Common collector configuration

Discussion in 'General Electronics Chat' started by elecidiot, Aug 1, 2010.

  1. elecidiot

    Thread Starter New Member

    Jun 29, 2010
    Hi all..
    How is the output resistance of transitors measured? Why is it is said that the output resistance of the common collector configuration is much lower than other configurations? Instead of writing equations what is the physical reason behind it? Why is the output resistance of the common emitter higher than that of common collector?
  2. hobbyist

    Distinguished Member

    Aug 10, 2008
    The impedance relationships in transistor circuits, are not referring to the transistor itself,
    but rather, the resistor values used to bias the transistor into it's proper operating region, for which the circuit is designed for.

    For the output impedances of a CC amp, there are several factors, that are calculated, such as output load, and output impedance of the signal source driving it.

    CE amp, the output impedance is determined by the load in parrallel with the collector bias resistor, for signal analysis.

    DC analysis, the output impedance for a CE. amp is the collector bias resistor alone.

    Example in a CE amp. The output impedance is large so that using emitter bias resistors, of proper values can give the amp. stage the proper gain in signal amplification needed, while with a CC amp. the voltage amplification is less than unity, but a higher current gain is available,
    That's why it depends on the bias resistors for each configuration.

    Iit gets more involved than this, but this is a brief introduction, to your questions.
    Last edited: Aug 1, 2010