Class A Amplifiers
The class A amplifier is typically used as the first amplification stage of a receiver or a transmitter where minimum noise is desired. This is achieved with a cost of relatively low efficiency. In a receiver the first stage in an amplifier chain handles low power levels, so the low efficiency of the first amplifiers actually wastes little power. Power amplifiers with different class designations are used in later stages. The variety of amplifier classes are described in  and will be covered more extensively in Chapter 9. The primary properties of importance to class A amplifier design are gain, bandwidth control, stability, return loss, and noise figure. Noise figure was considered in Chapter 7, but the other topics are described in the present one.
8.2 DEFINITION OF GAIN 
In low-frequency circuits, gain is often thought of in terms of voltage or current gain, for example, the ratio of the output voltage across the load to the input applied voltage. At radio frequencies it is difficult to directly measure a voltage, so typically some form of gain is used. But once the notion of power is introduced, there are several definitions of power gain that might be used.
1. Power Gain This is the ratio of power dissipated in the load, ZL, to the power delivered to the input of the amplifier. This definition is independent of the generator impedance, ZG. Certain amplifiers, especially negative resistance amplifiers, are strongly ...