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If I use a BJT transistor as a switch, the transistor operates in the saturation or cut-off region, not in the active region.
However, some sources use the formula IC = β·IB. However, this formula is only used if the transistor is in the active region. So what's wrong?

Second question: If I connect a very large resistor (1 MΩ) or a very small resistor (1 mΩ) to the collector, will it still produce the same current according to this formula?

Third question: What exactly should I do to bring this transistor into these modes?

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  • Take a look here https://electronics.stackexchange.com/questions/355899/how-is-possible-that-with-same-ibase-there-is-more-than-one-vce/355955#355955 and here https://electronics.stackexchange.com/questions/276146/a-question-about-vce-of-an-npn-bjt-in-saturation-region/276266#276266 and this https://electronics.stackexchange.com/questions/311243/bjt-base-current-calculation/311306#311306 – G36 Mar 19 '24 at 15:07

1 Answers1

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1.) Definition of saturation: The voltage drop across the collector resistor (caused by the collector current Ic) must be large enough to make Vb>Vc. That means: The base-collector-junction now is forward biased. As a consequence - the base current Ib will be much larger than expected for linear operation because now there will be an additional current through the forward biased B-C junction.

How do we know if the transistor is in saturation? As a good indication for this state we require that the base current Ib is app 10% (rule of thumb) of the collector current (B=Ic/Ib<10). Therefore, we use a base resistor Rb in series with the switching voltage Vs that allows such a large base current Ib>0.1*Ic.

Therefore: Rb=(Vs-0.7)/Ib. The minimum required collector current Ic results, of course, from the resistor Rc and the supply voltage Vcc.

2.) With a collector resistor as low as 1mOhm you hardly would be able to create a voltage Vc at the collector node which is smaller than the base voltage Vb (as required for saturation).

3.) This question was answered under 1.).

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