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A question about common emitter amplifier

  1. Feb 19, 2016 #1
    1. The problem statement, all variables and given/known data
    Hello my fellow PF mates! Today i received my electronic textbook from Russia. The book is great, however i cant explain myself one single equation, but i hope you will help me:-)

    The question is about the blocking capacitor in a common emitter amplifier. R1 and R2 are the voltage divider resistors (sorry, but i still havent learned how to post images) and C is the blocking capacitor.
    There is one relationship that i cant figure out.
    2. Relevant equations
    The textbook says that when choosing C, C>= 1/2пf(R1||R2)
    My question is why C should be equal or greater then 1/2пf(R1||R2)?

    3. The attempt at a solution
     
  2. jcsd
  3. Feb 19, 2016 #2

    gneill

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    Staff: Mentor

    What are the major contributing factors to the input impedance of the circuit?
     
  4. Feb 19, 2016 #3
    The input impedance is 8kOhms and the frequency is 200Hz, for this is used a capacitor of 0.1uF
     
  5. Feb 19, 2016 #4

    berkeman

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    Staff: Mentor

    What are the two functions of the input DC blocking capacitor in the CE amplifier circuit?

    http://webpages.ursinus.edu/lriley/ref/circuits/img119.gif
    img119.gif
     
  6. Feb 19, 2016 #5
    As far as i know, the blocking capacitor have 2 main uses. 1st they block DC and allow only AC to pass and 2nd they incidentaly serve as a high pass filter and must be calculated for the working frequency so that they dont cause any trouble. But as i see now, maybe i didnt formulated correctly my question -- How do we calculate that capacitor and what its value depends upon?
     
  7. Feb 19, 2016 #6

    berkeman

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    Staff: Mentor

    Correct. So to be a HPF, that input C and the two input base bias resistors have to have a certain relationship. And it looks like that relationship is the equation you posted in your first post... :smile:
     
  8. Feb 19, 2016 #7
    Thank you verry much for youre help! :-)
     
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