I'm trying to understand how mobile phone headsets work. I figured out that the electret mic gets supplied with 2.7V by the phone.

Opening a headset revealed the following PCB: enter image description here

I looked and measured and came up with the following circuit: enter image description here

Shouldn't there be at least one cap in series with the mic?


2 Answers 2


Electret microphones need a biasing voltage to operate: The 2.7 Volts you measured.

If there is a capacitor in series with the microphone, this would block DC, thus there would be no biasing voltage.

If the preamplifier / buffer inside the phone body needs the bias voltage removed, a capacitor would be placed in that part of the circuit, past the bias voltage provision stage, i.e. on the PCB inside the mobile phone handset.


The diode is there to clamp the voltage to 2.7V, it's a zener. The two caps are bypassing any AC on the line, one at low freq, the other at high freq.

  • 3
    \$\begingroup\$ You want the AC on the line - it's the microphone signal! \$\endgroup\$ Apr 27, 2013 at 22:54
  • \$\begingroup\$ @ConnorWolf - The caps could be removing non-audio frequencies. \$\endgroup\$
    – Joel B
    Mar 31, 2014 at 19:59

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