How does a Class AB amplifier combine aspects of Class A and Class B amplifiers?
1 Answer
A Class AB amplifier combines aspects of both Class A and Class B amplifiers to achieve a balance between efficiency and linearity. Let's briefly recap what Class A and Class B amplifiers are before understanding how Class AB combines their features:
Class A Amplifier:
A Class A amplifier operates in such a way that the output transistors conduct throughout the entire input signal cycle. In other words, they are biased to be "on" all the time, regardless of whether the input signal is present or not. Class A amplifiers offer excellent linearity, which means they faithfully amplify the input signal without introducing significant distortion. However, they are highly inefficient because the output transistors consume power continuously, even when there is no input signal variation.
Class B Amplifier:
A Class B amplifier operates in a push-pull configuration, where two complementary transistors (NPN and PNP) handle the positive and negative halves of the input signal, respectively. Each transistor conducts only during its respective half of the input signal cycle. Class B amplifiers are much more efficient than Class A amplifiers since the output transistors are only active when needed. However, there is a drawback called "crossover distortion," which occurs around the zero-crossing point when one transistor turns off, and the other turns on. This distortion can be audible in audio applications.
Now, here's how a Class AB amplifier combines aspects of both Class A and Class B:
Biasing:
A Class AB amplifier is biased so that the output transistors operate in a slightly turned-on state even when there is no input signal. This means that they conduct a small amount of current even during the zero-crossing point of the input signal. By doing so, the amplifier reduces the crossover distortion inherent in Class B amplifiers. The biasing point is carefully set to minimize distortion while also preserving efficiency.
Efficiency and Linearity:
During low-level signals, a Class AB amplifier operates in a manner similar to a Class A amplifier. The output transistors conduct through the entire input signal cycle, providing good linearity and low distortion. As the input signal increases in amplitude, the amplifier transitions into a Class B-like operation, with one output transistor turning off while the other handles the respective portion of the signal.
Reduced Power Dissipation:
Because of the biasing arrangement, Class AB amplifiers are more efficient than pure Class A amplifiers, but not as efficient as pure Class B amplifiers. The power dissipation is significantly reduced compared to Class A, making them more suitable for moderate to high-power applications.
Overall, the Class AB amplifier is a popular choice for many audio applications, as it strikes a balance between the linearity of Class A and the efficiency of Class B, while also mitigating the drawbacks of each.
Class A Amplifier:
A Class A amplifier operates in such a way that the output transistors conduct throughout the entire input signal cycle. In other words, they are biased to be "on" all the time, regardless of whether the input signal is present or not. Class A amplifiers offer excellent linearity, which means they faithfully amplify the input signal without introducing significant distortion. However, they are highly inefficient because the output transistors consume power continuously, even when there is no input signal variation.
Class B Amplifier:
A Class B amplifier operates in a push-pull configuration, where two complementary transistors (NPN and PNP) handle the positive and negative halves of the input signal, respectively. Each transistor conducts only during its respective half of the input signal cycle. Class B amplifiers are much more efficient than Class A amplifiers since the output transistors are only active when needed. However, there is a drawback called "crossover distortion," which occurs around the zero-crossing point when one transistor turns off, and the other turns on. This distortion can be audible in audio applications.
Now, here's how a Class AB amplifier combines aspects of both Class A and Class B:
Biasing:
A Class AB amplifier is biased so that the output transistors operate in a slightly turned-on state even when there is no input signal. This means that they conduct a small amount of current even during the zero-crossing point of the input signal. By doing so, the amplifier reduces the crossover distortion inherent in Class B amplifiers. The biasing point is carefully set to minimize distortion while also preserving efficiency.
Efficiency and Linearity:
During low-level signals, a Class AB amplifier operates in a manner similar to a Class A amplifier. The output transistors conduct through the entire input signal cycle, providing good linearity and low distortion. As the input signal increases in amplitude, the amplifier transitions into a Class B-like operation, with one output transistor turning off while the other handles the respective portion of the signal.
Reduced Power Dissipation:
Because of the biasing arrangement, Class AB amplifiers are more efficient than pure Class A amplifiers, but not as efficient as pure Class B amplifiers. The power dissipation is significantly reduced compared to Class A, making them more suitable for moderate to high-power applications.
Overall, the Class AB amplifier is a popular choice for many audio applications, as it strikes a balance between the linearity of Class A and the efficiency of Class B, while also mitigating the drawbacks of each.