Class AB Amplifier Biasing to Avoid Crossover Distortion

A Class AB push-pull amplifier is a type of amplifier configuration that aims to reduce the drawbacks of both Class A and Class B amplifiers. Specifically, it seeks to minimize crossover distortion while maintaining good efficiency.

Understanding Crossover Distortion

Crossover distortion is a type of signal distortion that occurs in amplifier circuits, particularly in complementary symmetry (push-pull) Class B amplifiers. It happens during the transition where the output signal changes from positive to negative, or vice-versa. At this point, the transistors switch conduction from one to the other. In a pure Class B amplifier, the transistors are biased exactly at their cut-off point, meaning they are 'off' when the input signal is near zero volts. This results in a small period where neither transistor is conducting, creating a noticeable 'gap' or distortion in the output waveform, especially noticeable at low signal levels.

Biasing in Amplifier Classes

• Class A: Transistors conduct for the entire input cycle (360°). They are biased in the middle of their active region. This provides very low distortion but is highly inefficient as current flows even without a signal.
• Class B: Transistors conduct for approximately half of the input cycle (180°). They are biased at the cut-off region. This improves efficiency significantly but introduces crossover distortion.
• Class AB: Transistors conduct for slightly more than half of the input cycle (just over 180°).

Class AB Biasing Strategy

To overcome the crossover distortion inherent in Class B amplifiers while retaining better efficiency than Class A, Class AB amplifiers employ a specific biasing technique. The transistors are biased slightly above cut-off. This ensures that each transistor is conducting a small quiescent current even when there is no input signal.

This minimal conduction ensures that as the input signal transitions through zero volts, one transistor is already starting to conduct before the other completely stops. This 'overlap' in conduction periods smoothly bridges the gap that causes crossover distortion in Class B amplifiers. By biasing the transistors slightly above cut-off, the amplifier effectively eliminates the dead zone, resulting in a cleaner output signal without the significant inefficiency of Class A amplifiers.

Evaluating the Options

• In cut-off region: This describes Class B biasing, which leads to crossover distortion.
• Slightly above cut-off: This is the defining characteristic of Class AB biasing, which successfully avoids crossover distortion by ensuring continuous (though minimal) conduction.
• Slightly below saturation: While transistors are conducting, biasing just below saturation isn't the specific method used to prevent crossover distortion in push-pull configurations. Saturation implies the transistor is fully 'on'.
• In saturation region: If transistors were biased in saturation, they would always be fully conducting, resembling Class A operation, but biasing slightly above cut-off is the specific technique for Class AB to balance distortion and efficiency.