1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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.


    Vout = ic*rL

    where rL = RL||Rc

    also Vi = ib*β*re

    but we know that ic = β*ib


    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


    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)


    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted