Question

Which of the following transistor configurations can be used as an amplifier?
A) Common base amplifier
B) Common emitter amplifier
C) Common collector amplifier
D) None of these

Answer 1

Hint:Transistor acts as an amplifier in the active region of its output characteristics. The active region, also called the linear region, lies between the saturation and cut-off region and it is the region where the emitter-base junction is forward biased while the collector-base junction is reverse biased.

Complete step by step answer.
Step 1: With the help of a graph, explain the transfer characteristics of a transistor when it acts as an amplifier.
An amplifier amplifies the input signal, i.e., it strengthens a weak input signal. A transistor acts as an amplifier in the active region of its output characteristics (a graph between the output voltage and the output voltage).

The above graph shows the output characteristics of a CE configuration. The three regions of importance are the saturation region, the active region and the cut-off region.
The active region depicts an increase in the collector-emitter voltage once saturation is attained. The input junction (base-emitter) is forward biased and the output junction (emitter-collector) is reverse biased in this region. Beyond this region, a transistor is most likely to have a breakdown.
The output characteristics of all the three transistor configurations have an active region.
Therefore, all three transistor configurations – can be used as an amplifier.

Additional information: There are three configurations for a transistor – common-emitter, common-base and common-collector.
As the name suggests, in the common-emitter configuration, the emitter is common to the base and the collector. Input will be supplied at the base terminal and output will be obtained at the collector terminal.
In the common-base configuration, the base is set to be the common terminal. Input will be supplied at the emitter terminal and output will be obtained at the collector terminal.
Lastly in the common-collector terminal, the collector is common to the base and the emitter. Input will be supplied at the base terminal and output will be obtained at the emitter terminal.

The common emitter configuration is the most widely used transistor configuration.

Note: The input characteristics control the output characteristics. In the CE configuration, the output current (collector) is controlled by the input current (base current).
The voltage gain of the CE amplifier is negative. This implies that the output voltage is the amplified version of the input but it is out of phase with the input.