Max. Collector Current Active BJT (Quick/Simple)

Discussion in 'Homework Help' started by jegues, Apr 20, 2011.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    In this example they say that the maximum current the collector can support while the transistor remains in active mode is approximately 0.5mA.

    Where does this come from? Why is it specifically 0.5mA? Can I assume this for all the BJT's I'm working with?

    Thanks again!
    Last edited: Apr 20, 2011
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Look at this simply example:


    The max collector current occur when Vce = Vce(sat)
    Ic_max≈ Vcc/Rc
    If Ib = 50uA then Ic = 5mA and Vce = 5V; VRc=5V
    Ib = 60uA ---> Ic = 6mA---> Vce = 4V---> VRc = 6V

    But if Ib = 110uA then Ic = β*Ib = 1.1mA and Vce = -1V ?? and VRc =11V ??
    Of course Ohms law and kirchhoff law must hold, so this situation is impossible.
    BJT is full open and it is in saturation mode, Ic = β *Ib dont hold anymore.
    But if we change Rc to 500Ω then for
    Ib= 11uA --->Ic = 11mA ---> Vce = 4.5V---> VRc=5.5V
    And saturation occurred when Ib > (Vcc/Rc) / β = 20mA /100 = 200uA
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  3. Audioguru

    AAC Fanatic!

    Dec 20, 2007
    Don't add another circuit, look at the circuit that was questioned:
    1) Its collector resistor is 10k and just before the transistor saturates its collector resistor has a max current of 5V/10k= 0.5mA.
    2) When it is saturated the collector voltage will be about +0.6V so the 10k collector resistor will have 5.6V across it.
    3) Then the max collector current in this circuit is 5.6V/10k= 0.56mA.

    The 10k resistor and the voltages in the circuit limit the active current to only 0.5mA, not the transistor.
  4. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    Very clear and to the point.

    Thanks again!