Electret microphone elements contain an FET (Field Effect Transistor) that serves to convert impedance and amplify the signal. Therefore, the microphone requires a DC voltage ranging from 1.5 to 6V. The current value of the microphone normally depends on the FET current, typically between 0.15mA and 0.5mA.
The FET acts as a constant current source, meaning that the current value remains stable despite changes in the microphone's external voltage and resistance. Hence, the microphone's current value can be considered equivalent to the FET current. The FET current value is influenced by its transconductance (analogous to a transistor's gain) and its inherent impedance.
It is generally understood that within a certain range, a higher normal current value for the microphone corresponds to lower impedance, greater amplification capability, and higher microphone sensitivity.
Specified Impedance Value
The manufacturer's specified impedance value for microphones is typically 2.2kΩ. However, in practice, the microphone's impedance is a range rather than a fixed point value. It usually lies between 700Ω and 3000Ω.
Important note: Measuring the microphone's impedance using a multimeter is inaccurate, as it only measures the FET's DC resistance.
The microphone's impedance value not only affects its sensitivity but also plays a critical role in the performance of the electrical device using the microphone. This means that the microphone's output impedance must match the input impedance of the amplifier in the device to achieve maximum sound transmission gain.
