A bipolar junction transistor (BJT) has three regions: the emitter, the base, and the collector.
It also has two junctions: the base-emitter junction and the collector-base junction. The active mode of a BJT is when the base-emitter junction is forward biased (meaning that the voltage at the base is higher than the emitter for an NPN transistor, and the opposite for a PNP transistor)and the collector-base junction is reverse biased (the voltage at the collector is higher than the base for an NPN transistor, and vice-versa for a PNP). In an NPN transistor, for example, theactive mode is achieved when:
• The base-emitter junction is forward biased meaning that the base voltage (Vb) emitter voltage (Ve), and the
• The collector-base junction is reverse biased meaning the collector voltage (Vc) base voltage (Vb).
In active mode, the BJT acts as an amplifier, where a small change in the base current can cause a large change in the collector current. This region is most useful for amplification in BJT circuits.