Power Amplifier Evolution#

How audio power amplifiers got from “one transistor, lots of heat” to “switching at megahertz, almost no heat.”

This section traces the design evolution of audio power amplifiers — not as a textbook taxonomy of classes, but as a chain of engineering problems and solutions. Each stage in the story solves the most pressing limitation of the previous design, and in doing so reveals a new problem that drives the next iteration.

The pattern is remarkably consistent:

Start simple — a single transistor amplifying a signal into a load.
Hit a wall — efficiency, distortion, stability, or some combination.
Solve the immediate problem — split the output stage, add bias, add feedback, switch to PWM.
Discover what the solution broke — crossover distortion, thermal runaway, switching noise.
Repeat.

This is how real circuit design works. Nobody sat down and invented the Class-AB amplifier from first principles. Engineers started with what they had, ran into limitations, and iterated. Understanding that progression builds better intuition than memorizing a table of amplifier classes.

Pages#

Single-Transistor Class A — Where it all starts: one transistor, full-time conduction, and a brutal efficiency problem.
Class B Push-Pull — Split the job between two transistors. Efficiency jumps, but now there’s a dead zone.
Class AB Output Stage — Bias the output transistors slightly on to kill crossover distortion — then wrestle with thermal stability.
The Complete Linear Architecture — Differential pair, voltage amplifier stage, driver, output: the multi-stage topology that dominates discrete analog amp design.
The Fork — Two divergent paths: refine linear amplifiers to perfection, or abandon analog output entirely.
Refining Linear Amplifiers — Better bias, better feedback, better devices — pushing Class AB toward vanishing distortion.
Class D Switching — Encode audio as pulse widths, switch the output stage, filter to recover the signal. Efficiency above 90%.
Lessons and Canonical Structure — What the evolutionary arc teaches about amplifier design: recurring themes and the canonical linear topology.