Bipolar Junction Transistor (BJT) Biasing

BIPOLAR JUNCTION TRANSISTOR BIASING

The application of a DC voltages, across the transistor terminals is called as biasing. Each junction of a transistor may be forward biased (or) reverse-biased independently. They are three different ways of biasing a transistor, which are also called as modes of transistor operation.

Active:

In this mode, the emitter-base junction of a transistor is forward biased and the collector-base junction is reverse biased as shown in fig.3.36.

In a forward biased active, the negative terminal of a battery is connected to N side and positive terminal to P side. The reverse biasing requires all the connections to be opposite to those for forward biasing.

Saturation:

In this mode, both the emitter-base and collector-base junctions of a transistors are forward-biased as shown its fig.3.37 the transistor has a very large value of current. The transistor is operated in this mode, act as closed switch.

Cut-off:

In this mode, both the emitter-base and collector-base junctions of a transistor are reverse biased as shown in fig.3.38. In this mode, the transistor has practically zero current. The transistor is operated in this mode, act as an open switch.

The three different modes are illustrate in table.

The transistor used as amplifier it operate in active mode and it act as switch, operate in saturation and cut-off mode. (ON and OFF).