EARLY EFFECT It is the variation in the effective width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage. A greater reverse bias across the collector–base junction, for example, increases the collector–base depletion width, thereby decreasing the width of the charge carrier portion of the base. Under increased collector–base reverse bias, the lower panel of Figure 1 shows a widening of the depletion region in the base and the associated narrowing of the neutral base region. The collector depletion region also increases under reverse bias, more than does that of the base, because the collector is more heavily doped relative to the base. The principle governing these two widths is charge neutrality. The narrowing of the collector does not have a significant effect as the collector is much longer than the base. The emitter–base junction is unchanged because the emitter–base voltage is the same. Base-narrowing has two consequences that affect the current: There is a lesser chance for recombination within the "smaller" base region. The charge gradient is increased across the base, and consequently, the current of minority carriers injected across the emitter junction increases. TRANSISTOR BIASING AND STABILIZATION What is the need for biasing? In order to produce distortion free output in amplifier circuits, the supply voltages and resistances establish a set of dc voltage VCEQ and ICQ to operate the transistor in the active region. These voltages and currents are called quiescent values which determine the operating point or Q-point for the transistor. The process of giving proper supply voltages and resistances for obtaining the desired Q- Point is called Biasing. The circuits used for getting the desired and proper operating point are known as biasing circuits. To establish the operating point in the active region biasing is required for transistors to be used as an amplifier. For analog circuit operation, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut -off region) when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary - switches from "on" to "off" state. Often, Q-point is established near the center of active region of transistor characteristic to allow similar signal swings in positive and negative directions. Q-point should be stable. In particular, it should be insensitive to variations in transistor parameters (for example, should not shift if transistor is replaced by another of the same type), variations in temperature, variations in power supply voltage and so forth. The circuit must be practical: easily implemented and cost -effective. DC OPERATING POINT (OR) QUIESCENT POINT: Application of DC voltages (bias) establishes a fixed level of current and voltage. For transistor amplifiers the resulting DC current and voltage establish an operating point on the
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EARLY EFFECT · 2019. 3. 8. · TRANSISTOR BIASING AND STABILIZATION What is the need for biasing? In order to produce distortion free output in amplifier circuits, the supply voltages
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EARLY EFFECT
It is the variation in the effective width of the base in
a bipolar junction transistor (BJT) due to a variation in the
applied base-to-collector voltage. A greater reverse bias
across the collector–base junction, for example, increases the
collector–base depletion width, thereby decreasing the width
of the charge carrier portion of the base.
Under increased collector–base reverse bias, the lower panel of
Figure 1 shows a widening of the depletion region in the base
and the associated narrowing of the neutral base region.
The collector depletion region also increases under reverse
bias, more than does that of the base, because the collector is
more heavily doped relative to the base. The principle
governing these two widths is charge neutrality. The
narrowing of the collector does not have a significant effect as the collector
is much longer than the base. The emitter–base junction is unchanged because the emitter–base
voltage is the same.
Base-narrowing has two consequences that affect the current:
There is a lesser chance for recombination within the "smaller" base region.
The charge gradient is increased across the base, and consequently, the current of minority
carriers injected across the emitter junction increases.
TRANSISTOR BIASING AND STABILIZATION
What is the need for biasing?
In order to produce distortion free output in amplifier circuits, the supply voltages and resistances
establish a set of dc voltage VCEQ and ICQ to operate the transistor in the active region. These voltages
and currents are called quiescent values which determine the operating point or Q-point for the
transistor. The process of giving proper supply voltages and resistances for obtaining the desired Q-
Point is called Biasing. The circuits used for getting the desired and proper operating point are known
as biasing circuits. To establish the operating point in the active region biasing is required for
transistors to be used as an amplifier. For analog circuit operation, the Q-point is placed so the
transistor stays in active mode (does not shift to operation in the saturation region or cut -off region)
when input is applied. For digital operation, the Q-point is placed so the transistor does the contrary
- switches from "on" to "off" state. Often, Q-point is established near the center of active region of
transistor characteristic to allow similar signal swings in positive and negative directions. Q-point
should be stable. In particular, it should be insensitive to variations in transistor parameters (for
example, should not shift if transistor is replaced by another of the same type), variations in
temperature, variations in power supply voltage and so forth. The circuit must be practical: easily
implemented and cost -effective.
DC OPERATING POINT (OR) QUIESCENT POINT:
Application of DC voltages (bias) establishes a fixed level of current and voltage. For
transistor amplifiers the resulting DC current and voltage establish an operating point on the