Amp FAIL!

Discussion in 'Homework Help' started by Theory, Nov 3, 2015.

  1. Theory

    Thread Starter New Member

    Nov 3, 2015
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    I have designed a class A amplifier ( its my assignment, note that im student). My problem is that the output ain't amplified instead it is smaller that the input. Whats the problem ? It is a Common Emitter btw.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Is there a connection between the resistors and the base?
    I do not see the connection dot.

    Bertus
     
  3. Theory

    Thread Starter New Member

    Nov 3, 2015
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    @bertus There is. My simulator look that way, Sorry sir. its Voltage divider sir. A common emitter configuration :)
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Get a voltmeter and measure the voltage at the base, emitter and collector,
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Since your simulator correctly put connection dots at the other nodes, I suspect there is no connection to the base of the transistor except the input coupling capacitor, hence no bias.

    At what frequency? You need to use .AC analysis (not .DC) and determine the gain vs. frequency (Bode Plot).[/user]
     
    Last edited: Nov 3, 2015
  6. Theory

    Thread Starter New Member

    Nov 3, 2015
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    300Hz to 30kHz Sirs! :) Okay my bad, so clumsy thanks sirs :)
     
  7. Theory

    Thread Starter New Member

    Nov 3, 2015
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    Sirs? What if im going to make this a 2 stage. Can i just attached same amp to it with a cap between them ? And btw thanks it works now. :D
     
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  8. #12

    Expert

    Nov 30, 2010
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    Yebbut...you might want to place about a nanofarad in parallel with R1 to limit the gain above 30KHz.
     
  9. Theory

    Thread Starter New Member

    Nov 3, 2015
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    Oh okay.
     
  10. Theory

    Thread Starter New Member

    Nov 3, 2015
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    Which cap actually is controlling my highest freq sirs?
     
  11. #12

    Expert

    Nov 30, 2010
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    The one that isn't installed in parallel with R1.
     
    Last edited: Nov 3, 2015
  12. Theory

    Thread Starter New Member

    Nov 3, 2015
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    @#12 Sir? C = 1/(2pifR), then f= 30kHz ( highest freq) and R will be R1 only?
     
  13. Theory

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    Nov 3, 2015
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  14. bertus

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    Theory likes this.
  15. Theory

    Thread Starter New Member

    Nov 3, 2015
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    Sorry for the rush ! Lel. Well Thanks a lot tho, admin @bertus
     
  16. Theory

    Thread Starter New Member

    Nov 3, 2015
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    Can someone show/tell me how and why Bypass capacitor is a high pass filter?
     
  17. Theory

    Thread Starter New Member

    Nov 3, 2015
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    What type of cap are being used in amplifiers? Some schematics have non-electrolytic, and some are using electrolytic. Can someone clarify this?
     
  18. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    It will depend on the capacitor value. If C is > 1μF we are going to use electrolytic capacitor. But if C < 1μF we can easily use non-electrolytic cap for example a metal film capacitors. And in general we are trying to avoid the ceramic capacitors in the signal path unless the cap is < 1nF and we are gonna to use Class 1 ceramic capacitors.

    As for your amp:

    C1 > 0.16/(300Hz * R3||R4||(hfe+1)*26mV/Ic) ≈ 0.16/(300Hz * 3kΩ) = 177nF, so we need to use 220nF.
    C3 > 0.16/(300Hz * 26mV/Ic) = 0.16/(300Hz * 26Ω) = 22μF
    C2 > 0.16/(300Hz * (R1 + Rload))
     
  19. #12

    Expert

    Nov 30, 2010
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    I handed you the answer (and now I regret doing that). What kind of component, where to place it, and a fair estimate of the size. This being the Homework Forum, why don't you use the formula that you already know (post #12) and find out if using only R1 results in about a nanofarad of capacitance being the right size? Is it easier to ask 6 times over the course of 4 days than to do one line of math?
     
  20. Theory

    Thread Starter New Member

    Nov 3, 2015
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    I actually did it sir, @#12 . I just asked for clarifications since i learned that inorder to control my highest freq, all i need to do is put a capacitor in parallel with Vce( goin to ground) . That's why doubts are coming.
     
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