BJT as an amplifier

Discussion in 'Homework Help' started by kareem3d, Jan 7, 2012.

  1. kareem3d

    Thread Starter New Member

    Jan 7, 2012
    10
    0
    hi
    i am using Multisim to design circuits and i have this ciruit where a transistor is working as an amplifier .
    but when i have designed it on Multisim the output voltage is not amplified nor inverted !
    here's the design and i used an oscilloscope to compare the output and the input voltages and it's not what i have expected
    [​IMG]

    Any help will be appreciated :)
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,754
    760
    You are missing some components like emitter bypass cap. :D
    Double check to see if the transistor is in the linear region

    Which means the proper biasing.
     
    Last edited: Jan 7, 2012
  3. kareem3d

    Thread Starter New Member

    Jan 7, 2012
    10
    0
    mmmm I made the emitter bypass cap.
    and the output voltage is amplified
    but now the output signal looks weird !
    here is the circuit after modification
    [​IMG]

    and here's the wave for both the output and input, ofcourse the input is the normal one (( with the same scale 1V/Div ))
    [​IMG]
     
  4. PepeSmooth

    New Member

    Jan 7, 2012
    3
    2
    Kareem,

    Here are the possible reasons:

    1. You must consider the DC gain of BC547B; I think hfe is 200 - 400 so as not to bring the transitor in cutoff and saturation. Your design is probably falls on the two region.
    2. C1 and C2 is to large for an ac transient variation, it will dump your signal significantly. (try nF range)
    3. The divider network R3 and R1 is not appropriate. Consider item 1.
    4. Reduce value R5 less than R2
    5. I also observed that you are playing with 60Hz frequency, are you sure that about this range of bandwith? too narrow

    ----> C1 and C2 try 0.01uF
    There is nothing wrong with the connection, just twick the component values.

    PepeSmooth
     
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  5. PepeSmooth

    New Member

    Jan 7, 2012
    3
    2
    Ahh yes emitter bypass cap..

    Youre waveform clearly shows - the cutoff and saturation
     
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  6. kareem3d

    Thread Starter New Member

    Jan 7, 2012
    10
    0
    thanks i changed the capacitors values and it worked , the output is a reproduction and inverted of the input , thanks for your help :)
     
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    You may think that changing the capacitor value has solved the problem, but the resistor values in your circuit are a long way from what would normally be expected - as follows:


    • You have the base voltage quite close to the positive supply, and therefore the emitter voltage is also close to the supply. This severely restricts the collector voltage swing. Normally, the emitter voltage is set at a fairly small proportion of the total supply, say 20% (or just high enough for reasonably stable bias).
    • The ratio of R5 and R2 can then be altered so that the collector quiescent voltage is about mid-way between the emitter voltage and the positive supply.
    • The base bias chain does not normally need to pass more than about 10 times the expected base current. In your current design, the base chain current must be of the same order as the collector current.
    Note also that 500mV input amplitude is much too big for a transistor stage with a fully bypassed emitter resistor. Either reduce the level, or if this amplitude is required, bypass only part of the emitter resistance so that the necessary signal handling is attained. The distortion will then be greatly reduced.
     
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