Transistor as an Amplifier
The transistor amplifier circuit is shown in the figure ,shows the common emitter npn amplifier circuit. VBB is connected in the input circuit in addition to the signal voltage. This d.c voltage is known as bias voltage and its magnitude is such that it always keeps the emitter-base junction forward biased regardless of the polarity of the signal source.
Operation(Transistor as an Amplifier)
During the positive half-cycle of the signal ,the forward bias across the emitter-base junction is increased. Therefore, more electrons flow form the emitter to the collector through the base. This causes an increase in collector current. The increased in collector current produces a greater voltage drop across the collector load resistance Rc. During the negative half-cycle of the signal, the forward bias across emitter-base junction is decreased. Therefore, collector current decreases. This results in the decreased output voltage(in the opposite direction).Hence an amplified output is obtain across the load.
Analysis of collector currents
when no signal is applied, the input circuit is forward biased by the batter VBB. Therefore, a d.c collector current Ic flows in the collector circuit. This is called Zero signal collector current. when the signal voltage is applied, the forward bias on the emitter-base junction increases or decreases depending upon whether the signal is positive or negative. During the positive half-cycle of the signal, the forward bias on emitter-base junction is increased, causing total collector current ic to increase. For negative half-cycle of the signal, the reverse bias on emitter-base junction is decreased, causing total collector current ic to decrease.
The figure shows the graph of total collector current ic versus time. Form the graph ,it is clear that total collector current consists of two components,
- The d.c collector current Ic 9zero signal collector current) due to bias batter VBB. This is the current that flows in the collector circuit in the absence of signal.
- The a.c collector current ,
The useful output is the voltage drop across collector load Rc due too the a.c component ic. The purpose of Zero signal collector current is to ensure that the emitter-base junction is forward biased at all times.
- Transistor connection: Common base(CB) common Emitter(CE) common collector(CE)
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