Biasing of Discrete BJT and MOSFET

BIASING OF DISCRETE BJT AND MOSFET

Introduction

BJT consists of 2 PN junctions. It has three terminals: emitter, base and collector. Transistorcan be operated in three regions, namely cut-off, active and saturation by applying proper biasing conditions.

     Active:

–  Most important mode, e.g. for amplifier operation and switching application

–  The region where current curves are practically flat.

Saturation:

–  Barrier potential of the junctions cancels each other out causing a virtual short.

–  Ideal transistor behaves like a closed switch.

Cutoff:

–  Current reduced to zero

–  Ideal transistor behaves like an open switch.

In order to operate transistor in the desired region we have to apply external d.c. voltages of correct polarity and magnitude to the two junctions of the transistor. This is nothing but the biasing of the transistor.

When we bias a transistor we establish a certain current and voltage conditions for the transistor. These conditions are known as operating conditions or d.c. operating point or quiescent point. The operating point must be stable for proper operation of the transistor. However, the operating point shifts with changes in transistor parameters such as β, Ico and V_BE. As transistor parameters are temperature dependent, the operating point also varies with changes in temperature.