This document discusses different classes of amplifiers, including: 1. Class-A amplifiers are the simplest design and can produce the most distortion-free output. However, they are very inefficient due to having current flowing through the output devices at all times. 2. Class-B amplifiers use two transistors or groups of transistors, with one reproducing the top half of the waveform and the other reproducing the bottom half. They are more efficient than class-A but can produce crossover distortion. 3. Class-AB amplifiers are a compromise between class-A and class-B. They have a small bias current flowing at all times to eliminate crossover distortion while being more efficient than class-A