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Design and analysis of a basic class D amplifier

Posted: 13 Jan 2006 ?? ?Print Version ?Bookmark and Share

Keywords:power amplifier? class D? PSpice?

In today's world, power amplifiers are used in many devices for a wide variety of applications. In general, an amplifier takes a low power input and regenerates the signal at several watts higher. Ideally, the input will be reproduced without any changes with an efficiency of 100%. Various types of power amplifiers exist and they are classified by the amount of time that the transistors conduct.

Class A
Class A amplifiers are always turned on. This means that the transistor provides power to the output even when no input signal is introduced. Hence, the transistor can become hot with most of the power provided lost as heat. Although efficiency is poor (around 20%), accuracy is quite high.

Figure 1.1: Class A amplifier

Class B
Class B amplifiers use two transistors. Each transistor is turned on for half of the time. One transistor operates during the positive cycle of the input, while the other is used for the negative cycle. Therefore in theory, both are never on at the same time. When there is no input, both transistors are turned off and no power appears at the output. For this reason, efficiency is better than class A amplifiers. However, due to the fact that transistors take some time to turn on, there is a moment when no power appears at the output. This powerless region is called the crossover region, as shown in Fig. 1.2, and introduces a relatively large amount of distortion. This class B amplifier has very good efficiency, but poor accuracy.

Figure 1.2: Crossover Region in Class B amplifier

Class AB
Class AB amplifiers are very similar to class B amplifiers, but their performance is improved by the addition of two diodes that eliminate the crossover region and allows both transistors to be turned on at the same time. The efficiency (around 50%) is not as high as class B because both transistors are turned on simultaneously, but accuracy is improved. It is the most commonly used audio power amplifier.

Figure 1.3: Class AB amplifier

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