Class AB Audio Amplifiers: Design, Efficiency & Applications

Class AB audio amplifiers are a hybrid design of Class A and Class B amplifiers. By setting a quiescent operating point to keep the transistors slightly on (biased above the cutoff point) throughout the signal cycle, they effectively address the crossover distortion problem of Class B amplifiers. Its core feature is that two complementary transistors (NPN/PNP or NMOS/PMOS) alternate between the positive and negative half-cycles of the input signal, achieving a smooth transition across the zero-crossing region.

Class A amplifiers maintain a constant on-state output transistor throughout the entire signal cycle. This results in excellent sound quality because the signal is uninterrupted, but efficiency is extremely low, with most energy dissipated as heat.

Class B amplifiers, on the other hand, use two transistors: one for the positive half-wave and the other for the negative half-wave. Each transistor conducts for only 180° of the signal half-cycle. This significantly improves efficiency, but the disadvantage is that the two transistors alternate between on and off at the signal's zero-crossing point, which can easily cause crossover distortion and affect sound quality.

Class AB amplifiers are designed to address the crossover distortion problem of Class B amplifiers while maintaining relatively high efficiency.

The core design principles are:

Biasing: Class AB amplifiers apply a small bias current to the output transistors, causing them to conduct slightly even when no signal is input. This way, when the signal crosses zero, both transistors remain slightly on, rather than completely off. This effectively avoids crossover distortion.

Push-pull: A push-pull design typically utilizes a pair of complementary transistors (such as an NPN and a PNP) to amplify the positive and negative half-cycles of the signal, respectively. When the signal is positive, one transistor conducts and amplifies; when the signal is negative, the other transistor conducts and amplifies. Due to the bias current, both transistors conduct for a short period of time (typically more than 180° but less than 360°) when the signal approaches zero, ensuring a smooth transition.

Class AB Audio Amplifier Efficiency
Class AB amplifiers have an efficiency between Class A and Class B.
Class AB amplifiers are more efficient than Class A amplifiers. Class A amplifiers continue to consume significant power even when no signal is present. However, Class AB amplifiers, due to the presence of bias current, consume much less power in idle mode than Class A amplifiers. Their efficiency further improves as signal levels increase.

Compared to Class B amplifiers, Class AB amplifiers introduce bias current, resulting in a certain amount of quiescent power consumption, making them slightly less efficient than pure Class B amplifiers. Class AB amplifiers typically have an efficiency between 60% and 70%. While not as efficient as Class D amplifiers (which typically reach over 90%), Class AB amplifiers offer significant advantages in sound quality.

Applications of Class AB Audio Amplifiers
Class AB amplifiers are popular in a variety of audio applications due to their excellent sound quality and relatively reasonable efficiency:

Home audio systems: These include amplifiers, receivers, and hi-fi systems. Class AB amplifiers deliver clear, low-distortion audio output, meeting the high-quality demands of audiophiles.

Car audio: The automotive environment demands both sound quality and power. Class AB amplifiers are widely used in car amplifiers, delivering a powerful and clear audio experience.

TV and multimedia systems: These amplifiers are integrated into consumer electronics such as TVs and computer speakers to drive built-in speakers or provide audio output.

Headphone amplifiers: Many high-performance headphone amplifiers use a Class AB design to ensure the sound quality and dynamic range required to drive headphones.

Professional audio equipment: Class AB amplifiers are also commonly used in studio monitors, instrument amplifiers, and PA systems for their reliability and sound quality.