How it works
Amplifier class is defined by the conduction angle of the output transistor — the fraction of the input cycle during which it conducts current. Class A conducts for the full 360°; the Q-point is centred, so the transistor always draws quiescent current even with no signal. Maximum efficiency is only 25% for transformer-coupled and 50% for direct-coupled designs. Class B conducts for exactly 180°; each transistor in a push-pull pair handles one half-cycle, giving maximum efficiency of 78.5%. However, crossover distortion appears at the zero-crossing where neither transistor fully conducts. Class AB biases transistors just above cutoff (conduction angle slightly over 180°) using a VBE multiplier or two diodes to eliminate crossover distortion while maintaining efficiency between Class A and B values. Class C conducts for less than 180°, achieves efficiency above 85%, but produces heavy distortion — used only in tuned RF power amplifiers where a tank circuit restores the waveform.
Key points to remember
Conduction angles: Class A = 360°, Class B = 180°, Class AB = 180°–360°, Class C < 180°. Maximum theoretical efficiency: Class A = 50% (RC-coupled), Class B = 78.5%, Class C > 85%. Class D (switching) can exceed 90% efficiency by driving transistors between saturation and cutoff using PWM — used in modern audio amplifiers and motor drives. Crossover distortion in Class B is reduced by biasing with two silicon diodes (≈ 1.4 V total) to provide a small forward bias to both transistors. Heat dissipation in Class A is constant and maximum at zero signal; in Class B, dissipation peaks at about 40% of maximum output power, not at full output.
Exam tip
The examiner always asks you to compare efficiency and distortion across Classes A, B, and C — state 50%, 78.5%, and >85% respectively, explain conduction angles, and describe how Class AB eliminates crossover distortion.