7.2 Amplifier Classes

There are several classes of amplifier operation. The class of an amplifier has nothing to do with the fidelity or quality of the amplifier. Rather, the class indicates the fundamental operational principle of the circuit. In general, as the class letter increases, the designs become more complicated but also more efficient. For audio and other linear applications, classes A, B and D are relatively common these days. Class C is largely relegated to high power telecommunications while classes G and H are essentially variations of class B.

The definition of class A is that signal current in the collector flows 360° out of the cycle. In other words, it flows for the entire cycle without interruption. All of the amplifiers that were presented in the prior chapter are class A designs. In class B, I_C flows for just 180°, and for class D, I_C is discontinuous; the transistor is used as a switch. Class B and D designs are examined in later chapters.