I need help about impedances in pre-amplifer.

Discussion in 'Homework Help' started by bradix23, Jan 26, 2011.

  1. bradix23

    Thread Starter New Member

    Jan 24, 2011
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    0
    Guys, sorry for this again. But, i am having problem about the impedances of my amplifier. Im using a JFET 2N3819 right now. We are required to use a FET. But, FET have high impedances and that is where my problem occur.

    I read something about "impedance matching". It says that the impedance of the load, which is for me a "speaker", must match the output impedance of my FET. Ive learned that the usual impedance of a speaker is 32, 16, 8, 4 Ohms.

    What will i do?...

    Hope you can help me...
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    A FET has a high input impedance. When you are building an amplifier to drive a speaker, the output impedance of the amplifier is what needs to match the rating of the speaker.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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  4. bradix23

    Thread Starter New Member

    Jan 24, 2011
    12
    0
    i figured out that i need a buffer amplifier to lower down the impedance of my FET.

    (@beenthere): i am working on a pre-amplifier that can run a low-power speaker, example 2-watt to 4-watt speaker. What should be my current output? Is 2N3819 not appropriate for my project?

    Also, i would like to ask advices about what BJT should in a pre-amplifier with a HIGH gain so that i can amplify the signal of a dynamic microphone so that it can drive a low power speaker...

    THANKS for the quick replies. :))
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Just to get us on the same page, a preamp is an impedance matching amplifier that is used to take a small signal from a microphone or tape head, add filtering to correct any non-linearities, and output a line level (roughly 1.25 volts RMS) to an amplifier. The amplifier is intended to drive low impedance loads like speakers.

    Your JFETS can't drive diddley. Power is the product of voltage and current. With a 20 ma limit, you would need a 200 volt supply to give 4 watts (4 watts/.02 amps = 200 volts). Did you read the data sheet?
     
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