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Old 28th April 2007, 08:48 PM   #1
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Default Question about microphone

I have seen the following circuit

http://www.zen22142.zen.co.uk/Circuits/rf/amtx.htm

Is it possible for anyone to explain the resistors R8 & R1.
This circuit is just an example, I have seen this sort of
arrangement on numerous circuits which use a microphone
for input.
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Old 29th April 2007, 01:41 AM   #2
bwaslo is offline bwaslo  United States
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Location: Cinciinati, OH
R8 works with C6 to filter the power supply for the microphone stage (so that it doesn't get messed up by fluctuations in the battery voltage when the other stage(s)' current varies). That's also known as "power supply decoupling", and the resistor and large capacitor allow the decoupling to be effective to lower frequencies.

R6 is the bias feed resistor for the FET in the electret microphone capsule. An electret capsule contains an internal FET, and that FET's current varies with the sound the capsule picks up. Since all the current to the FET comes through R6, then the varying current results in a varying voltage (V=I*R) that gets fed through C4 to drive Q2's input.
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Old 29th April 2007, 03:34 PM   #3
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Hello,

Can you explain about this technique (power supply
decoupling) how to calculate the values of the resistors
and capacitor.
Thanks in advance
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