The common emitter NPN amplifier circuit is shown in the figure below. The source VBB is applied to the input circuit in addition to the signal. The VBB 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 RC.
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 RC. 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 VBB applies no input signal. Therefore the DC collector current IC 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 IC. Whereas, the negative half cycle of the emitter-base junction voltage reduces the collector current. The collector current has two components. They are
- The DC collector current IC when no signal is applied. This is because of forwarding bias caused by the emitter-base junction by the bias VBB.
- The AC collector current ics due to a signal applied to the emitter-base junction.
The total collector current iC = ics + IC
The useful output is the voltage drop across the collector load resistor RC due to the AC component of the currents ics 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.