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 VBE. As transistor
parameters are temperature dependent, the operating point also varies with
changes in temperature.
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