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A doubt regarding saturation mode of BJT

  1. Feb 6, 2012 #1
    Hi all,
    I have a doubt regarding saturation mode of BJT. Saturation occurs when both emitter-base junction and base-collector junction gets forward biased. For forward biasing a silicon p-n junction only 0.7V is needed. So is for emitter-base junction. But not for base-collector junction. Because we know at saturation voltage across collector and emitter Vce is 0.2V. Using Kirchoff's voltage laws the voltage at base-collector p-n junction is less than 0.7V. Why is it lesser?

    -Devanand T
  2. jcsd
  3. Feb 6, 2012 #2
    0.7V is only an approximation.The more exact equation is:

    [tex]V_{BE}=V_T\; \ln [\frac {I_c}{I_s}][/tex]

    The Vbc diode obey the same equation. Actually I am working with transistors of Ic at 50uA range and the measured Vbe is about 0.5V!!!
    Last edited: Feb 6, 2012
  4. Feb 6, 2012 #3


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    Staff: Mentor

    No one said there is heavy current flow from B-C. It's just not reverse biased. For an NPN, VBC = +0.5v is definitely a bias in the forward direction. But does not correspond to significant current flow through a silicon diode.
  5. Feb 7, 2012 #4
    thankyou for your replies
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