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how can we use transistor as an amplifier?

A junction diode cannot amplify a signal. A transistor consisting of two p-n junctions, one forward-biased and the other reverse-biased, can, however, be used for amplifying a weak signal. The forward-biased junction has a low-resistance path whereas the reverse-biased junction has a high-resistance path. The weak input signal is applied across the forward-biased (low resistance) junction and the out put signal is taken across the reverse-biased (high resistance) junction. Since the input and output currents are almost equal, the output signal appears with a much higher voltage. The transistor thus acts as an amplifier.

When a transistor is to be operated as amplifier, three different basic circuit connections are possible, as illustrated in the Fig: These are (a) common-base, (b) common-emitter and (c) common-collector circuits. Each of these circuit configurations has specific advantages and limitations.

Whichever circuit configuration, the emitter is always forward-biased while the collector is always reverse-biased. This requires a positive emitter bias and a negative collector bias for the p-n-p transistor (opposite polarities are required for the n-p-n transistor).

(a) Common-Base (or Grounded-Base) Configuration: The input signal is applied across the emitter and the base, while the amplified output signal is taken across the collector and the base.

This circuit provides a very low input resistance, a very high output resistance and a current-gain of just less than 1. Still it provides a good voltage and power amplification. There is no phase reversal between the input and output signals.

(b) Common-Emitter(or Grounded Emitter) Configuration : The input signal is applied across the base and the emitter, while the amplified output signal is taken across the collector and the emitter

This is the most flexible and efficient of the three basic circuits. Its input resistance is somewhat higher and the output resistance is lower than those of the common-base circuit. It provides the highest voltage and power gains of the three circuits. It reverse the phase of the output signal with respect to the input.

(c) Common-collector (or Grounded-Collector) Configuration : The input signal is applied across the base and the collector, wile the amplified output signal is taken across the emitter and the collector.

This circuit provides a relatively high input resistance, a low output resistance and about the same current-gain as the common-emitter circuit. Its voltage-gain, however, is always less than 1, as in a cathode-follower circuit. Correspondingly, the power-gain is low.

The common-collector circuit is primarily used for impedance matching and as a buffer stage.