Electret microphones characteristics

[autir]

Quoting from http://www.epanorama.net/circuits/microphone_powering.html:

Basic electret microphone powering circuits

Basic circuit

        +---------------------------- battery +ve (3 to 12 Volts)
        |
      2k2 R1
        |
        o---------- 10uF ------o----- output
        |+                    |
    CAPSULE                  10k R2
        |-                    |
        +----------------------o----- GND, and battery -ve

This is the basic electret microphone powering circuit which you can use as generic reference when receivign circuits which use electret microphones. The putput impedance is determined by R1 and R2. If you leave out R2 the output impedance is roughly the resistance of R2.

-> How exactly is Zout calculated?
-> What is the internal Z of a typical electret capsule?
-> What is a typical Vp-p value produced from such a circuit?

Thank you.

[audioguru]

Hi Autir,
I think the circuit's R1 should be adjusted to match the supply voltage. The FET inside an electret mic draws about 500uA, so in order to have half the supply voltage across it then for a 3V supply, R1 should be about 3k ohms, for a 9V supply R1 should be about 9k ohms and for a 12V supply R1 should be about 12k ohms.

-> How exactly is Zout calculated?

It is best to measure it because the FET would be an extremely high impedance if it had more current and voltage, but in an electret mic circuit it is "starved". Every FET is different. Its Zout is in parallel with its load/supply resistor (R1 in your circuit).

-> What is the internal Z of a typical electret capsule?

They are all different. I measured one to be about 4.3k ohms when it was powered by 9V through a 10k resistor. Therefore the output impedance of the circuit was about 3k ohms.

-> What is a typical Vp-p value produced from such a circuit?

About 5mV to 10mV RMS which is 14mV to 28mV p-p, with a normal voice level at about 20cm away.
It is less with a lower supply voltage and/or less load/supply resistance.

I had a job installing and repairing large intercom systems. Suddenly all new systems had acoustical feedback and needed the gain reduced in the mic preamps. The manufacturer didn't know but was supplying electret microphones that were the same as before but with much more sensitivity. ;D

[autir]

I think the circuit's R1 should be adjusted to match the supply voltage. The FET inside an electret mic draws about 500uA, so in order to have half the supply voltage across it then for a 3V supply, R1 should be about 3k ohms, for a 9V supply R1 should be about 9k ohms and for a 12V supply R1 should be about 12k ohms.

Can you explain this a little bit more?  :-[  ;D

It is best to measure it

How? As a resistor?

Its Zout is in parallel with its load/supply resistor (R1 in your circuit).

Isn't Zout=R2//R1//Zmic ?

[audioguru]

Can you explain this a little bit more?

Ohm's Law says that 1.5V/500uA is 3k ohms, 4.5V/500uA is 9k ohms and 6V/500uA is 12k ohms.

How? As a resistor?

I injected a 100mV sine-wave through a variable resistor in series with a capacitor to the junction of the electret mic and its load/supply resistor. I measured the voltage at the junction and changed the variable resistance until the voltage measured half of the original 100mV. Then I measured the resistance of the variable resistor which equaled the impedance of the mic in parallel with its load/supply resistor. They made a voltage divider.

Isn't Zout=R2//R1//Zmic ?

You are correct, I forgot about R2 since it is usually not such a low value. :-[ ;D