BJT AC Analysis Problem

Discussion in 'Homework Help' started by Digit0001, Oct 13, 2011.

  1. Digit0001

    Digit0001 Thread Starter Member

    Joined:
    Mar 28, 2010
    Messages:
    89
    Hi

    Can someone explain to me how they got part c) finding the small signal; voltage gain in the following document.
    View attachment ques5_tut4.doc

    This is the AC equivalent circuit i used.
    ac_cct.jpg

    P.S
  2. Georacer

    Georacer Moderator Staff Member

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    Athens, Greece (GMT +2)
    Please don't upload .doc or .docx formats. If a simple .png image isn't enough, use the .pdf at least.

    Many members are reluctant to use the .doc format.

    Attached Files:

  3. t_n_k

    t_n_k AAC Fanatic!

    Joined:
    Mar 6, 2009
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    5,193
    Personally I think the solution to part (c) is overcomplicated.

    Since it is required to find Vo/Vs, I would simply assume a voltage source of value Vs is applied to the emitter.

    From there one can observe that

    v_{be}=-\frac{r_\pi}{r_\pi+R1||R2}v_s=-\frac{6.97k}{6.97k+37.5k}v_s=-0.157v_s

    Given that, one can then determine the output as a function of vs since

    v_o=-g_mv_{be}R_c||R_L=-17.93[mA/V](-0.157v_s)2.4[k\Omega]v_s=6.744v_s

    or Av=6.744 [which seems a better estimate to me than 6.73]

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