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Qusetion about dynamic emitter resistance

  1. Jul 7, 2009 #1
    I have a question about the nodal equations of BJTs - when analyzing a circuit to find out the input impedance, output impedance etc. is the term representing the intrinsic emitter resistance, Rbe (that is approximately 26mv/Ic) multiplied by the term beta+1? I'm looking at two references, and in one of them the output impedance of an emitter follower is calculated with the intrinsic emitter resistance and the external resistor both multiplied by beta+1, and another just has the external emitter resistance multiplied by beta+1 and not Rbe.
  2. jcsd
  3. Jul 7, 2009 #2


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    You might be asking about amplifiers where they have an unbypassed emitter resistor and take the output from a collector resistor.

    Call I the input signal current. (not the bias current)
    The input voltage in that case is I * base emitter resistance plus ( I * (Hfe +1)) times RE
    where RE is the emitter resistor.
    Looks complicated but it is just saying the base emitter junction and the emitter resistor are in series. The the base emitter junction has I going through it and the emitter resistor has this current plus an amplified version of it.

    Now if the emitter resistor is more than 100 ohms or so, the amplified current through it will make the voltage across the base emitter junction negligible so you can simplify things by ignoring it. This leads to the nice approximation that the gain of such an amplifier is equal to the collector resistance divided by the emitter resistance almost regardless of the transistor used.
  4. Jul 8, 2009 #3


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    Both are wrong (or you are mis-interpreting or mis-understanding the topology). For the output impedance of an emitter follower neither of the mentioned resistances should be multipied by beta+1.
  5. Jul 8, 2009 #4
    You are right, UART - I mistyped. In the case of the emitter follower the output impedance is just Rbe + Rsource / B + 1.
  6. Jul 8, 2009 #5


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    Yes that's correct bitrex (and possibly also including the external emitter resistance RE in parallel in the case where the load is seperate to RE).
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