# 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
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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!

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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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 New Member

Dec 20, 2007
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896
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
735
43
Very clear and to the point.

Thanks again!