A Class AB power amplifier is a push-pull output stage where both transistors conduct for slightly more than 180 degrees of the input cycle, eliminating crossover distortion while preserving most of Class B's efficiency. In a real audio circuit, this topology changes the design by introducing a small quiescent bias current—typically 10mA to 100mA—that keeps the output devices just above their turn-on threshold, trading a few percentage points of thermal efficiency for a massive drop in Total Harmonic Distortion (THD).
The Crossover Distortion Problem and the Class AB Solution
To understand why the AB class power amplifier exists, you have to look at the fatal flaw of pure Class B. In a standard Class B push-pull stage, an NPN transistor handles the positive half of the audio waveform, and a PNP transistor handles the negative half. However, the base-emitter junction of a standard bipolar junction transistor (BJT) requires roughly 0.65V to begin conducting. As the audio signal crosses the zero-volt axis, neither transistor is turned on. This creates a 'dead band' or notch in the output waveform known as crossover distortion, which sounds like harsh fizzing on high-frequency transients.
Think of a relay race where the baton handoff (the zero-crossing point) usually has a dead zone where neither runner is holding it. Class AB solves this by having both runners hold the baton simultaneously for a brief overlap before one lets go.
By applying a small forward bias voltage to the bases of both output transistors, we force them into a state of slight conduction even when there is no audio signal present. This quiescent current (I_q) bridges the dead zone. The circuit responsible for generating and maintaining this precise bias voltage is typically a Vbe multiplier (also called a bias spreader or rubber diode), which sits between the bases of the driver transistors.
Amplifier Topology Comparison and Operating Specs
When selecting an output stage for a project or analyzing a commercial schematic, it helps to see exactly where Class AB sits on the spectrum of efficiency and fidelity. The table below maps the core operating parameters of common audio amplifier classes.
| Amplifier Class | Conduction Angle | Max Theoretical Efficiency | Typical Real-World Efficiency | THD Profile | Thermal Design Requirement |
|---|---|---|---|---|---|
| Class A | 360° (Always On) | 25% (Transformer) / 50% (Inductive) | 15% - 25% | Ultra-low (< 0.05%) | Massive heatsinks; runs hot at idle |
| Class B | 180° (Half Cycle) | 78.5% | 60% - 70% | High (Crossover notch) | Moderate; scales with output power |
| Class AB | 181° to 270° | 78.5% | 50% - 65% | Very Low (< 0.1%) | Moderate to High; requires thermal tracking |
| Class D | Switching (PWM) | 100% | 85% - 95% | Low (Requires LC output filter) | Minimal; small surface-mount heatsinks |
| Class G/H | Variable (Rail Switching) | ~80% | 65% - 75% | Very Low (AB base topology) | Complex; multiple power supply rails |
Source data synthesized from standard semiconductor design principles outlined by Electronics Tutorials and All About Circuits.
Worked Example: Biasing and Thermal Math for a 50W Output Stage
Let's move from theory to the workbench. We are designing a 50W RMS AB class power amplifier driving an 8-ohm loudspeaker. We will use a complementary Darlington output pair: the ON Semiconductor MJL21193G (NPN) and MJL21194G (PNP).
1. Determine Rail Voltage and Peak Current
To deliver 50W into 8 ohms, the required RMS voltage is:
V_rms = sqrt(P * R) = sqrt(50 * 8) = 20V
The peak voltage is V_peak = 20V * 1.414 = 28.28V.
Accounting for transistor saturation voltage (V_ce(sat)) and emitter resistor drops, we need a power supply rail of roughly ±32V DC.
2. Calculate Maximum Transistor Dissipation
The worst-case power dissipation for a Class B/AB stage does not occur at maximum volume; it occurs when the output voltage is roughly 63% of the rail voltage. The formula for maximum dissipation per transistor is:
P_d(max) = V_rail^2 / (pi^2 * R_load)
P_d(max) = 32^2 / (9.869 * 8) = 1024 / 78.95 = 12.97W per transistor.
This means your heatsink must be sized to dissipate at least 26W total during peak sine-wave testing.
