Operating a NPN Transistor in Active Region

To understand how a NPN transistor operates in the active region, we need to examine the biasing of its emitter-base (EB) and collector-base (CB) junctions.

Emitter-Base Junction
The emitter-base junction is a forward-biased junction. This means that the voltage applied to the emitter terminal is higher than the voltage applied to the base terminal. Here's why:

1. When a positive voltage is applied to the emitter terminal with respect to the base, the electric field pushes the majority charge carriers (electrons for NPN transistor) from the emitter towards the base.

2. This forward biasing reduces the potential barrier at the junction, allowing electrons to easily cross from the emitter to the base.

3. The forward biasing also helps in injecting a large number of charge carriers into the base region, creating a surplus of electrons.

Collector-Base Junction
The collector-base junction is reverse-biased. This means that the voltage applied to the collector terminal is higher than the voltage applied to the base terminal. Here's why:

1. When a positive voltage is applied to the collector terminal with respect to the base, the electric field pushes the majority charge carriers (electrons for NPN transistor) away from the collector towards the base.

2. This reverse biasing increases the potential barrier at the junction, making it difficult for electrons to cross from the base to the collector.

3. The reverse biasing helps in collecting the electrons injected from the emitter and preventing them from reaching the collector terminal.

The Active Region
In the active region, the NPN transistor is biased such that both the emitter-base and collector-base junctions are biased correctly:

1. The emitter-base junction is forward-biased to allow a large number of electrons to flow from the emitter to the base.

2. The collector-base junction is reverse-biased to prevent the majority charge carriers (electrons) from flowing from the base to the collector.

3. This biasing configuration ensures that the transistor operates as an amplifier, where a small input current at the base controls a larger current flowing through the collector.

4. The active region is the region of operation where the transistor exhibits the desired amplification characteristics and is commonly used in various electronic applications.

Therefore, to operate a NPN transistor in the active region, its emitter-base junction should be forward biased, and its collector-base junction should be reverse biased, which is represented by option 'A'.