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Type of transistor configuration on a circuit

  1. Apr 16, 2005 #1
    OK, I need some help here. I can´t really recognize the type of transistor configuration on a circuit (common emitter, common base, common colector).
    I`ll some tips that help me identify the configuration.

    Check the circuit attached.

    Attached Files:

  2. jcsd
  3. Apr 17, 2005 #2


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    Assuming you already know how to identify (convention for naming) each transistor terminal, the terminal that is common to both the input and output of the circuit defines the description (common-emitter, common-collector, common-base).

    illustration ---> http://people.deas.harvard.edu/~jones/es154/lectures/lecture_3/bjt_amps/bjt_amps.html

    With that in mind, it should be straight forward deciding how T1 and T2 are used in your circuit.
    Last edited: Apr 17, 2005
  4. Apr 17, 2005 #3
    Although you can have common emitter, common base, and common collector you NEVER input a signal on a collector. I have never seen it anyway, and that is what is being attempted in this circuit.
  5. Apr 18, 2005 #4


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    I believe the fellow in the bjt_amps reference just wanted to illustrate the naming of these three kinds of circuits (without adding too many additional circuit elements).

    I agree with you, in practise you would not find a input (Vin) directly attached to the collector (often a DC supply Vdc is connected),
    however in the AC analysis the collector would be common to both Vin and Vout nodes.
    The following reference illustrates this idea.
    ---> http://www.allaboutcircuits.com/vol_3/chpt_4/6.html

    Also in practise, the common-emmitter may have impedance or Vdc in series with the emitter before reaching a common Vin, Vout node. Similarly for common-base, there may be impedance or Vdc in series with the base before reaching a common Vin, Vout node. (you can see this illustrated at some of the links on the above reference).
  6. Apr 18, 2005 #5
    The word 'common"

    This word ONLY applies to the circuit equivalent of a single transistor and it means your reference point for voltages ( i.e that point is considered at voltage zero ) .
    In a more complex circuits with several devices the common point is usually what is refered to as Ground ( often - ve supply ) but not always .
    Generally the 'common point ' is simply the zero voltage reference point
    by which all others are measured .
    The reference point in no way effects what is occuring in a circuit so 'common emitter' 'common collector' or 'common base ' in no way changes the behaviour -- it only has to do with the way YOU see things if you consider such a point as a reference .
    Example all the usual oscillator configurations are actually the same but viewed with different reference points .
    As an example in the Sun / planets system , it is a lot simpler to consider the Sun as a reference as opposed to the earth .
    So 'common collector ' means viewing a specific collector as zero voltage
    and everything measured with respect to this -- that's all -- for ONE transistor this is not too complex but you would not wish to do this for a whole circuit .
    Ray --- it's usually the English as opposed to the science which is confusing
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