Amplifier
In general, an amplifier is an electronic circuit designed by using an active device (e.g. a bipolar junction transistor (BJT), vacuum tube, or field-effect transistor (FET)) that changes, usually increases, the amplitude of an “input signal,” usually voltage or current. For example, in a public addressing system, an amplifier is used to magnify the electrical signal produced by a microphone to operate loudspeakers to make the human voice louder.
However, the relationship between the input and the output signals of an amplifier is usually expressed as a function of input frequency, commonly known as the transfer function of the amplifier. In this section, we will discuss the principle of operation of a BJT-based amplifier, which can be used to amplify the signals with smaller amplitudes (with respect to the DC biasing currents and voltages of the transistor circuit discussed in the preceding chapters).
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