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Region of operation for BJT

  1. Feb 24, 2013 #1
    1. The problem statement, all variables and given/known data
    The terminal voltages to ground are measured on a particular BJT. In what region does the device operate if these terminal voltages are equal to the following values:

    a) Vc = 6V, Vb = 0.6V, Ve = 0V
    b) Vc = 6V, Vb = 2V, Ve = 1.4V
    c) Vc = 6V, Vb = 6V, Ve = 5.4V
    d) Vc = 6V, Vb = 4V, Ve = 3.6V
    e) Vc = 3.6V, Vb = 4V, Ve = 3.4V

    2. Relevant equations
    No idea which equations apply to this.

    3. The attempt at a solution
    I sort of just guessed because I honestly have no idea:

    a) Cut-off
    b) Constant current
    c) Constant Current
    d) Constant Current
    e) Saturation

    I don't know why, but I am having a hard time understanding some of the concepts in the book we are using (Microelectronic Circuits and Devices 2nd, Horenstein) so these problems are giving me trouble. The book seems to be really vague.
     
  2. jcsd
  3. Feb 25, 2013 #2

    rude man

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    I would first need to know how your text defines the different regions. Do they show a graph of i on the y axis vs. Vce on the x axis and then define the various regions?
     
  4. Feb 25, 2013 #3
    Yes, they do I on y-axis and Vce on x-axis.
     
  5. Feb 25, 2013 #4

    rude man

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    Do they superimpose a piecewise-linear graph on top of one or more of the i-V curves? And is the family of curves for various values of Vbe or ib?
     
  6. Feb 25, 2013 #5
    I've attached an image of the graph shown in the book.
     

    Attached Files:

  7. Feb 25, 2013 #6

    rude man

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    OK, so there are just three regions : cutoff, saturation and active.

    So - how about looking carefully at the graph and making your decisions? BTW there are no numbers given on your graph but assume that "saturation" implies Vbe > Vce and "cutoff" implies Vbe < 0.6V. Make your choices & I'll help you if needed.

    Keep in mind that all the voltages given are with respect to "ground". So for example in case (b), Vce = 6 - 1.4 = 4.6V, not 6V.

    EDIT: not to spoil your day, but you got only 2 out of 5 right so far. :redface:
     
    Last edited: Feb 25, 2013
  8. Feb 25, 2013 #7
    Here's what I came up with:

    a) Constant current (Vce > Vbe)
    b) Constant current (Vce > Vbe)
    c) Saturation (Vce = Vbe)
    d) Cutoff (Vbe < 0.6)
    e) Saturation (Vce < Vbe)
     
    Last edited: Feb 25, 2013
  9. Feb 25, 2013 #8

    rude man

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    OK except possibly for (c). I say "possibly" because here Vce = Vbe puts us right on the borderline between active and saturation. (Actually most people would consider that saturation.) So you have done well unless you were just lucky ...
     
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