3. Set the Quiescent Bias Current (I_q)
To eliminate the crossover notch without pushing the amp into Class A territory, we target an I_q of 50mA. The power dissipated at idle per transistor is:
P_idle = V_rail * I_q = 32V * 0.05A = 1.6W.
Total idle heat for the pair is 3.2W, which is easily managed and keeps the silicon junctions warm enough for stable thermal tracking.
4. Calculate the Required V_bias
The Vbe multiplier must provide enough voltage to turn on both output transistors and push 50mA through their emitter resistors (typically 0.22 ohms each).
Assuming a Vbe of 0.7V per Darlington pair at this low current:
V_bias = (2 * Vbe) + (I_q * (R_e1 + R_e2))
V_bias = 1.4V + (0.05A * 0.44 ohms) = 1.4V + 0.022V = 1.422V.
You will adjust the trimmer potentiometer in your Vbe multiplier circuit until your multimeter reads exactly 22mV across one of the 0.22-ohm emitter resistors (since V = I * R -> 0.05A * 0.22 = 11mV, wait, 50mA * 0.22 = 11mV. Let's correct the bench measurement: you measure 11mV across a single 0.22Ω emitter resistor to confirm 50mA of quiescent current).
The transistor used in your Vbe multiplier circuit must be physically bolted to the same heatsink as the output devices. As the MJL21193/4 transistors heat up under load, their Vbe requirement drops by roughly -2mV/°C. If the bias transistor is not thermally coupled to the heatsink, it will continue supplying the original 1.422V, forcing the hot output transistors to draw massive amounts of current, leading to catastrophic thermal runaway and exploded silicon.
Where You Meet Class AB in Practice (And What It Isn't)
You will find the AB class power amplifier topology dominating in applications where audio fidelity is paramount but the massive heat sinks of Class A are physically or financially unviable.
- Mid-to-High-End AV Receivers: Brands like Denon and Marantz frequently use discrete Class AB output stages for their main front-left and front-right channels to ensure clean, high-current delivery to complex speaker loads.
- Powered Studio Monitors: The analog amplification stages in classic monitors (like the Yamaha HS8 or older KRK Rokit generations) rely on Class AB for its predictable phase response and lack of high-frequency switching noise.
- Guitar Pedal Buffers and Headphone Amps: Low-voltage Class AB stages are standard for driving low-impedance headphones without the hiss associated with early Class D chips.
Common Confusions on the Datasheet
Makers and junior engineers frequently confuse Class AB with its modern derivatives. If a schematic features an AB class power amplifier output stage but also includes a secondary, higher-voltage power supply rail that switches in only during loud bass transients, you are actually looking at Class G or Class H. These are not entirely different topologies; they are simply Class AB amplifiers with dynamic rail-tracking to improve efficiency during high-power peaks.
Conversely, do not confuse Class AB with Class D. Class D is a switching topology that uses Pulse Width Modulation (PWM) at hundreds of kilohertz. While modern Class D chips (like the Texas Instruments TPA3255) have largely conquered the subwoofer and portable Bluetooth speaker markets due to >90% efficiency, Class AB remains the benchmark for linear, non-switching analog fidelity in critical listening environments.
Frequently Asked Questions
Q: Why do my Class AB output transistors get warm when no music is playing?
A: This is by design. The quiescent bias current (I_q) flowing through both transistors simultaneously generates idle heat. A properly biased Class AB amp should feel warm to the touch (around 40°C - 50°C on the heatsink) at idle. If it is too hot to touch at idle, your Vbe multiplier is set too high, and you are drifting into Class A operation or thermal runaway.
Q: Can I use MOSFETs instead of BJTs for a Class AB stage?
A: Yes, lateral MOSFETs (like the Exicon ECX10N20) are highly prized in Class AB designs because they lack the secondary breakdown limitations of BJTs and have a negative temperature coefficient for current, making them inherently immune to thermal runaway. However, their gate threshold voltage is much higher (requiring a V_bias of roughly 3V to 4V instead of 1.4V), meaning your Vbe multiplier circuit must be redesigned to provide a larger voltage drop.
