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Load line of transistor question

  1. Nov 20, 2012 #1
    In order to draw the load line of transistor, we have to find out the saturation current (where the voltage is minimum or zero) and voltage of collector-base when collector current is zero.

    In order to calculate maximum saturation current, we have to forward biased the base-emitter, and reverse biased the base -collector and then get the voltage of collector-emitter to be zero(V of collector minus voltage of emitter equals to zero), right??

    Thank you
  2. jcsd
  3. Nov 20, 2012 #2
    I think 'we have to forward biased the base-emitter, and reverse biased the base' instead of both forward because when we forward both we can't get a constant curve at saturation region, correct?
  4. Nov 20, 2012 #3


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    the base-emitter junction is always forward biased as long as the transistor is not in full "cut-off" (where the collector current is zero). so, from the POV of the load line, the base-emitter is forward biased, usually by about 0.6 volts.
  5. Nov 21, 2012 #4

    what is POV?
    The base-emitter is usually 0.7V.
    How to draw a load line?
  6. Nov 21, 2012 #5


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    Staff: Mentor

    POV = "point of view"
  7. Nov 21, 2012 #6


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    Staff: Mentor

    Maybe this will help:

    Last edited by a moderator: Sep 25, 2014
  8. Nov 21, 2012 #7
    First of all there is not one load line for a transistor.

    There are many and they depend upon the circuit configuration.

    Having said that, for a resistive load the load line is a straight line so you only need to find two points and join them to be able to draw one.

    To draw a load line for R ohms on the collector current (y axis) v collector voltage (x axis) curves:

    1)Locate the point of the voltage axis corresponding to the supply voltage.

    2)Locate the point on the current axis which the supply voltage would drive through R ohms.

    3) Join these two points to form the load line for R ohms.

    go well
  9. Nov 21, 2012 #8
    Thank you, understand already.

    Last edited by a moderator: Sep 25, 2014
  10. Nov 22, 2012 #9


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    okay. whatever. from long ago, when i was a graduate teaching assistant (and i was teaching a lab), it was most often 0.6v for silicon.

    well, your transistor curve is that of a function [itex]I_c[/itex] as a function of [itex]V_{ce}[/itex] for various [itex]I_b[/itex]. so given an [itex]I_b[/itex], you still don't know what [itex]I_c[/itex] is without also knowing [itex]V_{ce}[/itex]. the load line is another equation that relates [itex]I_c[/itex] and [itex]V_{ce}[/itex]. and that equation comes from the rest of the circuit (assuming no [itex]R_e[/itex]:

    [tex] V_{cc} = V_{ce} + I_c R_c [/tex]


    [tex] I_c = \frac{1}{R_c} ( V_{cc} - V_{ce} ) [/tex]

    plotting that equation on top of the transistor curves is your load line.
  11. Nov 22, 2012 #10
    when there is [itex]R_e[/itex] ,we can use the equation in this (transistor load line)
    But for this case , our Ic=Ib, correct?
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