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Collector Current through BJT Circuit

  1. Nov 22, 2009 #1
    1. The problem statement, all variables and given/known data

    Consider the following circuit
    http://img109.imageshack.us/img109/80/circuit1.jpg [Broken]

    IL, current through R3 (unknown resistance must be found)

    2. Relevant equations
    Vth=(Vcc/Rtotal)*R1 (V tThevenin)

    Rth=R1//R2 (R Thevenin)

    Vbe= 0.7 volts (silicon diode)
    Beta (transistor constant)=100
    Ie=Ib + IL

    3. The attempt at a solution

    I used the following Equation to try and obtain an answer.

    Vcc= (Ib*Rth)+ (Ie*R4) + Vbe + Vth

    Rth=2.5k Ohms

    Substituting gives:

    10=(IL/Beta)*(2.5k) + 1000*(IL*((Beta +1)/Beta)) + 0.7 + 5

    This yields an answer of IL=4.15mA.

    My tutor said this is a wrong answer, the correct one is 4.3mA
    What is wrong in my reasoning/Working?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 22, 2009 #2


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    Science Advisor

    I like your answer better.

    The tutor has just taken 0.7 V from the 5 V at the center of the two 5K resistors and said this must be the voltage across the emitter resistor.

    But the base current pulls this voltage down a little and that makes the difference between your answers.
  4. Nov 22, 2009 #3

    I just realised what he did,

    He assumed Ie=IL and Ib=0 since it is very small.
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