There are five classes of amplifiers: A, B, AB, C and D commonly used today. With the exception of “D”, all the others are termed linear amplifiers because the output stages do not operate near the power supply rails. Class A amplifier topology is the most linear, therefore lowest distortion. The output transistors continuously conduct current throughout the complete audio cycle. The down side of this amplifier is the efficiency of 20%, with heat management being a significant challenge. As a result, it requires large heat sinks due to 80% of the power input being dissipated as heat in the amplifier. A Class B amplifier’s output stage is in a push-pull configuration. One device conducts in the positive region, and other in the negative region. This improves the efficiency somewhat to about 50%. One disadvantage is a condition called cross-over distortion which results from the time it takes the output transistors to switch off one amplifier and turn on the other. Class AB amplifiers are used in all of today’s single chip audio ICs that are not Class D topology. To minimize the cross-over distortion, the output stage can be biased in a combination of Class A and B topology. It uses 2 symmetrical output transistors, each conducting for approximately ½ the cycle. Modern Class AB amps are commonly between 50–60% efficient. Class C amplifiers are commonly used for RF transmission, and not generally used for audio applications, due to very high distortion figures.