Q. Explain about Transistor Amplifiers?

Amplifiers are circuits that produce an output signal which is larger than, but proportional to, an input signal. The input and output signals can be both voltages or currents, or one or the other, as in voltage-in current-out and current-in voltage-out amplifiers. The amplifier gain is just the network's transfer function, which is the ratio of output-to-input complex signals in the frequency domain as found by complex analysis.

Amplifiers find extensive use in instrumentation applications. Sometimes, amplifiers are used for reasons other than gain alone. An amplifier may be designed to have high input impedance so that it does not affect the output of a sensor while at the same time giving a low output impedance so that it can drive large currents into its load, such as a lamp or heating element. In some other applications, an amplifier with a low input impedance might be desirable.

The first step in designing or analyzing any amplifier is to consider the biasing. The biasing network consists of the power supply and the passive circuit elements surrounding the transistor that provide the correct dc levels at the terminals. This is known as setting the Q point (quiescent or operating point) with no signal applied. A good bias circuit must not only establish the correct dc levels, but must maintain them in spite of changes in temperature, variations in transistor characteristics, or any other sources of variation.