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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.

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2 Answers 2

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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\$ Mar 11, 2015 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, 2015 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, 2015 at 9:13
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Usually the output dynamic impedance in electret microphones are mentioned in datasheets regardless of the biasing resistor Rd. So the justification of Andy aka doesn't seem convincing "for me". But 100% it's not rd (drain resistance) because it's normally high, where the electret microphone impedance is typically 2k ohm or less. So how can one justify that? I search for that but the resources satisficing answers are so scarce. The only justification I found is that there is a shunt capacitor (Cs) usually added inside the electret mic between JFET drain and ground for RF decoupling usually(10-100nf) so: Mic(ro)=rd//(1/WCs) measured @1kh i.e 1/WCs=(1.6-16)kohm @1khz and (.16-1.6)khz @ 10khz .So for precise design consider the output impedance seen by preamplifier as the (mic datasheet impedance)//(biasing resistor). I think the following link is helpful: https://www.ti.com/lit/ug/tidu765/tidu765.pdf?ts=1612331325542&ref_url=https%253A%252F%252Fwww.google.com%252F

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