitivity.
Electret Condenser Microphone (Electret Condenser Microphone) uses electret material that can retain a permanent charge, so there is no need to supply power to the capacitor. However, the general electret microphone assembly has built-in electronic circuits to amplify the signal, so it still needs to be powered by a low voltage (the normal voltage is 1.0V-10V). This type of microphone is widely used in consumer electronics

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