Current of Electret Microphone Capsules
An electret microphone capsule contains a built-in FET (Field Effect Transistor), which functions as an impedance converter and signal amplifier. Therefore, a DC bias voltage is required for operation, typically ranging between 1.5V and 6V.
The operating current of the capsule is determined by the FET and usually falls within 0.15 to 0.5 mA. The FET acts as a constant current source, meaning the current remains mostly stable even when the external voltage or resistor value changes. Thus, the microphone's operating current can be considered equal to the FET current.
The FET current is related to its transconductance (amplification gain) and internal impedance. Generally, within a certain range, a higher operating current corresponds to lower impedance, higher amplification capability, and greater microphone sensitivity.
Impedance of Electret Microphone Capsules
Manufacturers often mark electret microphone impedance as 2.2kΩ, but in reality, the impedance varies within a range, typically from 700 ohms to 3000 ohms. Measuring impedance with a multimeter is not accurate, as it only reads the FET's DC resistance.
The microphone's impedance affects not only its sensitivity but also the performance of the connected audio circuitry. To achieve maximum signal gain, the microphone's output impedance must match the input impedance of the amplifier or receiving device.
Noise in Electret Microphone Capsules
Noise in electret microphones consists of internal self-noise and external interference. Since the FET operates at a very low current, internal noise is minimal.
However, poor grounding of the microphone's metal housing or improper sealing are common causes of noise. In high-frequency devices such as mobile phones, if the contact resistance between the microphone shell and PCB exceeds 1 ohm, it can lead to high-frequency modulation interference-commonly heard as buzzing or mosquito-like sounds.
