Electret microphones have a small volume, simple structure, good
electroacoustic performance, low cost, and are widely used.
Electret microphone, also known as an electret microphone,
consists of acoustic electric conversion and impedance
conversion. The electret diaphragm is a key component of
acoustic electric conversion. It is a very thin plastic film,
with one side being an evaporated metal film. Then, standing
behind a high-voltage electric field, opposite charges are
stationed on both sides. The vapor side of the diaphragm faces
outward and communicates with the metal casing. In electret
microphones, field-effect transistors are used for
preamplification, so a certain bias voltage is required for
normal operation of electret microphones, which generally does
not exceed 10V.
There are two connection methods between the electret microphone
and the circuit: active pole output and drain pole output. The
source output is similar to the emitter output of a transistor.
Three outgoing lines are required. The leakage pole d is
connected to the positive pole of the power supply. The resistor
Rs is connected between the source s and ground to provide
source voltage, and the signal is output from the source through
capacitor C. The braided wire is grounded to provide shielding.
The output impedance of the source output is less than 2k, the
circuit is 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 input of a transistor. Only two
outgoing lines are needed. The drain resistor RD is connected
between the drain d and the positive pole of the power supply,
and the signal is output from the drain d through capacitor C.
The source pole s is grounded together with the braided wire.
The drain output has a voltage gain, so the sensitivity of the
electret microphone is higher than that of the source output,
but the dynamic range of the circuit is slightly smaller.
The size of Rs and RD should be determined based on the power
supply voltage. It can generally be selected between 2.2-5.1k,
such as a power supply voltage of 6V, Rs of 4.7k, and RD of 2.
2k. In the output circuit, if the power supply is grounded to
the positive pole, d and s can still be used as source and drain
outputs. In the preamplifier stage of the sound control circuit,
there are two different connections between the source output
and drain output of the electret microphone. Finally, it should
be noted that whether it is the source output or the drain
output, the electret microphone must provide DC voltage in order
to operate, as it is equipped with FETs internally.
The working principle of an electret microphone is as follows:
when the electret diaphragm encounters acoustic vibration, the
distance between the electret diaphragm and the metal plate will
change. Due to the relatively small capacitance formed between
the electret diaphragm and the metal plate (usually in the tens
of wave method), its output impedance value is very high, about
tens of megaohms or more.
Such a high impedance cannot directly match the input of a
typical audio amplifier, so a junction field-effect transistor
is connected to the microphone for impedance conversion. The
voltage at both ends of the capacitor is taken out by a high
impedance field-effect transistor and amplified simultaneously
to obtain the output voltage signal corresponding to the sound
wave.
Electret microphones can convert sound signals into electrical
signals. It has small size, light weight, simple structure, wide
frequency response, high sensitivity, anti vibration and low
price, and is suitable for electronic devices such as recorders,
wireless microphones, and voice switches.
