CE Amplifier Input Impedance question

Discussion in 'Homework Help' started by ECE101, Nov 26, 2011.

  1. ECE101

    Thread Starter New Member

    Nov 25, 2011
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    0
    I'm trying would the input impedance would be for the circuit shown below..

    I understand the input signal will see the Beta*Re resistance, but if I assume the constant current source is a resistor, would it just be:

    Rb || Beta*Re?

    I've seen it defined as Re + (1 + Beta)*RE

    where RE is an emitter resistor, as there is no RE in this diagram I'm not really sure how to derive the input impedance.

    Any advice would be appreciated!

    [​IMG]
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The input resistance Rin=Vin/Iin.

    Find Vin and Iin and divide them.
     
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  3. ECE101

    Thread Starter New Member

    Nov 25, 2011
    29
    0
    Thanks for the reply milk3!

    I've just found the answer and

    Rin = Beta/Gm (or Beta*Re as I suspected)
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    The input resistance

    Rin = RB|| ( Beta + 1) * re

    re = 26mV/Ic ---> dynamic emitter resistance ( looking into the emitter to base).
    And this intrinsic resistance (re) acts as if is is in series with the emitter.

    gm = 1/re
     
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