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Transistor how to find total amplification

  1. Jul 1, 2010 #1
    [PLAIN]http://img43.imageshack.us/img43/9491/47613590.jpg [Broken]

    find Aus which is Vout/Vs = Vout/Vi * Vi/Vs

    where Vi = ib*β*re and Vout = VL in this case

    also we know that

    Rc=2kΩ , Rb=430kΩ , Rs=1kΩ, RL=4kΩ, Vcc=10V, β=100.

    MY ATTEMPT FOR A SOLUTION

    Vout = ic*rL

    where rL = RL||Rc

    also Vi = ib*β*re

    but we know that ic = β*ib

    hence

    Au = Vout/Vi = (β*ib*rL)/ib*β*re = rL/re

    as i said rL = RL||Rc = 8000/6000 = 1.34 KΩ

    I need to find re know, we know that re = 25mV/IE

    BUT we dont know IE, I should analyze the dc circuit in order to find IE

    in the DC circuit we have

    10 - 0.7 = Ib*430000

    hence Ic = β*(9.3/430000) = 2.15mA which is almost equals to IE since β is large enough

    we know IE now

    so

    re = 25mV / 2.16mA = 11.57 Ω

    hence Au = rL/re = 1340/11.57 = 115.8

    now I need to find Vi/Vs

    taking the ac circuit I get that Rs is in series with β*re||Rb

    hence we have a voltage devider here and

    Vi = Vs*(β*re||Rb)/(β*re||Rb+Rs)

    hence

    Vi/Vs = (β*re||Rb)/(β*re||Rb+Rs)

    β*re||Rb = 1157||430000 = 1153 Ω

    hence Vi/Vs = 1153/(1153+1000) = 0.53

    so Aus = 0.53*115.8 = 62

    am I correct?

    thanks in advance
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
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