BJT question because i can't remember what i learned 10 years ago

Discussion in 'General Electronics Chat' started by automagp68, Oct 25, 2015.

  1. automagp68

    Thread Starter Member

    Nov 13, 2011
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    Im considering the situation where Beta = infinity

    here is my issue

    Everyone knows thats

    Its assumed when beta = infinity ib = 0

    Ie = (b+1)(ib)

    Also

    Ie = ib+ic

    Hence most people say when Ib=0
    Ic = ie

    These two equations are contradictory.

    From the first equation we see that if ib = 0 then Ie = 0

    from the second equation we see that if ib = 0 then Ic = ie

    How is this possible.

    I know beta = infinity is commonly referred to as saturation
    But i can't recall why these two equations are contradictory like this.

    They must agree and for the reasons stated above they do not
    Someone help me remember why please?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Beta is never infinity....
    Try:
    1. beta = big number = Ic/Ib,
    2. Rearrange: Ib = Ic/(big number)
    3. Ie = Ib + Ic
    4 Subst. 2 into 3: Ie=Ic(1+1/(big number))
     
  3. automagp68

    Thread Starter Member

    Nov 13, 2011
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    0
    I know beta is never really infinity. Its a theoretical problem commonly given to students

    How is this handled when a problem specifically states beta = infinity

    I know what your getting at above
    aka ic = ie
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    That might be similar to an op-amp problem/exercise.

    AFAICR: the gain of an op-amp is considered infinite (for all intents and purposes) for some parts of the mathematical considerations.
     
  5. be80be

    Senior Member

    Jul 5, 2008
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    A lot of people say this because it doesn't matter for the part your trying to figure.
    But some parts of the figure need the value you trying to get. Maybe that more unstandable. Maybe not.
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You are interested in what happens to the ratio of the currents as the β gets bigger. That is what Limits are used for
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
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    For sufficiently large β it's okay to think of Ie as the same as Ic. Even when β is only 100, that assumption introduces less than 1% error.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Eh? If Ib=0 then Ie=0 and Ic=0. Both equations are satisfied.
     
  9. automagp68

    Thread Starter Member

    Nov 13, 2011
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    And that is the problem Alec

    Normally Beta = infinity is a saturation excersise.

    If Ie = 0 = IC

    then we are talking about cut off
     
  10. crutschow

    Expert

    Mar 14, 2008
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    It really makes no sense to talk about a Beta of infinity, since then you have no way to control the current in the transistor (the collector current would either be 0 or infinity since even an infinitesimal base current causes infinite collector current).
    I think discussing the operation of a transistor with infinity Beta is as pointless as arguing about how may angels can sit on the head of a pin. :rolleyes:
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    Beta decreases when Vce is low.
     
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