# electret microphone dynamic impedence

how much is the dynamic impedance/resistance seen from a electret microphone? I am trying to design a microphone preamp and need to know seen resistance in small signal from the signal source (microphone in this case). This is very important because I should match poles and zeros so they will disable each other.

## 1 Answer

Pretty much the normal rule is that the output impedance is the bias resistor. This is because the output is usually a jfet's drain and in normal bias configurations for a jfet it operates as a high compliance output hence output impedance is largely dictated by the external bias resistor.

• That's my understanding, and the low pole set by the AC coupling Cap. – George Herold Mar 11 '15 at 0:30
• The seen resistance from the drain is R_d || (rd+R_s(1+u)) where rd is Va/Id and u is gm*Vgs. According to [wikipedia](en.wikipedia.org/wiki/Electret_microphone) inside an electret microphone there is no resistor in source and then seen resistance from the stage must be R_bias || rd . As we don't know how much Va here, we can't calculate rd – Nixmd Mar 11 '15 at 6:45
• @Nixmd you can assume the jfet will be acting in the constant current part of its curves – Andy aka Mar 11 '15 at 9:13