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learning BJT's - Saturation Mode - Doubt

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frs89

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Hi

I'm learning BJT's, I have a beginner's doubt.

The collector current in active mode is usually bigger or lower than when in saturation mode? I look in a ic-Vce plot that ic grows with Vce and drops when Vce-->0 i.e the CBJ is forward biased.
In an example that I looked on Sedra is said that the ic current is aprox 4.3mA, then because the max current in active mode is aprox 0.5mA the Bjt is clearly in the saturation mode. Please I need some clarification.

Thanks




 

If we have this simple schematic

89_1259576519.png


And assume for simplification that β is constant and equal 100.
So the max Ic current that can flow in collector is equal:

Ic_max ≈Vcc/Rc=(Vcc-Vce_sat)/Rc≈12mA

So if base current will be larger then
Ib≥12mA/100=120uA the BJt will be saturated.
For example for Ib=60uA ---> Ic=6mA and Vce=6V--->Ie=6.06mA

And for Ib=1mA--->Ic≈12mA and Vce=Vce_sa ≤ 0.2V and Ie=13mA

So collector current can change form 0A to approximate 12mA
Form cut-off through active region to saturation.
 

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    frs89

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Beta is not used when a transistor is saturated.
Beta is used when a transistor is linear with plenty of collector to emitter voltage.

Most transistor datasheets show the max saturation voltage loss when the base current is 1/10th the collector current even if the beta is 300.
 

In saturation mode, gate control is lost and the transistor operates like a closed switch
 

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