*We have two independent variables here and . For different value of , the input characteristics as a function of is as follows.
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*Similarly, for different values of , the vs. characteristic is shown below. The line passing through
and
is known as the load line and its intersection with the
curve determines the quiscent point or operating point Q. Note that in the active region, is almost independent of (i.e. nearly constant) and depends mostly on . The ratio
is a constant for the active region and is known as .
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*In the common emitter circuit shown above,
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*For the cutoff region, and
V.
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*In the saturation region,
V and
.
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*We will denote the operating point by and .
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*The fact that in the active region, variations in result in proportional varitions in and hence forms the fundamental principle of amplifier. For the trabsistor to remain in the active region throughout this variation, variations in should be maximum A.
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*In the CE configuration above, with the change in temperature, changes and hence, so does Q. is an increasing function of T and therefore as T increases, increases causing further heating of the transistor and thereby further increasing . This is known as thermal runaway. To avoid this, we stabilize the circuit my introducing an emitter resistance.
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*Now, as T increases, increases and so does . This increases and therefore reduces as the base voltage increases. This decrease in resluts in decrease of , thereby compensating the effect of temperature and stablization of the operating point.
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