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?

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[dpneber]

Part 5? Didn't see that until the video was over. Part 1 here I come! Thank you, both!

[SabrinaVideo]

09:24 This part is GOLD IN MY EYES. This microphones series are marvellous.

[hubercats]

Hi Dave - Thanks very much for taking the time to share your expertise through your numerous videos. In a correspondence with Alan (w2aew), I believe he mentioned that you have done work in the area of underwater acoustics. I spent about 45 years working on underwater acoustic signal processing algorithms. Now that I’m (mostly) retired, I’m trying to learn more about the hardware side of underwater acoustics. Recently I’ve been trying to come up with a 40 dB gain preamp for a hydrophone as related to some student mentoring I’m involved in and I’ve been focusing in on designs that employ JFET front ends. I’m curious to know if you’ve produced any videos that may be helpful to me. Thanks again! - Jim

[motokokusanagi]

Great series, Dave! Even though you say Mail Bag is everyone's favorite, practical electronics lectures are far more interesting and useful. That's why your channel is one of my favorite ones. I missed fundamentals videos so much)

[kevinbeckenham]

Doug Ford is a brilliant presenter and lecture; I look forward seeing Doug becoming a T.V presenter of a science program one day on T.V, can't wait for that to happen.

[enginerdy]

Thanks Dave, Doug, This series is absolutely amazing!
I would love to see more in depth / master-class type stuff on your channel. I'm learning some really useful stuff here.

[kludgeaudio]

The microphones that don't have leak resistors don't use a normal JFET, they use what the friendly people at Toshiba call a FET-IC.  They have parasitic diodes fabricated on the surface of the transistor in order to provide a leak resistor as well as a source bias resistor.  Sadly this does not improve linearity or noise performance, but it sure is cheap to make!

[joostjepek]

Hi Dave, I did a rerun off some your vid's, kinda missed this one.. But this set is one of your best guest appearances !!
So informative an clear on info! Fun to see the interaction of you both...

[AntonioBarba_TheKaneB]

Doug is *really* good at teaching, keep goin on!

[jjqformerlyjailbreak]

Had to be about 1 year ago I forced myself through this video. ATT I did t understand any of it. Now with a full year of research I can design my own circuit, test it and implement it into another circuit.
For those just now playing along at home. Keep it up. In the beginning it’s like learning a new language. Kinda cool.

[luistorh]

Thanks! I found the part of choosing the bias resistor for the JFET in cases of low supply very useful! Good advices!

[bain]

Very informative. Enjoyed it greatly Dave. I'm surprised they aren't more views than they are on this. Thanks for sharing these series of vids. 

[TheHeraldOfChange]

Would love to see you guys extend this series with a discussion on Piezo Electric Oscillators as microphones, especially since many people are using them for acoustic guitar pickups.

[EvelienSnel]

This explains exactly why the microphone in the cheap-ass headset I bought does not want to work with my semi-professional mixer. It gives me no output voltage and when measured, it likes to draw a fixed current of about 200 µA, no matter what voltage I apply. (Well, I'm glad I did not go higher than 10 V.)

Looks like I've got to build myself a small pre-amp.

No, I am not going to sing :) I just want to be able to talk to the musicians over the monitor channel in case of emergency.

[OtusAsio]

I like this very much...even if I will never build a microphone, it gives me knowledge about electronics...Thank`s

[gulassuppe]

Nice job Dave! Your vids are always educational and very entertaining!

[MrTridac]

I love the way you guys chat together. Hilarious :D

[vasileceteras]

Those cheap mics can be modded (or at least some of them) to get access to the  source pin and use the fet in common drain connection (repeater). Look up "Linkwitz Mod"

[was]

I don't get the jfet circuit. Wouldn't the microphone deliver a small AC signal to the gate that certainly is in cutoff region or does it also have a permanent bias voltage because of the charge?

[gustavlicht]

If you have current modulation per pressure, then why not use a transimpendance amplifier?