This document discusses the stabilization of operating point in BJTs. It defines the load line and operating point, and explains that the operating point depends on factors like current gain, temperature, and supply voltage variations. An increase in current gain or temperature will shift the operating point towards saturation. It also describes different bias circuit types - fixed bias, collector base bias, fixed bias with emitter resistance, and voltage divider bias - and provides their basic equations.
2. BJT load line
It is plotted from DC equivalent circuit
In this collector current at is determined at
saturation and collector emitter voltage at cut-off.
3. Operating point or Q-point
It is the point at which transistor is biased
It is determined on DC load line
4. Operating point stability
It depends on various factor
Variation of current gain
Variation of temperature
Variation of reverse saturation current
Variation of base emitter voltage
Improper regulation of supply voltage
5. Effect of variation of current gain
For higher current gain operating point shifts
towards saturation.
6. Effect of variation of temperature
For higher temperature, keeping base current
constant operating point shifts towards saturation.
7. Types of bias circuit
Fixed bias
Collector base bias
Fixed bias with emitter resistance
Voltage divider bias
8. Fixed bias
Base current is given by
Collector current is
DC equivalent circuit
9. Collector base bias
This arrangement is more sensitive to
temperature variation.
Base current is
DC equivalent circuit
10. Fixed bias with emitter resistance
It has additional resistive element on emitter
terminal.
It improves stability.
Base current is
DC equivalent circuit
11. Voltage divider bias
Potential divider circuit is implemented.
Voltage across resistance R2 is taken as base
voltage.
Base voltage is
DC equivalent circuit is
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