Common collector configuration in BJT

Discussion in 'General Electronics Chat' started by vasanth1991, Aug 13, 2013.

  1. vasanth1991

    Thread Starter New Member

    May 18, 2009
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    Why output impedance in common collector configuration of BJT is low?

    Output impedance for both Common collector and common emitter configuration is measured across collector and emitter terminal then why there is difference in value. for common emitter terminal output impedance is high but for commom collector configuration output impedance is low.
    please help me...
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    782
    The two circuit types are quite different.

    In the common collector amplifier, the output comes from the emitter terminal. So the CC output impedance is that value seen "looking back" into the emitter terminal. This is generally a low value.

    For the common emitter stage the output is taken from the collector terminal. In the CE case the output impedance is therefore that value seen "looking back" into the collector terminal. This is considerably higher than that of the CC output impedance.
     
    Last edited: Aug 13, 2013
  3. WBahn

    Moderator

    Mar 31, 2012
    17,716
    4,788
    The output impedance is NOT always measured across the collector and emitter terminals. In fact, it almost never is. The output impedance is the impedance (generally small signal impedance) between the two terminals that constitute the output port -- in most simple configurations this is between the output and ground.

    Then you need to understand what the concept of output impedance is. Basically, it says how much the current will change as a result of forcing the voltage to change (or the other way around).
     
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