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BJT pls help!

Discussion in 'General Electronics Discussion' started by sahil_time, Jan 6, 2012.

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  1. sahil_time

    sahil_time

    2
    0
    Nov 1, 2011
    Now as you can see in the picture a common base configuration of BJT.
    The green markings in the picture are resistances. :p
    My question is:
    By KVL : VBB = VBE + IE*RB which means that IE is esentially a constant because
    VBB , VBE, RB are constants.
    Now If Voltage Vcb is increased Early effect states that depletion region
    of CB junction penetrates deeper into the Base region. Which means charge gradient in base increases and hence IE increases . But KVL says that IE should be a constant....

    What is the contradiction?

    Thankyou in advance :)
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,025
    2,138
    Nov 17, 2011
    Some ideas, but question still open

    Oh my god,
    I used to study these things, but it's been so long ...

    I'd say that the narrowing of the base comes with an corresponding decrease in Vbe, thus IE*RB+VBE=VBB.

    Another explanation could be taken from the Wikipedia article (http://en.wikipedia.org/wiki/Early_effect): If "the current of minority carriers injected across the emitter junction increases" you need less base current for the same emitter current. The difference flows on to the collector, increasing Ic.

    I hope someone with deeper understnding of BJT technology can help.

    Harald
     
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