# Electret Microphone

Discussion in 'General Electronics Chat' started by jaydnul, Apr 8, 2015.

1. ### jaydnul Thread Starter Member

Apr 2, 2015
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Does the actual microphone capsule (connected to the transistor gate and ground) produce an AC voltage? Do the voltages go negative or is the minimum voltage zero? I'm having a hard time understanding how this circuit would work if the voltages did go negative... the transistor would be in the exact same state if the voltage was negative OR zero, right?

2. ### JohnInTX Moderator

Jun 26, 2012
2,394
1,051
The short answer is that you get an AC signal swinging around some baseline (how its determined is not shown in the diagram) corresponding to the sound waves on the mic.
This document in the Wiki article that the diagram came from has all the detailed info you need.
Have fun.

3. ### jaydnul Thread Starter Member

Apr 2, 2015
90
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For some reason I can't get that specific pdf to load, but I think I get the basics. The microphone has a constant static charge, and that sets the baseline.

Considering the voltage on the gate is always positive, a BJT wouldn't work for this application, because the charge would eventually leave the microphone completely, right? With a FET, no charge is leaving.

4. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
2,845
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It's not so much that a BJT would 'kill' the electret as it is that the low (b-e) impedance would excessively load/attenuate the signal...

Best regards
HP

5. ### jaydnul Thread Starter Member

Apr 2, 2015
90
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I see, makes sense now. One last question now that we're sort of on the topic. Do most audio signals start at a baseline like this and only produce positive voltages that vary in magnitude?

6. ### Sinus23 Member

Sep 7, 2013
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No most audio signals swings both ways Like a pendulum. Like a speaker cone from + to - . There is if the DC offset isn't stronger in amplitude than the AC signal.

Edit:Typo or two

Last edited: Apr 9, 2015
Hypatia's Protege likes this.
7. ### Hypatia's Protege Distinguished Member

Mar 1, 2015
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I concur with Sinus23 (post #6) -- Note that this is accomplished, in 'single-ended' Amps, via reactive coupling (CIP magnetic transformers or capacitors) thus it is that the signal's 'DC component' is blocked or 'filtered off', if you will...

As a point of interest, please note that certain microphone technologies (two examples being Piezo and Magnetic) produce AC output signals...

Best Regards
HP

Last edited: Apr 9, 2015
8. ### Sinus23 Member

Sep 7, 2013
178
465
Even though it's just the wikipedia page it's a good start and from it you should find keywords to google for about the subject of transistor biasing.

http://en.wikipedia.org/wiki/Biasing

9. ### jaydnul Thread Starter Member

Apr 2, 2015
90
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Perfect. Thanks guys

Jul 31, 2013
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The representation of the components inside the dotted line could possibly be incomplete. Anyway there will be a standing dc current through the FET and consequently a fixed dc on the capsule output. The actual level will be set by the choice of the external resistance shown. However the variation (ac component) in the voltage caused by sound on the mic element will be small _ I'm guessing 20mv max so the bias DC level can be quite small and the capsule will work OK. The "output" will be AC but may have a different DC offset depending on what it is to be connected to. The polarity of the capacitor shown in the diagram may not necessarily be correct.

Last edited: Apr 10, 2015
11. ### racarrasco New Member

Dec 5, 2015
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So I was wondering, if I have the same resistor connected to the ground lead, and the output also on the ground lead, have I just created a follower? Because right now this is how I have it configured and I still see a signal on the oscilloscope.