Common Emitter Amplifier

The common emitter NPN amplifier circuit is shown in the figure below. The source V_BB is applied to the input circuit in addition to the signal. The V_BB battery provides the forward bias voltage to the emitter-base junction of the transistor. The magnitude of the forward bias voltage should be such that it should keep the emitter-base junction always in the forward biased regarding the polarity of the signal source.

Operation of Common Emitter Amplifier

When a signal is applied across the emitter-base junction during the positive half cycle the forward bias across this junction increases. This increases the flow of electrons from the emitter to a collector through the base, thus increases the collector current. The increasing collector current induces more voltage drops across the collector load resistor R_C.

The negative half cycle decreases the forward bias voltage across the emitter-base junction. The decreasing collector-base voltage reduces the collector current in the whole collector resistor R_C. Thus, the amplified load resistor appears across the collector resistor.

Collector Current Analysis

The graphical representation of collector current is shown in the figure above. The emitter-base junction remains in forwarding bias even when the bias battery V_BB applies no input signal. Therefore the DC collector current I_C flows in the circuit. And this current is called zero signal collector current.

When the voltage signal applies during the positive half cycle, the forward bias on emitter-base junction increases which increases the total collector current I_C. Whereas, the negative half cycle of the emitter-base junction voltage reduces the collector current. The collector current has two components. They are

1. The DC collector current I_C when no signal is applied. This is because of forwarding bias caused by the emitter-base junction by the bias V_BB.
2. The AC collector current i_cs due to a signal applied to the emitter-base junction.

The total collector current i_C = i_cs + I_C

The useful output is the voltage drop across the collector load resistor R_C due to the AC component of the currents i_cs which through the load because of the applied signal. The purpose of the DC component of the collector current is to keep the emitter-base junction in forward biasing at all times.