BJT High frequency analysis

Discussion in 'Homework Help' started by Digit0001, Jun 2, 2012.

  1. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    Hi
    Can someone tell me what i have done wrong in the following question?
    The question and my solutions are below:
    View attachment question10.pdf
    qns10_sol.jpg

    The answer is supposed to be fH=2.97MHz

    P.S
     
  2. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    0
    so can anyone help me on this question?
     
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Well I don't have a time to do any calculations. But for my in this circuit there is no millers effect.
     
  4. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    ok, but from my solutions i have Q1 => C-B and Q2 => C-E is this correct?
     
  5. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Yes Q2 works as a CE amplifier but the gain of this amplifier is close to one.
    And Q1 work as CB amplifier and CB amplifier is free from miller effect
     
  6. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    i have worked it out, forgot to use the gain for common emitter in order to work out the miller's effect
     
  7. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Interestingly, I've found a more careful analysis / simulation gives the upper -3dB point as ~3.8MHz [rather than 2.97MHz] which suggests the approximations applied aren't particularly robust. The model I used was the Hybrid-Pi. Rather than applying the Miller theorem I left Cμ [2pF] from the two BJT's collector-to-base terminals. Cbe was 5.96pF per the relationship fT=gm/(2∏(Cμ+Cbe). Also used gm=20mA/V and r∏=5k.
     
    Last edited: Jun 4, 2012
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