1. Not finding help here? Sign up for a free 30min 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!

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
    Vcc=10v
    IL=Beta*Ib
    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

    Vth=5v
    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

    vk6kro

    User Avatar
    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
    THanks,

    I just realised what he did,


    He assumed Ie=IL and Ib=0 since it is very small.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook