# Electret condenser bias voltage

I have an electret condenser from PUI Audio (https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf). The Vs is recommended to be 3V but my circuit and only supply maximum of 2.5V.

The suggested RL value is 2.2kOhm, but we are using 3kohm instead because it works better when we tested it (no sound clipping). From what I read, the electret condenser has an open drain FET inside of it. How do I figure out what the minimum voltage Vs could be for this to still be functioning correctly? I'm just unsure whether using 2.5V is sufficient when the recommended value is 3V.

The output impedance at 1kHz is 2.2kOhm. Does that mean it's the maximum impedance or is it just what the impedance is at 1kHz and that varies depending on the frequency? How would I know what the maximum impedance would be? At 1kHz, I assume the RL = 2.2kohm just forms a voltage divider of 1/2 ; therefore, the voltage at the output would just be Vs/2. So I should choose the RL such that the output voltage would be within the range of what my ADC requires for the input voltage.

The spec sheet indicates that the microphone will operate from VS = 1V to 10V. You can operate at 2.5V but expect a reduction in maximum output voltage level. While not clarified in the specifications, the RL = 2K probably is the output impedance seen at between output and ground terminals. Be sure that VS is a very clean voltage. Any fluctuations here will be heard. Include a large decoupling capacitor from the supply end of the resistor to ground. Keep the area of the dc loop small.

There is no dc output from the microphone. With RL = 2k and VS = 3V the microphone draws 500 microamps. Since this usually is fixed by the gate bias inside the microphone, it should be reasonably insensitive to VS and RL variations, but don't take my word for it. Make some measurements.

The ceramic cartridge does not turn the gate on and off, but definitely modulates the gate bias voltage to control the drain current.

The FET operates by varying the current draw according to the ac signal on its gate. The ac voltage signal is created by the changing current in RL. If RL is larger the amplitude of the signal will be greater, but may result in clipping for quieter signals.

The specs say that the output level is reduced -3dB if the supply voltage is reduced from 3.0V to 2.0V and obviously the clipping sound level is also reduced.

The output of a microphone is usually AC, not DC. The DC output from the microphone will have a range of DC voltages since the "500uA" current is not regulated.

• I should add that the the ADC has an input specs of 0.7 Vrms, so it's expecting an AC input signal. You mention 500 uA (0.5 mA) current and that's the current through the RL because of the FET as the microphone turns on and off the base/gate via the sound pressure differentials? Does the FET just try to maintain the current and vary the voltage going to the output? Commented Sep 17, 2022 at 18:29