1. Class A Amplifiers: In a Class A amplifier, the output transistor conducts for the
full $360^\circ$ of the input cycle. The operating point (Q-point) is set exactly in the middle of the load line. Because the transistor is always "on" and operating in its linear region, the output waveform is a very faithful reproduction of the input, resulting in the least distortion.
2. Comparison with other classes:
• Class B: Conducts for only $180^\circ$. This causes significant "crossover distortion" where the signal crosses zero.
• Class AB: Conducts for slightly more than $180^\circ$ to reduce crossover distortion, but it still has more distortion than Class A.
• Class C: Conducts for much less than $180^\circ$. While highly efficient, it produces massive amounts of distortion and is mainly used for RF applications with tuned circuits.
3. The Trade-off: While Class A provides the highest fidelity (least distortion), it is the least efficient (max theoretical efficiency is 25% to 50%) because it draws full current even when no signal is present.