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BJT impedance question

  1. Jan 22, 2012 #1
    In the equivalent diagram of BJT, Vbe and rbe represent the base voltage and resistance.
    gmVbe is a current source, which represents the current gain.

    Since gm = Ic/Vt

    Vt/Ic = 1/gm
    Does this mean the impedance of the current source is 1/gm ?
    I know this sounds a bit weird. It's because of a hw problem posted here -
    https://www.physicsforums.com/showthread.php?t=569541
    (the input resistance of the diode connected BJT is (rbe||1/gm).
     
  2. jcsd
  3. Jan 22, 2012 #2
    You're confusing up things. gm means trans-conductance. it is derived as gm = dIc/dVbe = Ic/Vt. This Vt is thermal equivalent of voltage and is constant for a particular temperature.
    It only means gm is proportional to Ic. And it does mean the current source has 1/gm impedance.
     
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