A Class-AB amplifier is a type of electronic amplifier that combines the features of both Class-A and Class-B amplifiers to achieve a balance between efficiency and linearity. It's commonly used in audio amplification applications where both power efficiency and signal fidelity are important.
To understand the operation of a Class-AB amplifier, let's break it down:
Biasing: Like Class-A amplifiers, Class-AB amplifiers have a biasing setup. Biasing is the process of providing a small, constant current to the output transistors (typically bipolar junction transistors or BJTs) even when no input signal is present. This biasing ensures that the transistors are operating in their active region (part of the characteristics where the transistor acts as an amplifier) even for small input signals.
Operating Quiescent Point: The biasing establishes an operating point, known as the quiescent point or Q-point, on the transistor's load line. This point is typically located near the center of the transistor's active region.
Class-B Operation: In a pure Class-B amplifier, each transistor handles only half of the input signal waveform. One transistor amplifies the positive half of the waveform, while the other handles the negative half. This division of labor improves efficiency but introduces a distortion known as crossover distortion at the point where the two halves switch over.
Crossover Distortion: In Class-B amplifiers, when the input signal crosses zero, there's a small gap during which both transistors are off. This gap leads to distortion, as the output momentarily drops to zero or close to it. Crossover distortion can be audible in audio applications and is undesirable.
Class-AB Operation: Class-AB amplifiers address the issue of crossover distortion by allowing a small overlap between the conduction regions of the two transistors. In other words, both transistors are slightly turned on around the zero-crossing point of the input signal. This overlap ensures that there's a continuous flow of current through the output stage, reducing the distortion caused by the gaps in pure Class-B amplifiers.
Efficiency and Linearity: The slight overlap of conduction regions allows Class-AB amplifiers to maintain relatively low distortion while also improving efficiency compared to pure Class-A amplifiers. Class-AB amplifiers are not as efficient as Class-B amplifiers but provide much better linearity and reduced distortion.
In summary, a Class-AB amplifier combines the benefits of Class-A (low distortion) and Class-B (higher efficiency) amplifiers by biasing the transistors to operate in their active region even for small signals, thereby reducing crossover distortion. This makes Class-AB amplifiers suitable for applications where both signal fidelity and power efficiency are crucial, such as audio amplification in speakers and headphones.