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I come again to get help from the experts.

I am using this BJT transistor in the circuit.

http://www.fairchildsemi.com/ds/2N/2N3904.pdf

Here is the microphone that I am hooking up in the circuit.

It's an electret condenser microphone from digikey.

http://www.cui.com/product/resource/cma-4544pf-w.pdf

Here is my design

I am following the tutorial from this link to build the amplifier. It's exactly the same circuit but with different values to make it work with microcontroller. For that, I have used the supply voltage of 5V instead of 9V as in the tutorial. I have also tweaked the values of resistors to make the correct biased voltage.

http://www.youtube.com/watch?v=Br3uOM0IVjc

enter image description here

The problem is that I get a lot of noise. I checked with oscillosope and it is biased correctly, i.e., midway between the 5V. But you can see a very noisy signal. I hooked the circuit to a computer speaker and you can hear the noise after you hook up the circuit. The output works correctly though, i.e, when you play a sound in the microphone you can hear clearly in the speaker but during idle there is a noise that is audible.

What is the reason of this noise and how to minimize it while the microphone is sitting idle? Thank you for the help.

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  • \$\begingroup\$ What are you powering the circuit from? Have you added a decoupling capacitor across the circuit? \$\endgroup\$ – JIm Dearden Jun 26 '13 at 23:52
  • \$\begingroup\$ i am powering the circuit directly from 5v arduino pins. is that a problem? \$\endgroup\$ – user734861 Jun 27 '13 at 0:30
  • \$\begingroup\$ could be - I'd be looking to power a circuit from the power supply rather than a pin output. \$\endgroup\$ – JIm Dearden Jun 27 '13 at 0:34
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The noise is very unlikely to be coming from any of the components in the circuit. The biggest culprit is likely to be the power supply. Add a 22uF and 0.1 uF (helps with high frequencies) capacitor and a 100 ohm resistor that feed the electret part of the circuit and a 22uF across the 5V supply. enter image description here

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  • \$\begingroup\$ worth giving +1 point. \$\endgroup\$ – Standard Sandun Jun 27 '13 at 0:49
  • \$\begingroup\$ thanks a lot JIm Dearden, adding a cap in parallel to the power supply definately reduced the noise a lot. But 47 uF doesn't seem enough. I put one big 470 uF cap and it reduced the noise a lot. i am testing other suggestions you provided. \$\endgroup\$ – user734861 Jun 27 '13 at 0:50
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    \$\begingroup\$ @user734861 you may have to experiment with values but at least you have a strategy for reducing/eliminating the source of the noise. \$\endgroup\$ – JIm Dearden Jun 27 '13 at 0:52
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    \$\begingroup\$ holy cow! thanks a lot jim that reduced all the noise. your the boss. \$\endgroup\$ – user734861 Jun 27 '13 at 0:58
  • \$\begingroup\$ out of curiosity what does adding 100 ohm at the point you specified achieve? \$\endgroup\$ – user734861 Jun 27 '13 at 1:01

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