In using an electret mic, I understand that you need to supply it some voltage for it to operate, and that you connect a coupling capacitor at its output to remove the DC biasing component of the signal.

My question is, what would the output be when you don't use a coupling capacitor? Would it be an AC signal with a DC offset when the mic detects a sound? And when it does not detect a sound, is the voltage at the output equal to the supply voltage?


The DC content of the output will remain the same with or without the presence of any acoustic stimulus. Inside the microphone is a JFET (usually) that takes a constant DC bias current: -

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This means that the DC voltage dropped across the external resistor (up to V+) is also a constant DC voltage. When sound is received, the constant DC current is "modulated" by the sound and this superimposes an alternating signal voltage across the standing DC voltage on the external resistor.

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  • \$\begingroup\$ So when no sound is received, the voltage across the drain and source is just a dc voltage? If so, how does one calculate Vds (voltage across drain and source) when you don't know the current through the resistor (which is the drain current)? \$\endgroup\$ – user128233 Oct 29 '16 at 10:03
  • \$\begingroup\$ Correct. Why would you want to calculate the voltage BTW - it has no effect on any AC coupled amplifier that follows. \$\endgroup\$ – Andy aka Oct 29 '16 at 10:06
  • \$\begingroup\$ Ah my bad. You see, I'm trying to use an electret mic to switch a transistor whose collector pin is connected to the trigger pin of a 555. Right now, I'm still thinking on how to use that mic to switch a transistor. Now, considering what you said about the drain voltage having the same dc voltage regardless of picking up a sound, I am forced to use a coupling capacitor to filter out that dc component. However, now I need to figure out a way to turn on the transistor when the filtered output is just in the mV range. Do you have any suggestions? Thank you btw for answering! \$\endgroup\$ – user128233 Oct 29 '16 at 10:26
  • \$\begingroup\$ See my answer here: electronics.stackexchange.com/questions/65891/… - instead of the LED you can use a pull up resistor and attach the collector to the 555. This is a circuit that I used many years ago in a telecom product. \$\endgroup\$ – Andy aka Oct 29 '16 at 11:04
  • \$\begingroup\$ If you are trying to detect sound (as per your subsequent clap question), you might find that the circuit I linked avoids using the 555 and can directly interface to a flip flop. \$\endgroup\$ – Andy aka Oct 29 '16 at 11:38

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