A Class AB amplifier is a type of electronic amplifier commonly used to amplify audio signals. It is designed to combine the advantages of both Class A and Class B amplifiers while minimizing their drawbacks. Class AB amplifiers are widely used in applications where efficiency and reduced distortion are important, such as in audio equipment like power amplifiers and audio receivers.
Here's how a Class AB amplifier operates:
Biasing: The first step in the operation of a Class AB amplifier is biasing. Biasing involves setting up a DC voltage level (called the bias voltage) at the input of the amplifier. This is done to ensure that the amplifier operates in its active region even when no input signal is present. In Class AB amplifiers, the bias voltage is set such that the output transistors (typically, there are two transistors, one for the positive half of the signal and one for the negative half) are slightly turned on, but not fully conducting.
Amplification: When an AC audio signal is applied to the input of the amplifier, it rides on top of the DC bias voltage. As the input signal varies, it modulates the conduction of the output transistors. For the positive half of the input signal, the transistor responsible for amplifying positive voltages (NPN transistor) begins to conduct more, allowing current to flow from the power supply through the load (usually a speaker) and producing an amplified positive half of the signal at the output. Conversely, for the negative half of the input signal, the other transistor (PNP transistor) becomes more conductive, producing the amplified negative half of the signal at the output.
Crossover Distortion Reduction: One of the drawbacks of Class B amplifiers is crossover distortion, which occurs when there is a gap or discontinuity between the positive and negative halves of the signal at the output. Class AB amplifiers aim to minimize this distortion by having both transistors slightly turned on at all times due to the biasing. This ensures that there is a small overlap region where both transistors are conducting, reducing the likelihood of a complete cutoff during signal transitions.
Efficiency Improvement: Compared to a pure Class A amplifier (where transistors are always conducting), Class AB amplifiers are more efficient because the transistors are only conducting when needed, i.e., during signal transitions. This reduced conduction time helps to save power and minimize heat generation.
In summary, a Class AB amplifier operates by biasing its output transistors to operate in their active region even when there is no input signal. When an input audio signal is applied, the transistors amplify the signal by conducting more during the appropriate half-cycle of the input signal. The biasing and overlapping conduction regions help reduce crossover distortion and improve efficiency compared to other amplifier classes.