Q. What are transistor amplifiers? What is meant by efficiency of an amplifier? What is amplifier coupling?

The AMPLIFIER is a device that enables an input signal to control an output signal. The output signal will have some (or all) of the characteristics of the input signal but will generally be larger than the input signal in terms of voltage, current, or power. A transistor amplifier is a current-control device. The current in the base of the transistor (which is dependent on the emitter-base bias) controls the current in the collector.

Once current is controlled it can be used to give a voltage gain or a power gain. The efficiency of an amplifier refers to the amount of power delivered to the output compared to the power supplied to the circuit. Since every device takes power to operate, if the amplifier operates for 360° of input signal, it uses more power than if it only operates for 180° of input signal. If the amplifier uses more power, less power is available for the output signal and efficiency is lower. Since class A amplifiers operate (have current flow) for 360° of input signal, they are low in efficiency.