Voltage divider in common emmitter amplifier (hybrid pi model)

Discussion in 'Homework Help' started by Steve|, Nov 5, 2008.

  1. Steve|

    Thread Starter Member

    Nov 4, 2008
    17
    0
    [​IMG]
    Hi there, I'm having problems in being able to derive this expression - I don't understand why rpi/(rpi+rb) is multiplied by the other expression

    Breaking the left part of the circuit down into something that looks like this:

    [​IMG]
    then... using the voltage divider rule

    Vout = Vs(rpi+rb||R1||r1)/(rpi+rb||R1||R2 + Rs)

    But of course we want the voltage over Rpi.. which is where I'm stuck.

    Sorry if this seems trivial...
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Take Vout as an input voltage to the voltage divider formed by Rb and Rpi and use the voltage divider formula to find Vpi

    Vpi=(Vout*Rpi)/(Rpi+Rb)
     
  3. Steve|

    Thread Starter Member

    Nov 4, 2008
    17
    0
    Hmm, I think I might have figured it out.. is this correct?

    Since I need to find the voltage going through Rpi+Rb, this is actually my Vout and therefore to express the voltage over rpi =

    rpi/(rpi+rb) * Vout
    = rpi/(rpi+rb) * Vs(rpi+rb||R1||r1)/(rpi+rb||R1||R2 + Rs)
     
  4. Steve|

    Thread Starter Member

    Nov 4, 2008
    17
    0
    Ah was typing it while you posted.. thanks mik3 :)
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You are right, i have just posted the same answer!
     
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