The transistor amplifier circuit shown in the figure shows the common emitter npn amplifier circuit. VBB is connected to 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 from the emitter to the collector through the base. This causes an increase in collector current. The increase 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 the emitter-base junction is decreased. Therefore, collector current decreases. This results in a decreased output voltage(in the opposite direction). Hence an amplified output is obtained 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 on whether the signal is positive or negative. During the positive half-cycle of the signal, the forward bias on the emitter-base junction is increased, causing the total collector current to increase. For the negative half-cycle of the signal, the reverse bias on the emitter-base junction is decreased, causing the total collector current to decrease.
The figure shows the graph of total collector current ic versus time. From the graph, it is clear that the 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 a signal.
- The a.c collector current,
The useful output is the voltage drop across collector load Rc due to 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.