What's a Power Amp? Theory, Classes, and Sizing Guide

A power amplifier is an electronic circuit designed to deliver high current and voltage to a low-impedance load, converting a low-power input signal into a high-power output. While a digital-to-analog converter (DAC) or a microphone preamplifier might generate a pristine 2V RMS audio signal, that signal lacks the current delivery required to physically move a heavy speaker cone, drive an RF antenna, or spin a motor. The power amp provides that missing muscle, acting as the final stage in a signal chain before the transducer.

What a Power Amp Actually Changes (And Common Confusions)

In any real circuit installation, a power amp fundamentally changes the impedance and current profile of the signal. A line-level signal from a mixer or DAC is high-impedance (typically 10kΩ to 100kΩ) and low-current (measured in milliamps). If you connect an 8-ohm speaker directly to that line-level source, the voltage will instantly collapse to near zero because the source cannot supply the required amperage. The power amp takes that high-impedance voltage signal and buffers it into a low-impedance, high-current output capable of sourcing or sinking multiple amps without the voltage sagging.

Amplifier Topologies and Efficiency Classes

Not all power amps are built the same. The "Class" of an amplifier dictates how its output transistors are biased, which directly impacts efficiency, heat dissipation, and total harmonic distortion (THD). Below is a data-dense breakdown of the classes you will encounter on the bench or in commercial gear.

Class	Theoretical Max Efficiency	Real-World Efficiency	Typical THD+N	Common IC / Example
Class A	25% (50% w/ transformer)	15% - 20%	< 0.01%	Discrete JLH1969, LME49830 (biased)
Class AB	78.5%	50% - 65%	0.01% - 0.1%	LM3886, TDA7294, Crown XLS 1502
Class B	78.5%	60% - 70%	High (Crossover distortion)	Rarely used standalone in audio
Class D	100%	85% - 95%	0.05% - 0.2%	TPA3116D2, TAS5805M, Hypex NC400
Class G/H	90%+	70% - 80%	0.02% - 0.05%	LM4702 (w/ rail switching), PLX Series

Row-by-Row Notes for the Bench:

• Class A: The output transistors are always fully conducting, meaning they burn maximum power even when no audio is playing. You need massive heatsinks. Audiophiles love them for the lack of crossover distortion, but they are thermal nightmares in enclosed racks.
• Class AB: The workhorse of the 20th century. It uses two transistors (push-pull) with a slight bias current to eliminate the dead-zone crossover distortion of Class B. As detailed in standard semiconductor texts, the trade-off is moderate heat generation.
• Class D: Often mistakenly called "digital" because of the PWM switching, Class D is actually an analog switching topology. The input is compared to a high-frequency triangle wave (often 400kHz+), driving the output MOSFETs fully on or fully off. This minimizes the time spent in the linear (high-dissipation) region, yielding massive efficiency. Analog Devices provides an excellent deep-dive into the LC output filter requirements needed to strip the switching carrier frequency before it reaches the speaker.

Worked Example: Sizing a 50W RMS Audio Stage

Let's run the actual math for designing or selecting a power amp to deliver 50W RMS into an 8-ohm speaker. This is a common target for high-fidelity desktop monitors or small PA monitors.

1. Voltage Requirements:
Using the power formula $P = V^2 / R$, we solve for RMS voltage:
$V_{RMS} = \sqrt{P \times R} = \sqrt{50 \times 8} = \sqrt{400} = 20V_{RMS}$

2. Peak Voltage and Current:
Audio is an AC waveform. The peak voltage is $V_{RMS} \times \sqrt{2}$ (approx 1.414).
$V_{Peak} = 20V \times 1.414 = 28.28V$
Peak Current ($I_{Peak}$) = $V_{Peak} / R = 28.28V / 8\Omega = 3.53A$

3. Power Supply Sizing:
Your DC power rails must exceed the peak voltage to account for transistor saturation voltage ($V_{CE(sat)}$ or $R_{DS(on)}$ drops) and power supply ripple under load. For a Class AB amp, you need at least ±32V DC rails. For a single-supply Class D amp (like the TPA3116D2), you need a clean 24V to 30V DC supply capable of delivering at least 4A continuous current (accounting for 90% efficiency: $50W / 0.90 = 55.5W$ drawn from the wall; $55.5W / 24V = 2.3A$, plus headroom for dynamic peaks).

4. Thermal Dissipation (The Killer Metric):
If you use a Class AB chip like the LM3886 (running at ~55% efficiency at this power level), drawing 90W from the supply to output 50W means 40W is dissipated as heat. You will need a heatsink with a thermal resistance ($\theta_{SA}$) of less than 1.2°C/W to keep the junction temperature below 125°C in a 25°C room.
If you use a Class D module (92% efficient), you draw 54.3W to output 50W. Only 4.3W is dissipated as heat. A tiny extruded aluminum fin or even the PCB copper pour is sufficient. This is why Class D has entirely taken over the subwoofer and portable PA market.

Where You Meet This in Practice

Power amplifiers aren't just for audiophiles spinning vinyl. If a circuit needs to move physical mass or radiate electromagnetic energy, a power amp is involved.

• DIY Audio and Home Theater: Builders frequently choose "chip amps" (like the TI TPA3255) for active crossover projects. The practical advantage here is that the power amp is mounted directly behind the speaker driver, eliminating long, lossy speaker wire runs and damping factor degradation.
• RF Transmission: In ham radio or IoT LoRaWAN nodes, the final stage before the antenna is an RF power amplifier (PA). Unlike audio, RF PAs must be strictly matched to a 50-ohm impedance. A mismatch here doesn't just cause clipping; it causes standing waves (high VSWR) that reflect power back into the transistor, instantly destroying the silicon die.
• Motor Control and Robotics: An H-bridge motor driver (like the DRV8871 or L298N) is functionally a DC power amplifier. It takes a low-current logic or PWM signal from a microcontroller (like an Arduino or ESP32) and amplifies it to source 2A to 10A into the highly inductive load of a DC motor coil. The flyback diodes on these boards are there specifically to handle the inductive kickback that audio amps don't have to worry about.

For further reading on modern amplifier topologies and selection criteria, the Texas Instruments Amplifier Overview provides excellent parametric search tools and application notes for matching the right power stage to your specific load requirements.