CE Amplifier Input Impedance question

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

  1. ECE101

    Thread Starter New Member

    Nov 25, 2011
    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!

  2. mik3

    Senior Member

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

    Find Vin and Iin and divide them.
    ECE101 likes this.
  3. ECE101

    Thread Starter New Member

    Nov 25, 2011
    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
    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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