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impedance of amplifier

Discussion in 'Electronics Homework Help' started by bhuvanesh, Mar 17, 2015.

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  1. bhuvanesh


    Aug 29, 2013
    i am studying bjt amplifiers,i am often getting through impedance of input and outputs . Its messing up with me
    example:common collector configuration have high input impedance and low output impedance
    does that input impedance is impedance between the terminal( base and collector)?
    it is also said as impedance matching circuit.why common base and common emitter are not being said impedance matching circuits.Thank you in advance
    Last edited by a moderator: Mar 18, 2015
  2. witsender


    Dec 12, 2013
    Hi Bhuvanesh :)
    Transistor circuits are either common base, common emitter or common collector.
    In the case of a common emitter circuit whose input is at the base of the transistor, the input impedance is measured between base and emitter. Output impedance for such a circuit is measured between collector and emitter.
    Hope this helps...
  3. bhuvanesh


    Aug 29, 2013
    my question is why we are calling common collector configuration as impedance matching network
  4. LvW


    Apr 12, 2014
    Well - a common collector stage can placed BETWEEN a signal source having a high source impedance and another stage having rather small input impedance.
    Thus, the common-collector stage acts like a "matching network" between source and load.
    Without this stage (large input resistance, small output resistance) we would have a huge and unwanted voltage drop across the source output impedance.
  5. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    "why we are calling common collector configuration as impedance matching network"

    There are three uses for an amplifier. To increase the voltage (the amplitude of the voltage)
    To increase the current. To increase both the voltage and current.
    Since a common collector configuration does not increase the voltage of the signal, we have to talk about the feature or quality that it will provide.
    The only thing it does is increase the current.
    So we call it a CURRENT AMPLIFIER.
    It just happens to have a large input impedance due to the way the transistor is connected and the gain of the transistor increases the input impedance by a factor equal to the amplification of the transistor.
    When a load is placed between the emitter and 0v rail, the transistor increases the impedance or resistance of the load by a factor of about 100 to 200, depending on the gain of the transistor.
    This means the input of the stage has a very high impedance and when you connect a signal to the input, the amplitude of the signal will not be reduced (attenuated) or it will be attenuated a very small amount.
    However if you connect the signal to the load, the amplitude of the signal will be reduced ENORMOUSLY.
    This means the stage has the effect of "changing the load" by a factor of 100 or 200 and the input signal thinks it is delivering its signal to the load.
    The transistor is converting or changing the load into a higher impedance or resistance.
    We call this feature MATCHING.
    You can also call it CONVERTING or INCREASING.
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