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Common Emitter Amplifier turns into AM radio

Discussion in 'General Electronics Discussion' started by Mozly, Dec 10, 2012.

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  1. Mozly


    Dec 10, 2012
    I was testing an electret mic when I noticed something strange happen.

    I had the electret mic connected in the wrong way to a Common emitter amp circuit that then goes to my speakers. I noticed that I started picking up AM radio station 1650 when I would touch the mic's casing with my fingers (the casing is also the negative terminal).

    I would like to know:
    What is the phenomenon at work?
    Can I pick up different stations?
    Does the station I pick up depend on the capacitance of my body only?

    Here are some more details as to what I did as well as the circuit.

    The above picture is the circuit diagram the only difference is that the mic's polarity is inverted.

    I also noticed that when I touched a big metal object like my computer case, the AM signal was greatly attenuated.
  2. Raven Luni

    Raven Luni

    Oct 15, 2011
    The phenomenon at work is circuit resonance. I'm not an expert and I expect someone will correct me but I'd da the capacitor is working in conjunction with an inductance in your circuit - most likely the speaker to form an LC tank - basically all the energy is being more concentrated into a single frequency which just happens to be that of the radio station. The transistor will rectify this signal and give you the audio.
  3. BobK


    Jan 5, 2010
    Actually, resonance is probably not needed. Any diode can be an AM detector, and you will just hear the strongest station at the location. In this case, the diode is parf of the transistor.

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