What is a Class AB amplifier?
1 Answer
A Class AB amplifier is a type of electronic amplifier used in audio and radio frequency applications. It is designed to combine the benefits of both Class A and Class B amplifiers, addressing some of their respective inefficiencies.
Class A amplifiers are known for their high-quality output but suffer from low efficiency, as they continuously draw current and generate heat even when there is no input signal. Class B amplifiers, on the other hand, are more efficient but suffer from crossover distortion—a distortion that occurs when the signal transitions between the positive and negative halves of the waveform, leading to an audible distortion at the crossover point.
Class AB amplifiers attempt to strike a balance between these two classes, offering improved efficiency while still maintaining relatively good signal quality. The basic operation of a Class AB amplifier is as follows:
Biasing: Class AB amplifiers incorporate a small bias current in the output transistors to ensure that they are slightly conducting even in the absence of input signals. This biasing eliminates the crossover distortion issue found in Class B amplifiers.
Signal Amplification: When an input signal is applied to a Class AB amplifier, it uses both the positive and negative halves of the input waveform to drive the respective output transistors. During the positive half-cycle of the input waveform, the positive transistor conducts current, while the negative transistor remains slightly biased off. Conversely, during the negative half-cycle of the input waveform, the negative transistor conducts current, while the positive transistor remains slightly biased off.
Transition: As the input signal transitions from positive to negative (or vice versa), both output transistors are active for a brief moment to ensure a smooth handover, minimizing crossover distortion.
By allowing both output transistors to work during the transition, Class AB amplifiers significantly reduce the crossover distortion compared to Class B amplifiers while also improving efficiency compared to Class A amplifiers.
Class AB amplifiers are commonly used in audio applications, such as stereo amplifiers, where good audio quality and reasonable efficiency are important. However, it's worth noting that there are also other amplifier classes like Class D (digital) and Class T (a proprietary digital amplifier technology from Tripath) that provide even higher efficiency, albeit with different trade-offs in terms of complexity and potential distortion.
Class A amplifiers are known for their high-quality output but suffer from low efficiency, as they continuously draw current and generate heat even when there is no input signal. Class B amplifiers, on the other hand, are more efficient but suffer from crossover distortion—a distortion that occurs when the signal transitions between the positive and negative halves of the waveform, leading to an audible distortion at the crossover point.
Class AB amplifiers attempt to strike a balance between these two classes, offering improved efficiency while still maintaining relatively good signal quality. The basic operation of a Class AB amplifier is as follows:
Biasing: Class AB amplifiers incorporate a small bias current in the output transistors to ensure that they are slightly conducting even in the absence of input signals. This biasing eliminates the crossover distortion issue found in Class B amplifiers.
Signal Amplification: When an input signal is applied to a Class AB amplifier, it uses both the positive and negative halves of the input waveform to drive the respective output transistors. During the positive half-cycle of the input waveform, the positive transistor conducts current, while the negative transistor remains slightly biased off. Conversely, during the negative half-cycle of the input waveform, the negative transistor conducts current, while the positive transistor remains slightly biased off.
Transition: As the input signal transitions from positive to negative (or vice versa), both output transistors are active for a brief moment to ensure a smooth handover, minimizing crossover distortion.
By allowing both output transistors to work during the transition, Class AB amplifiers significantly reduce the crossover distortion compared to Class B amplifiers while also improving efficiency compared to Class A amplifiers.
Class AB amplifiers are commonly used in audio applications, such as stereo amplifiers, where good audio quality and reasonable efficiency are important. However, it's worth noting that there are also other amplifier classes like Class D (digital) and Class T (a proprietary digital amplifier technology from Tripath) that provide even higher efficiency, albeit with different trade-offs in terms of complexity and potential distortion.