How are power amplifiers classified? Explain Class A, Class B and Class AB amplifiers. Give an example for each class of power amplifier.

Power amplifiers are electronic devices that increase the power of a signal. They are commonly used in wireless communications and broadcasting, audio equipment, and other applications where a small signal needs to be amplified for driving a high-power load such as a loudspeaker or antenna. Power amplifiers are classified based on their mode of operation, which is determined by the angle of conduction or the portion of the input signal cycle during which the amplifier conducts. The most common classes of power amplifiers are Class A, Class B, and Class AB.

Here is a step-by-step approach to explain each class, along with a comparison table and examples:

Class A Amplifiers

1. Conduction Angle: Class A amplifiers conduct over the entire 360 degrees of the input cycle.
2. Operation: In Class A amplifiers, the output transistors are always on, which means they are conducting current throughout the entire cycle of the input signal.
3. Efficiency: They are the least efficient, with a theoretical maximum efficiency of 25-30% for inductive loads and around 50% for resistive loads.
4. Linearity: They offer the best linearity and the least amount of signal distortion.
5. Heat Generation: Due to constant current flow, they generate a lot of heat and require substantial cooling.
6. Usage: Class A amplifiers are typically used in applications where sound quality is critical, such as high-fidelity audio.

Example: A simple Class A amplifier can be built using a single transistor where the load is connected in series with the collector of the transistor.

Class B Amplifiers

1. Conduction Angle: Class B amplifiers conduct over 180 degrees of the input cycle.
2. Operation: In Class B amplifiers, there are two output transistors, and each transistor conducts for exactly one-half of the signal cycle. One transistor amplifies the positive half-cycle, while the other amplifies the negative half-cycle.
3. Efficiency: They are more efficient than Class A amplifiers, with a theoretical maximum efficiency of about 78.5%.
4. Linearity: They have more distortion than Class A amplifiers, especially at the crossover point of the waveforms from the two transistors (crossover distortion).
5. Heat Generation: They generate less heat than Class A amplifiers and thus require less cooling.
6. Usage: Class B amplifiers are often used in applications where efficiency is more important than audio fidelity, such as in some power amplification applications.

Example: A push-pull amplifier using a pair of transistors, where one transistor amplifies the positive half of the waveform and the other amplifies the negative half.

Class AB Amplifiers

1. Conduction Angle: Class AB amplifiers conduct more than 180 degrees but less than 360 degrees of the input cycle.
2. Operation: Class AB amplifiers are a hybrid of Class A and Class B. They have two output transistors like Class B amplifiers, but both transistors are allowed to conduct slightly more than half the time. They are biased so that there is a small amount of current that flows through both transistors, even when there is no input signal, reducing crossover distortion.
3. Efficiency: They are more efficient than Class A but less efficient than Class B, with a typical efficiency between 50% and 70%.
4. Linearity: They offer better linearity than Class B amplifiers with minimal distortion.
5. Heat Generation: They generate more heat than Class B but less than Class A amplifiers.
6. Usage: Class AB amplifiers are commonly used in audio amplifiers where a good balance between efficiency and audio fidelity is required.

Example: An audio amplifier using complementary transistors where both devices conduct more than half the time, reducing the crossover distortion found in Class B designs.

Here's a comparison table summarizing the differences:

Feature	Class A	Class B	Class AB
Conduction Angle	360 degrees	180 degrees	>180 degrees, <360 degrees
Efficiency	25-30% (inductive), 50% (resistive)	~78.5%	50-70%
Linearity	Best	Good, with crossover distortion	Better than Class B
Heat Generation	High	Moderate	Moderate to High
Usage	High-fidelity audio	Power applications	Audio amplifiers
Example	Single transistor amplifier	Push-pull amplifier	Complementary transistor amplifier

In summary, Class A amplifiers are best for high-quality audio but are inefficient and generate a lot of heat. Class B amplifiers are efficient but suffer from crossover distortion. Class AB amplifiers strike a balance between the two, offering better efficiency than Class A and better audio quality than Class B.