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Thread: Common Emitter Amplifier - AC Impedance in

  1. #1
    Banned bugmenot Bad bugmenot Bad
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    Default Common Emitter Amplifier - AC Impedance in

    It's often recommended to improve the AC gain of a common emitter amplifier by putting a bypass capacitor paralell to Re, thus effectively decreasing the Ze for the signal and increasing the gain.

    My question is: Won't this also lower the Z input (AC) for the amplifier? If Ze(AC) is only 100Ω or so, won't that lower Zin(AC) to around 10k (assuming a β of 100)?


  2. #2
    MrAl Excellent MrAl Excellent MrAl Excellent MrAl Excellent MrAl Excellent MrAl Excellent MrAl Excellent MrAl Excellent
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    Hello,

    Lowering Ze will lower Zin, that's life.

  3. #3
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
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    Adding a capacitor across the emitter resistor increases the voltage gain by removing the negative feedback provided by the emitter resistor. Then the distortion becomes extremely high at fairly high levels.
    Uncle $crooge

  4. #4
    silvarblade Good silvarblade Good
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    well audioguru sums it up the bypass capacitor is chosen to reduce the i/p impedance at signal levels hence giving more gain but its value should be carefully chosen.its not preffered where you need very good signal.not a popular technique.if you want a large voltage gain use a follower instead else use class b amp.
    I just cant come up with a good one!

  5. #5
    audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent audioguru Excellent
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    My calculation of the input impedance of the 2N3904 was double the amount shown on its datasheet. I was wrong and used the much higher DC current gain, hFE instead of the lower AC current gain, hfe.
    Uncle $crooge

  6. #6
    khathorzhe Newbie
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    Default Low-Cost Digital Echo/Reverb Microphone Preamplifier Schematic Diagram

    hI!!!!!!!

    do u hav a schematic diagram of Low-Cost Digital Echo/Reverb Microphone Preamplifier?

    plssss... give meh a rply about this!!!!!!1

    thanks!!!!!!

  7. #7
    Torben Excellent Torben Excellent Torben Excellent Torben Excellent Torben Excellent Torben Excellent Torben Excellent Torben Excellent Torben Excellent
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    Quote Originally Posted by khathorzhe View Post
    hI!!!!!!!

    do u hav a schematic diagram of Low-Cost Digital Echo/Reverb Microphone Preamplifier?

    plssss... give meh a rply about this!!!!!!1

    thanks!!!!!!
    Hi there!

    Just a couple of hints to help you get useful answers here:

    1) Please use complete words and sentences here, as well as you can. It is understood that English is not everybody's first language and that's OK, but you don't need to use 6 exclamation marks or spell "please" as "plssss".

    2) Please start your own thread for new topics instead of hijacking someone else's thread. First pick the appropriate forum on the main page, then click the "New Thread" button near the top left-hand side of the page.


    Torben
    Curiosity was framed. Ignorance killed the cat.

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