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How impedence matching is done with any transistor configuration ?

  1. Sep 14, 2010 #1
    how impedence matching is done with any transistor configuration ????

    hello friends....we all know that common collector transistor have higher input impedence and lower o/p impedence ....and this is very good for impedence matching....but i dont know how impedence matching is done by them....

    can anyone help me out ????

    thanks in advance....:smile:
  2. jcsd
  3. Sep 19, 2010 #2
    Re: how impedence matching is done with any transistor configuration ????

    Let’s say you have a 50K ohm microphone you want to use with a line amp with a 600 ohm input impedance. It is the job of the preamp to do this transformation. What does the preamp do?
    If you connect the microphone directly into the line amp you’ll find you don’t hear anything even if you turn the volume up. Why not? If the amplitude of the microphone is 10 mV the input voltage the amp sees is 600/(50K + 600) = 0.119 mV or about 1/84 of the microphone’s open circuit output voltage. Suppose instead we use an impedance matching transformer. The impedance ratio has to be 50K:600 which results in a voltage or turns ratio of 83:1. The transformer will also reduce the microphone’s voltage by a factor of about 83.

    In a common collector, also known as an emitter follower, the input impedance is approximately equal to the beta times the value of the emitter resistor. If we determine that the beta is 50 then the emitter resistor needs to be about 1K ohm for an input impedance of 50K ohms. If the quiescent or DC voltage on the emitter resistor is about Vcc/2 then the output impedance will be about half the emitter resistor or about 500 ohms, which is usually close enough. With a beta of 50, the output voltage will be about 49/50 * input voltage, or about 83 times better than either of the previous two methods.
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