An electret is a stable dielectric material with a permanently embedded static electric charge
(which, due to the high resistance and chemical stability of the material, will not decay for hundreds of years).
Electret microphones are very common in personal electronics due to their small size, excellent frequency
response, and reasonable cost. An "electret" is a thin, Teflon-like material with a fixed charge bonded to
its surface. The electret is housed between two electrodes, and the structure forms a capacitor which
contains a fixed charge. Air pressure variations (sound waves) move one of the electrodes of the capacitor
back and forth, changing the distance between the two electrodes, and modulating the capacitance of the
structure. Because the charge on the microphone is fixed, varying the capacitance causes the voltage on
the capacitor to also change, satisfying the equation:
Q = C * V
Where Q is charge, C is capacitance, and V is voltage. Therefore the microphone capacitor acts as an AC coupled voltage source. Because the charge on the microphone capacitor must be fixed, the amplifier circuitry directly in contact with it must have extremely high input impedance. Most electret microphones have an internal JFET which buffers the microphone capacitor. The voltage signal produced by sound modulates the gate voltage of the JFET, labeled VG in Figure 2 causing a change in the current flowing between the drain and source of the JFET (IMIC). An extremely high resistance, RG, may be included to bias the gate of the JFET.
Rpull-up is an external resistor, R5 on the image below.
Signal from the microphone is applied to C4 1.5μF capacitor. R3 - R4 resistor network adds a DC shift to the signal from the microphone. The resultant signal is applied to the non-inverting input of the U1 operational amplifier.OPA364: