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Common Base Amplifier

  1. May 17, 2013 #1
    1. The problem statement, all variables and given/known data
    The problem statement, given solutions, and my small signal drawing are attached


    2. Relevant equations
    Rout, Rin, Av


    3. The attempt at a solution
    To begin with, our text seems to alternate between using approximate equations and exact derived equations without stating why at any given point. Also, circuit elements are frequently ignored as you will see below.

    Looking at my small signal drawing, I don't understand why r is not being considered in any of the calculations. By my calculations, r and R4 are the same size: r = β0/gm = 100 kΩ and R4 is given as 100 kΩ. I see that RI is only 50 Ω. Should r not be considered in the parallel combination in the equation for Rout?

    Also, Rc is not considered at all. I know it is not given, does that mean I am supposed to assume there isn't a bias resistor on the collector? Or would I normally need to try and solve for Rc somehow?

    The problem is, I have found several errors in the text and in the given solutions so far, and the text gives up to three different equations for each characteristic at times (the derivation and up to two different approximations). This is making me doubt everything I do, and making my homework take forever.

    Regards
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. May 18, 2013 #2

    rude man

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    Homework Helper
    Gold Member

    In 'real life' you would say:

    A_v = R_L/(R_l + 25mV/25uA) = 200e3/1050 = 190, R_in = 50 + 25mV/25uA = 1050 ohms, R_out = infinity (this is not including R_L in the computation), A_i = +1.

    But that obviously would not satisfy the problem issuer!

    So comfort yourself in the thought that on a job you would probably never use all the variables & constants given in your answer sheet ...
     
  4. May 19, 2013 #3
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