neelandan
Joined: 26 Mar 2004
Posts: 28
Helped: 1
Location: Terra Firma
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 21 Aug 2008 6:29 Re: condenser and electret mics |
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| Audioguru wrote: |
The "amplifier" transistor in the photo does not have a base bias resistor.
But then the original circuit doesn't either because the value of 4.7M is much too high. |
<Galileo> But it works!</>
For each clap, the microphone produces a decaying oscillatory waveform. If the transistor is in cutoff, it will be driven ON. If it is in saturation, it will get driven OFF. If it is in the active region, it will be driven between ON and OFF. With a sufficiently loud clap, of course.
Without a bias resistor, the leakage of the (electrolytic) coupling capacitor biases the transistor at the threshold of conduction. This particular circuit will work with a wide range of values for that bias resistor, or no bias resistor at all.
It will not, however, work with a low leakage input capacitor and NO bias resistor.
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