input and output characteristics of common emitter configuration
The graph between the variation of voltage and variation of currents when emitter of a transistor is common to both input and output circuits are known as common emitter characteristics of a transistor.
Circuit diagram to study input and output characteristics of common emitter configuration of a transistor is shown in the figure.
Characteristics of a transistor in common emitter configuration are of two types:
- input characteristics.
- output characteristics.
The graphs showing the variation of base current IB (input) with the variation of emitter-base voltage (VEB) at a constant collector-emitter voltage (VCE) are called input characteristics.
The set of input characteristics are shown in the figure.
The input characteristics of a PNP transistor are just like the characteristics of a forward-biased diode when the collector of the transistor is short-circuited to the emitter and the emitter is forward biased.
When VBE= 0, IB = 0 because in this case both the junction i.e. emitter-base junction and collector-base junction are short-circuited. As the value of VCE increases, then due to the Early effect, the width of the base region decreases, and hence the recombination of holes and electrons in the base agent decreases. As a result of this, the recombination current ( i.e. base current IB ) decreases at a given Value of VBE .
The graphs showing the variation of collector current IC (output) with the variation of collector-emitter voltage at constant base current IB (input) are known as output characteristics of a transistor in the common-emitter configuration.
A set of output characteristics are shown in the figure.
output characteristics of a transistor in common emitter configuration are divided into three regions:
- Active region.
- cutoff region.
- saturation region.
In this region, collector junction is Reverse Biased and emitter junction is forward biased. This region lies above IB = 0 and to the right of the ordinate VCE= a few tenths of a volt. For a given value of IB the value of IC increases due to early effect as (VCE) increases.
Note. Transistor is operated in an active region if it is used as an amplifier.
The cutoff region lies below IB = 0. However, it is clear from the figure that the collector current has a significant value under this condition. In order to cut off the transistor, the emitter junction has to be made slightly reverse biased in addition to IB.=0.
In this region, both emitter and collector junctions are forward biased equal to cut in voltage. Saturation region lies close to the zero voltage axis where all the curves coincide and fall rapidly toward the origin.