EEVblog #611 – Electret Microphone Design

Part 5 – Doug Ford explains the design of Electret Microphone circuits, and all the associated traps:
Transconductance, correct biasing, and temperature effects, leaky JFETS, non-linear clipping & headroom, current noise and voltage noise, and pseudo bias resistors.
Also, the non-intuitive concept of higher resistance = less noise, and how this applies to electret biasing.
And what is a soggy circuit board and why is it important?

Forum HERE

About EEVblog

Check Also

EEVblog #1334 – Mystery Dumpster Teardown

Mystery dumpster teardown time! With the most amazing mechanical mains power switch you’ll ever see! ...


  1. Thank you Doug and Dave for bringing this educational and entertaining series of talks. Doug’s understanding and appreciation of the “physics of the real world” and how to make electronic circuits that make the best use of what’s there, is inspirational.

  2. To add to the thoroughness of the discussion, I have some comments on this part.

    First, biasing the jFET to 0v gate-to-source is usually not done in circuits in which the gate voltage varies more. This is because when the gate-to-source voltage exceeds about 0.6 volts (one diode drop), the gate-source pn junction becomes a forward-biased diode, and you don’t get very good audio quality that way! I’m guessing that in an electret mic capsule, the maximum excursion of the diaphragm does not result in larger than a +0.6 volt potential on the gate, so this is never a problem.

    Second, asymmetry of the waveform is caused by the Vgs vs. Id transfer characteristic of the jFET, which is similar to that of a triode vacuum tube. (This is especially true if running on a low drain voltage, as with a low battery.) One thing microphone manufacturers never seem to talk about is that many simple preamp circuits in microphones add some harmonic distortion. This may be considered an enhancement and part of the microphone’s “character”, but if you want a really accurate sound, you need a design involving negative feedback.

    Aside from these small niggles, I’ve been enjoying the series very much, and thanks!

  3. I’m all micro phoned out now, it’s all starting to meld into to one.

  4. Hello Dave,
    The whole microphone series has been fascinating! Who’d thought so much would go into the design and theory behind the microphone? And also thanks to Doug Ford for taking the time in doing this.
    All the best,


  5. Wow, how many parts are there?

  6. I took appart a cheap electec mic and indeed did not find a resistor.

    What I did find is a K596 jfet, from some chinese manufacturer.
    I looked at the datasheet and found that the resistor is part of the jfet itself.

  7. Hi Dave, will the topic of dielectric absorption as it applies to DC blocking capacitor selection for ‘sound quality’ be discussed?

Leave a Reply

Your email address will not be published. Required fields are marked *