Explain the working of BJT Amplifiers?

The purpose of electronic amplifiers is essentially to increase the amplitude and power of a signal so that either useful work is done or information processing is realized. The output signal power being greater than the input signal power, the additional power is supplied by the bias supply.

Thus, the amplifier action is one of energy conversion in which the bias power is converted to signal power within the device. A single-stage amplifier is one in which there is only one amplifying element. By combining several single-stage amplifier circuits, a multistage amplifier is produced. Audio amplifiers are designed to amplify signals in the frequency range of 30 to 15,000 Hz perceptible to the human ear. A video amplifier is designed to amplify the signal frequencies needed for television imaging (see Chapter 15). An amplifier, in general, is then made up of a cascade of several stages. A stage usually consists of an elementary amplifier, which normally has only one transistor. The cascade is formed by making the output of the first stage as the input of the second stage, the output of the second stage as the input of the third stage, and so on. This section is devoted to the study of three basic forms of amplifier stages which use a BJT.