Finding the Gain/Input/Output Resistance of cascase amplifier

Discussion in 'Homework Help' started by Digit0001, Sep 19, 2011.

  1. Digit0001

    Thread Starter Member

    Mar 28, 2010
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    0
    Hi

    Can someone explain to me how they got the answer? I don't understand why they have only used a few of the values to find the gain.

    tut1_pic.JPG

    P.S
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Try to draw this two-port amplifiers in series (cascase amplifier).
    And then you will see that the output resistance of the first stage form voltage divider whit input resistance of a second stage. And we have similarly situation when we connect second stage output resistance to the third stage input resistance.
    http://forum.allaboutcircuits.com/showthread.php?p=153702#post153702 ( post 8)
     
  3. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    ok i understand this part, however i don't understand why isn't the first input resistance and the third output resistance not included in the gain calculation.
     
  4. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    That is because they don't form any voltage divider. They are just resistances "hanging in the air" that don't drop any voltage on them and consequently don't suppress the signal level.

    If you connect a load on the output of that system, or a source on its input, then further calculations about the gain are needed again.

    Is that clear?
     
  5. Digit0001

    Thread Starter Member

    Mar 28, 2010
    89
    0
    thanks i understand now
     
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