The electret microphone has the characteristics of small size, simple structure, good electroacoustic performance, and low price. It is widely used in circuits such as cassette recorders, wireless microphones and voice control.
The electret microphone is composed of two parts: acoustic-electric conversion and impedance conversion. Its internal structure is shown in Figure 1. 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 of the diaphragm
One side is separated from the metal pole plate with a thin insulating lining ring. 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 value 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 with a diode compound between the internal source and the gate, as shown in Figure 2. 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.
There are two ways to connect the electret microphone to the circuit: source output and drain output, as shown in Figure 3. The source output is similar to the emitter output of a transistor. Three lead wires are required. Drain D is connected to the positive terminal 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 of Figure 3, if the power supply is positive and grounded, just swap D and S, and it can still be source and drain output. Figure 4 shows two different connections of the source output and drain output of an electret microphone in a preamplifier stage of a voice control circuit. Finally, we must explain one point, whether it is source output or drain output, electret microphone Must provide DC voltage to work, because it is equipped with a field effect tube.