Electret mic output

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What does the output of an electret mic look like? Assume it will be configured as shown in this first schematic. Is it centered around VCC/2 and varies above and below according to the sound? Is no input = 0V output? I'm asking because I need to sense just the amplitude of the sound and I was wondering if I would need to take into account negative voltages (rectify it?) or a constant bias. I want to sense when the amplitude goes above a certain threshold. Of course, even if a negative output or biased output is made, the level would go above the threshold amplitude within half a wavelength, but I was really just wondering. Thanks![/url]

Math is cool.
jevinskie.com

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Usually, the output is the open drain of a junction FET biased in the linear range. So there will be a small AC component on top of an unspecified DC offset, hence the blocking cap in the diagrams you link to. You will have to amplify the AC component before you measure it. The size of the AC component is a bit hard to say, as the data sheets quote it in terms like "-50dB relative to 0dB=1V/Pa". What that means is that at a sound pressure level equivalent to 1 Pascal, the output is 50dB below 1V, or 10 microvolts. One Pascal isn't much - it's 1/7000 PSI. I have no sense of how much loudness that is - sorry! - but in my experience, average speech levels give an output in the order of 10mV.

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Right.

DC level is unspecified. It may (will?) change with temperature. Often, you have to supply the pullup resistor. So, the mic, itself, looks rather like a current source with the current varying around some average level.

Yes, you need gain like peret says, and, yes, you need a capacitor. The load impedance, beyond the pullup, needs to be fairly large, otherwise you loose signal.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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This is what the microphone typically looks like inside. The junction FET needs a negative voltage on the gate to turn it off, so when you supply voltage through external resistor R3, enough current flows in R2 to develop about minus half a volt between gate and source. Since no more and no less is needed, this current stays constant over a wide range of supply voltages. The voltage gain of the FET in this mode is simply R3/R2 so all other things being equal, you will get about 10 times the output with a 10k R3 than with a 1k. The limiting (high) value for R3 is that you want a minimum voltage drop of 0.75 to 1 volt across the microphone, which will usually draw in the order of 300 microamps; 10k usually works with a 5V supply.

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As usual, wikipedia answers the question of sound pressure level:

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

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One Pa is not much pressure, but as a sound pressure that is 94 dB, or so loud that you want to protect your ears.