Power Amplifiers: Class A, B and AB With easy examples.

A power amplifier delivers real power, voltage and current, into low-impedance loads like loudspeakers. Small-signal stages worry about voltage gain; power stages worry about efficiency, heat and distortion at watts or kilowatts. Classes A, B, AB and C differ in what fraction of the cycle the transistors conduct, and that choice sets the trade-off between quality and wasted heat.

Every speaker and antenna is driven by one: home stereos, concert PAs, phone audio, radio transmitters. Class choice decides battery life and heat-sink size, so it is the heart of real amplifier design.

Key Concepts

• Q-point: The DC bias point with no signal; its placement defines the class.
• Class A: Conducts the full 360 degrees, Q-point centered. Lowest distortion, but dissipates power even idle. Max theoretical efficiency: 25 percent resistive (50 percent transformer-coupled).
• Class B: Push-pull pair, each device conducts 180 degrees. Max efficiency pi/4 ≈ 78.5 percent, but both devices are off near zero crossing, causing crossover distortion.
• Class AB: Small idle bias, conduction between 180 and 360 degrees. Crossover distortion gone, efficiency far above Class A. Nearly all hi-fi amps are AB.
• Crossover distortion: The kink near zero crossing in Class B: each base-emitter junction needs ~0.7 V before conducting.
• Push-pull: Complementary NPN/PNP (or n/p MOSFET) pairs take turns sourcing and sinking load current.
• Efficiency: Load power over DC supply power; the rest becomes heat.

Formulas

• Sine load power: Pout = Vpeak^2/(2 x RL).
• Class B max: pi/4 = 78.5 percent at near rail-to-rail swing.
• Class A max: 25 percent resistive; 50 percent transformer-coupled.
• Class B supply power: Pdc = (2 x Vcc x Vpeak)/(pi x RL) for ±Vcc supplies.
• Limits: Ideal transistors, perfect sines. Real saturation voltage and reactive speaker loads always reduce practical efficiency.

Worked Example

Class B push-pull on ±12 V driving 8 ohms with a 10 V peak sine. Step 1: Pout = 100/16 = 6.25 W. Step 2: Pdc = (2 x 12 x 10)/(pi x 8) = 240/25.13 = 9.55 W. Step 3: efficiency = 6.25/9.55 ≈ 65 percent. Step 4: 3.3 W becomes heat, needing modest heat sinks. Class A at best 25 percent would burn over 18 W for the same output, which is why high-power audio abandoned it.

Common Misconceptions

• "Class A is always best." Best linearity, worst efficiency. Class AB keeps the linearity and fixes the efficiency.
• "Crossover distortion is clipping." Clipping hits the rails at large signals; crossover is a dead zone at zero crossing from the 0.7 V threshold, harshest on quiet passages.
• "Bigger supply means more power." Power is limited by load, transistor ratings and thermals together; raising supply alone destroys devices.

Real-World Applications

• Hi-fi and PA: Class AB push-pull dominates stereos, guitar amps and concert rigs.
• Portable audio: Efficient AB or D stages protect battery life in phones and headphones.
• Radio: RF power finals, often Class C for FM, feed antennas with watts to kilowatts.
• Motor drives: Push-pull stages drive motors under the same efficiency/thermal rules.

Classes are conduction angles: A conducts 360 degrees with best linearity but ≤25 percent efficiency, B conducts 180 per device for ≤78.5 percent but crossover distortion, AB adds a touch of bias to erase the kink while staying efficient. The example is concrete: 6.25 W into 8 ohms from 12 V rails runs 65 percent in Class B, wasting 3.3 W, where Class A would burn most of the power as heat.