A Class AB amplifier is a type of power amplifier that combines the low distortion of Class A designs with the improved efficiency of Class B designs. It uses two transistors in a “push-pull” arrangement, each biased to conduct slightly more than half the input signal. This small overlap is what makes Class AB the dominant amplifier topology in consumer audio equipment like stereo receivers and home theater systems.
How Push-Pull Amplification Works
To understand Class AB, it helps to see where it fits between its two parent designs. In a Class A amplifier, a single output transistor stays on for the entire signal cycle, all 360 degrees of it. This produces a very clean, linear output, but the transistor is always conducting and always burning power, even when no music is playing. Maximum efficiency tops out at just 25 to 50 percent, depending on the biasing method. The rest becomes heat.
Class B takes the opposite approach. It uses two complementary transistors, one handling the positive half of the signal and the other handling the negative half. Each transistor conducts for exactly 180 degrees, then shuts off completely. When there’s no input signal, the idle current is zero and no power is wasted. Efficiency jumps dramatically, but there’s a catch: at the moment one transistor hands off to the other, neither is fully conducting. This creates a brief gap in the output waveform called crossover distortion, an audible glitch that makes the sound harsh or gritty, especially at low volumes.
Class AB solves this by biasing both transistors just slightly above their shutoff point. Instead of waiting until the signal crosses zero to wake up, each transistor is already conducting a small amount of current at all times. The result is that both transistors are on simultaneously during the crossover region, smoothly passing the signal from one to the other with no gap. Each transistor conducts for more than 180 degrees but less than 360, typically described as a conduction angle somewhere between those two extremes.
Why the Bias Current Matters
The defining feature of a Class AB amplifier is its quiescent current, the small amount of current flowing through the output transistors when no audio signal is present. In a pure Class B design, this current is zero. In Class A, it’s large enough to keep the transistor conducting at all times. Class AB sits between them: just enough idle current to keep both transistors slightly on, eliminating the dead zone where crossover distortion occurs.
This bias point is set carefully during design. Too little bias and you’re essentially back to Class B with its crossover artifacts. Too much and you’re creeping toward Class A territory, burning excess power as heat. The sweet spot preserves the clean, linear output of Class A during the critical zero-crossing region while letting the amplifier operate in a more efficient, Class B-like mode for the rest of the signal cycle.
Keeping the Bias Stable
One practical challenge with Class AB amplifiers is that the bias point can drift as the transistors heat up. As temperature rises, transistors naturally want to conduct more current, which generates more heat, which increases current further. Left unchecked, this positive feedback loop can lead to thermal runaway, where the transistors overheat and destroy themselves.
Designers counteract this by using temperature-sensitive components in the bias circuit. A common approach places diodes or a dedicated bias-spreading circuit in direct thermal contact with the output transistors. As the transistors warm up, these components warm up too, automatically reducing the bias voltage to compensate. This keeps the quiescent current stable across a wide range of operating temperatures.
Efficiency and Heat
Class AB amplifiers are significantly more efficient than Class A, but they still convert a meaningful portion of their input power into heat. The theoretical maximum efficiency of a Class B amplifier is about 78.5 percent, and Class AB falls somewhere between that figure and the 25 to 50 percent ceiling of Class A, depending on how much bias current is used and how hard the amplifier is being driven.
In practice, this means Class AB amplifiers need heatsinks, sometimes quite large ones, bolted to the output transistors to dissipate waste heat. The output transistors are the primary source of thermal energy, and without adequate cooling, the amplifier’s performance degrades and its lifespan shortens. This is one of the main reasons Class AB amplifiers tend to be heavier and bulkier than newer alternatives. The heatsinks add both weight and manufacturing cost.
Class AB vs. Class D
The main competitor to Class AB in modern audio is Class D, which works on a fundamentally different principle. Instead of using transistors as variable resistors that linearly follow the audio signal, Class D rapidly switches transistors fully on and fully off at very high frequencies, then filters the output to reconstruct the audio waveform. Because the transistors spend almost no time in their resistive, heat-generating middle zone, Class D amplifiers run far cooler and can achieve much higher efficiency.
For years, Class AB held a reputation for superior sound quality, particularly in high-fidelity applications. Class D designs struggled with distortion that rose at higher audio frequencies and were limited to roughly CD-quality dynamic range (about 16 bits of resolution) in typical pulse-width modulation implementations. Modern Class D amplifiers have largely closed this gap. Their linearity now often exceeds that of analog-based power amplifiers, and some designers argue that Class D’s ability to maintain flat distortion characteristics across the entire audio band gives it a measurable advantage.
Class AB still has loyal advocates, particularly among audiophiles who value the specific distortion characteristics of analog output stages. The distortion signature of an amplifier is, in effect, its sonic fingerprint. Different amplifier topologies produce different harmonic distortion profiles, and listeners can prefer one over another even when both measure well on paper. Class AB amplifiers are also more sensitive to component quality and matching. Tight tolerances and well-matched transistors can yield excellent results, but semiconductor linearity varies with temperature, placing a practical ceiling on performance that switching designs can sidestep.
Where Class AB Is Used
Class AB remains the standard topology in a wide range of consumer audio products, including stereo receivers, home theater systems, and car audio amplifiers. Its combination of good sound quality, proven reliability, and well-understood design principles makes it a safe choice for manufacturers. You’ll also find Class AB stages inside many professional audio amplifiers, guitar amplifiers, and powered studio monitors, though Class D is rapidly gaining ground in all of these categories thanks to its smaller size, lighter weight, and reduced need for bulky heatsinks.
The trend in commercial products is clearly moving toward Class D, driven primarily by cost and thermal advantages. Class AB amplifiers require large heatsinks that add size, weight, and expense. Class D amplifiers that produce equivalent output power can be built into much smaller enclosures with minimal cooling. Still, Class AB remains a workhorse technology with decades of refinement behind it, and for many applications, its performance-to-cost ratio is hard to beat.