INTRODUCTION

Comparison Study of audio amplifier performances using Class AB, B and D topologies in portable audio devices.

Amplifier plays an important role in an audio system. As it simply amplify the input audio signal to a certain power level to drive the speaker to bring the original desired signal to live. In recent market, Class AB amplifier seems to be dominating the audio market. Class B operation is opposite of Class A.

Figure 1: Class B, AB and D Amplifier

Class B amplifiers are those in which the output transistors only conduct during half (180 degrees) of the signal waveform. To amplify the entire signal, they use two transistors, one conducting for positive output signals and the other conducting for negative outputs. Class B amplifiers are much more efficient than Class A amplifiers, but they have high distortion due to the crossover point when the two transistors transition from on to off.

Class AB amplifiers combine Class A and Class B to form an amplifier with more efficiency than Class A but with lower distortion than Class B. This performance is achieved by biasing both transistors to conduct near zero signal output-the point where Class B amplifiers introduce nonlinearities. They then transistor to Class B for large excursions.

Class D amplifiers are those that output a switching waveform at a frequency for higher than the highest audio signal that needs to be reproduced. The lowpass-filtered, average value of this waveform corresponds to the actual required audio waveform. This amplifiers are highly efficient (often up to 90% or higher) because the output transistors are either fully turned on or off during operation. Modern Class D amplifiers achieve fidelity comparable to Class AB.