Power Amplifiers Basics: Turning Tiny Signals Into Big Sound

Small-signal amplifiers boost voltage, but a loudspeaker needs real power: voltage times current. A power amplifier is the final stage that takes a small signal and delivers watts into a low-impedance load such as a speaker, motor, or antenna. Without it, your phone's music would be inaudible.

Power amplifiers are classed by how much of the input cycle their transistors conduct: class A, B, AB, and C. Each trades efficiency against distortion, and choosing the right class is the central design decision, because a few percent of efficiency at tens of watts decides between a cool chip and a smoking one.

Key Concepts

• Class A: conducts the full 360°; most linear but least efficient, 25% maximum with resistive load (50% transformer-coupled).
• Class B: each transistor conducts 180° in push-pull; maximum theoretical efficiency 78.5%.
• Crossover distortion: near zero crossing neither class-B transistor conducts (about 0.7 V is needed to turn on), creating a harsh dead zone.
• Class AB: a small idle current keeps both transistors slightly on, removing crossover distortion; nearly class-B efficiency with near class-A linearity. Dominates audio.
• Class C: conducts under 180°; very efficient but distorted, used only with tuned LC circuits in RF transmitters.
• Efficiency (η): AC load power divided by DC supply power; the rest becomes heat.
• Heat sink: finned metal carrying waste heat away; at tens of watts it is not optional.
• Complementary pair: matched NPN and PNP transistors working push-pull.

Formulas

• Output power: Pout = Vrms2/RL = Vpeak2/(2RL) = Vpp2/(8RL) watts.
• Efficiency: η = (Pout/Pdc) × 100%.
• Class B maximum: π/4 ≈ 78.5% at maximum unclipped swing.
• Heat to remove: Pdissipated = Pdc − Pout.

Worked Example

A class B amplifier drives an 8 Ω speaker with a 10 V peak sine wave. Find the output power.

1. Vrms = 10/√2 = 7.07 V. Pout = 7.072/8 = 50/8 = 6.25 W.
2. Near 78.5% efficiency, the supply delivers about 6.25/0.785 = 7.96 W and roughly 1.7 W becomes heat.
3. The 10 V peak needs rails above 10 V plus saturation drops, so ±15 V supplies are comfortable.

Common Misconceptions

• Double the watts means double the loudness. Hearing is logarithmic: doubling loudness needs roughly ten times the power.
• Class A is best everywhere. Its terrible efficiency means huge heat sinks and power bills; class AB rules audio, class C rules RF.
• Heat sinks are optional for small amplifiers. Even a 5 W chip can destroy itself without cooling when driven hard.

Real-World Applications

• Home and car audio use class AB (and now class D switching) amplifiers to drive speakers.
• Stadium PA systems deliver hundreds of watts from high-power stages.
• Radio and TV transmitters use class C for efficient RF power.
• Servo drivers use power stages to move heavy loads precisely.

To recap: power amplifiers deliver watts into low-impedance loads. Class A is linear but wasteful, class B is efficient but distorts at crossover, class AB fixes that and dominates audio, class C serves RF. Efficiency math and heat sinking decide whether the design survives.