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.


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.

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  • \$\begingroup\$ That's my understanding, and the low pole set by the AC coupling Cap. \$\endgroup\$ – George Herold Mar 11 '15 at 0:30
  • \$\begingroup\$ 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 \$\endgroup\$ – Nixmd Mar 11 '15 at 6:45
  • \$\begingroup\$ @Nixmd you can assume the jfet will be acting in the constant current part of its curves \$\endgroup\$ – Andy aka Mar 11 '15 at 9:13

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