Class C Amplifier : Expert Help needed !

Discussion in 'The Projects Forum' started by haruwn, Jan 30, 2009.

  1. haruwn

    Thread Starter Member

    Jan 30, 2009
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    Hi.... m emloying a Class C Amplifier Design for the amplification of an fsk signal
    I simulated the whole thing on EWB 5 , worked pretty well, input 30dB, where voltage rms ws 20mV @ 10MHz
    and i obtained about 250mV in the simulation.
    Now doing this all practically, gave input of 2V rms and results were quite disappointing, my signal rather got attenuated !
    I tested the results on 2N3904 BJT !
    any suggestions !
    I have attached the simulation print screen with results.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Have you a reason for using such a small value for output coupling capacitor?

    hgmjr
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Remember a Class C amplifier is always a TUNED amplfier. Is the final tank circuit tuned to your input frequency, (or vice versa)?

    ERic
     
  5. haruwn

    Thread Starter Member

    Jan 30, 2009
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    Infact this is a Class A amplifier associated with the tank circuit as in Class C, i calculated the values of all the components using the AC and DC analysis separately and ws perfect in theory as wel.....
    input, bypass, output reactance ws setted to te critical frqncy of 1 MHz while the tank ckt is resonated on 10 MHz.....
    all this is good, whts wrng with the real circuit....... ?
     
  6. haruwn

    Thread Starter Member

    Jan 30, 2009
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    Ofcourse i tuned it by watchful calculations ....... any othr help ???
    should i change the transistor ........ my group fella was like, 2N3904 has lesser bandwidth...... ? may be this is the issue ?
    any recommendations from ppl who had bn wrkin on BJTS ?
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think the tank circuit will not do much with the 4k7 resistor in series.
    The gain is also reduced by the emitter resistor without a decoupling capacitor.

    Greetings,
    Bertus

    PS Do not use SMS like text, this will make your text unreadable.
    Use proper english.
     
  8. haruwn

    Thread Starter Member

    Jan 30, 2009
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    dsnt help !
     
  9. haruwn

    Thread Starter Member

    Jan 30, 2009
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    oK, Can anybody please ammend, or modify the circuit so that i could attain a gain = 10 ?
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Try increasing the tank capacitor to 2.56 pFd. I am assuming that your input signal is 10MHz.

    hgmjr
     
  11. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    using your figures for the tank circuit, I'm showing the resonant frequency being around 17 MHz
     
  12. hgmjr

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    Jan 28, 2005
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    What is the frequency of your input signal. The scope makes it looks like 10 MHz.

    hgmjr
     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Actually, it should be exactly 16 MHz
     
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