Power Amplifier Evolution on EE Notebook

Single-Transistor Class A

Mon, 01 Jan 0001 00:00:00 +0000

Single-Transistor Class A#

The simplest possible audio power amplifier: one transistor conducting all the time, linearly reproducing the input signal. This is where the story starts — and where the efficiency problem first becomes obvious.

The Circuit#

A single NPN BJT in common-emitter configuration, biased at a quiescent point roughly halfway between the supply rails. The load (speaker or resistor) sits in the collector circuit. An input signal swings the base voltage, modulating the collector current and producing an amplified version of the input across the load.

Class B Push-Pull

Mon, 01 Jan 0001 00:00:00 +0000

Class B Push-Pull#

The efficiency fix for Class A: split the output stage into two transistors, each handling half the signal. One conducts on positive swings, the other on negative swings. Neither conducts at idle. The efficiency jumps dramatically — but a new kind of distortion appears.

The Circuit#

A complementary pair — one NPN and one PNP — configured as emitter followers. Both emitters connect to the load (speaker). The NPN handles the positive half-cycle: when the input goes positive, the NPN turns on and sources current into the load. The PNP handles the negative half-cycle: when the input goes negative, the PNP turns on and sinks current from the load.

Class AB Output Stage

Mon, 01 Jan 0001 00:00:00 +0000

Class AB Output Stage#

The fix for crossover distortion: bias the push-pull output transistors slightly into conduction so the dead zone disappears. This turns out to work extremely well — but introduces a new set of problems related to bias stability and thermal behavior.

The Circuit#

Same complementary push-pull topology as Class B, but with a bias network inserted between the bases of the NPN and PNP output transistors. This network holds the bases apart by approximately 2 × V_BE (about 1.2-1.4V), ensuring both devices are slightly on at idle.

The Complete Linear Architecture

Mon, 01 Jan 0001 00:00:00 +0000

The Complete Linear Architecture#

The Class AB output stage is a good power delivery mechanism — but it can’t stand alone. It needs a front end to provide gain, reject noise, and enable feedback. The result is the canonical multi-stage linear amplifier: a topology so successful that nearly every discrete audio power amplifier built since the 1970s follows this pattern.

The Architecture#

Four stages, each with a distinct job:

The Fork

Mon, 01 Jan 0001 00:00:00 +0000

The Fork#

By the late 20th century, the Class AB linear amplifier was mature. The multi-stage architecture worked. Distortion was vanishingly low. Feedback tamed every nonlinearity. The remaining problem — efficiency — was fundamental to the approach.

Two paths diverged:

Path 1: Refine the Linear Amplifier#

Accept the efficiency limitation and push everything else to the extreme. Better bias circuits. Better matched devices. Cascoded stages for higher gain. Current mirrors for better linearity. MOSFET output devices for easier thermal management. Error-correction circuits that supplement feedback.

Refining Linear Amplifiers

Mon, 01 Jan 0001 00:00:00 +0000

Refining Linear Amplifiers#

Once the basic multi-stage Class AB architecture was established, decades of refinement pushed linear amplifier performance to extraordinary levels. The efficiency problem was accepted as fundamental; everything else was optimized.

Better Input Stages#

Current Mirror Loading#

Replacing the simple resistor load on the differential pair with a current mirror doubles the gain of the input stage and improves balance. The mirror forces equal collector currents in both halves, converting the full differential signal into a single-ended output without losing half the signal.

Class D Switching

Mon, 01 Jan 0001 00:00:00 +0000

Class D Switching#

The radical departure: instead of using output transistors as continuously variable current sources (linear operation), use them as switches — fully on or fully off. Encode the audio signal as a pattern of switching, then filter the output to recover the audio.

The Core Idea#

A PWM (pulse-width modulation) Class D amplifier works in three stages:

1. Modulator#

The audio input is compared against a high-frequency triangle or sawtooth wave (typically 300kHz-1MHz). When the audio signal is higher than the triangle, the output is high; when it’s lower, the output is low. The result is a rectangular wave whose pulse widths encode the audio signal.

Lessons and Canonical Structure

Mon, 01 Jan 0001 00:00:00 +0000

Lessons and Canonical Structure#

The evolution from single-transistor Class A to modern Class D is a case study in how engineering design actually progresses. Each stage solved one problem and revealed the next. Looking back, several themes recur throughout the arc.

Recurring Engineering Themes#

Efficiency vs. Linearity#

This is the central tension. A transistor operating in its linear region provides smooth, faithful amplification — but it dissipates the voltage difference between supply and output as heat. A transistor operating as a switch wastes almost no power — but a switch is inherently nonlinear.