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Homework Help: Understanding why R_E is in parallel with g_m in a BJT

  1. Oct 10, 2013 #1
    1. The problem statement, all variables and given/known data
    Why is R_E in parallel with 1/g_m1?
    I drew the small signal model but it does not make sense to me

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 10, 2013 #2
    I get that V_A is given by a voltage divider, but why is R_E in parallel with 1/g_m1?
  4. Oct 14, 2013 #3

    rude man

    User Avatar
    Homework Helper
    Gold Member

    Q3 is just a diode so gm3 can't play a role.

    What is r23? What is VA? You need to define some of these parameters. I assume gm = β but am not sure ...

    I analyzed the circuit assuming β → ∞. Why? Because any circuit that is β-dependent is not a good circuit.

    So I got what looks like your expression if gm → ∞. I know that's not what you are trying to find out - sorry.
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