1. Working Principle Diagram of the Microphone Capsule
Below is the schematic diagram of the working principle of an electret condenser microphone capsule:
Electret Condenser Microphone Working Principle Diagram
- FET (Field Effect Transistor): The key component in MIC, responsible for impedance conversion.
- C: A capacitor whose capacitance changes with the vibration of the diaphragm. This is the main component for converting sound into an electrical signal.
- C1, C2: Capacitors used to suppress radio frequency interference, each targeting different frequency bands.
- RL (Load Resistor): Determines the sensitivity of the microphone. Larger resistance leads to higher sensitivity.
- VS (Supply Voltage): Provides working voltage for the MIC.
- C0 (DC Blocking Capacitor): Used at the signal output terminal.
2. Operating Principle of Electret Condenser Microphone
One side of the microphone diaphragm is coated with a thin layer of metal and placed opposite to the metal of the electret plate, separated by an insulating sheet. These two parallel metal plates with a dielectric between them form a capacitor.
According to electrostatics, the capacitance of a parallel-plate capacitor can be expressed as:
C = ε × S / L ......(1)
Where:
ε = dielectric constant of the material,
S = surface area of the diaphragm and electret plate,
L = distance between diaphragm and electret plate.
Therefore, the capacitance is directly proportional to ε and S, and inversely proportional to L.
Additionally, capacitance also relates to charge (Q) and voltage (V) as:
C = Q / V ......(2)
From equations (1) and (2), we get:
ε × S / L = Q / V ......(3)
For a specific electret condenser microphone, ε, S, and Q are constant (Q is determined during the charging process). When sound pressure acts on the 2μm-thick diaphragm, the diaphragm vibrates and causes a variation Δd in the distance L. According to equation (3), this results in a corresponding change in voltage ΔV, thus converting sound into an electrical signal.
Since this signal is extremely weak and has high internal impedance, it cannot be used directly. Therefore, impedance conversion is required.
The FET (field effect transistor) is a voltage-controlled device. The output current at the drain terminal is controlled by the voltage between the gate and source terminals.
The two terminals of the capacitor are connected to the FET's source (S) and gate (G). When there is a change ΔV between S and G, it causes a change ΔID in the drain current. This change in current produces a voltage change ΔVD across the load resistor RL. The voltage change can then be output through capacitor C0. Since this voltage change is caused by sound pressure, the microphone achieves the complete conversion of sound to an electrical signal.
