Class AB Amplifier Theory: Crossover Distortion, Bias, and Thermal Runaway

A Class AB amplifier is a push-pull audio circuit where both output transistors conduct slightly more than half a cycle, overlapping in the middle to eliminate crossover distortion while keeping heat manageable. What this topology changes in a real circuit is the thermal and power reality of audio amplification: it allows you to push 100W+ into a speaker without requiring liquid cooling or massive, impractical heatsinks, bridging the gap between the pristine but wasteful Class A and the efficient but distorted Class B.

The Core Mechanism: Eliminating Crossover Distortion

In a pure Class B amplifier, one transistor handles the positive half of the audio waveform and the other handles the negative half. The problem is that silicon bipolar junction transistors (BJTs) require about 0.6V to 0.7V at the base-emitter junction to start conducting. When the audio signal crosses the 0V center line, there is a brief dead zone where neither transistor is fully on. This creates crossover distortion, which sounds like harsh, gritty static on quiet passages.

Class AB solves this by applying a small DC bias voltage to the bases of both transistors, keeping them just barely turned on even when there is no audio signal. Think of it like a relay race baton handoff: instead of the first runner stopping completely before the second runner starts (Class B), they run alongside each other for a few steps while passing the baton. This overlap ensures the signal is never dropped at the zero-crossing point.

By the Numbers: Efficiency, Heat, and a Worked Power Example

To understand why Class AB dominates analog audio, we need to look at the power math. Let us calculate the heat dissipation for a typical discrete Class AB amplifier driving a standard bookshelf speaker.

Worked Numeric Example

• Target Output Power ($P_{out}$): 50W into an 8Ω load.
• Supply Rails ($V_{cc}$): ±35V DC.
• Peak Output Voltage ($V_{peak}$): $\sqrt{2 \times 50W \times 8\Omega} = 28.28V$.
• Peak Output Current ($I_{peak}$): $28.28V / 8\Omega = 3.535A$.

The DC power drawn from the supply ($P_{dc}$) for a push-pull stage is calculated as:

$P_{dc} = \frac{2 \times V_{cc} \times I_{peak}}{\pi} = \frac{2 \times 35V \times 3.535A}{3.1416} = 78.86W$

The power dissipated as heat by the transistors ($P_{diss}$) is the difference between what the supply provides and what the speaker receives:

$P_{diss} = P_{dc} - P_{out} = 78.86W - 50W = 28.86W$

At maximum output, the efficiency is $50W / 78.86W = 63.4\%$. While this means 28.86W of heat must be sunk to the chassis, it is vastly superior to a Class A amplifier of the same output, which would dissipate over 100W of heat continuously just sitting at idle.

Where You Meet Class AB in Practice

You will find Class AB topology everywhere high-fidelity analog audio is required without the switching noise of Class D.

1. Powered Studio Monitors: Speakers like the Yamaha HS8 or KRK Rokit series often use Class AB for their high-frequency tweeter amplifiers, where switching noise from Class D would be highly audible.
2. AV Receivers: Mid-range to high-end home theater receivers (Denon, Marantz) use massive toroidal transformers and banks of discrete Class AB output transistors to drive multiple channels cleanly.
3. Guitar Amplifiers: The classic push-pull output stage of a tube amp (using EL34 or 6L6 tubes) operates in Class AB, providing the characteristic harmonic distortion and sag that guitarists rely on.
4. Monolithic ICs: If you are building a DIY chipamp, the legendary LM3886, TDA7294, and LM4780 are all integrated Class AB designs.

Bench Scenario: The Thermal Runaway Trap in Bias Setting

Setting the quiescent bias current ($I_q$) on a discrete Class AB amplifier is a rite of passage, and it is where most hobbyists destroy their first prototype.

The Setup

You are prototyping a discrete amplifier using an MJL21193 (NPN) and MJL21194 (PNP) complementary output pair on ±40V rails. You have 0.22Ω emitter degeneration resistors. Your target quiescent current is 50mA per device to ensure the overlap zone is clean.

The Numbers

To verify 50mA, you measure the voltage drop across the 0.22Ω emitter resistor. Using Ohm's law ($V = I \times R$), your target voltage is $0.05A \times 0.22\Omega = 11mV$.

The Outcome and What Went Wrong

You power the amp with no load, connect your multimeter across the emitter resistor, and slowly turn the blue bias trimmer potentiometer. The meter reads 5mV, then 8mV, then 11mV. You stop adjusting. Five minutes later, you glance at the meter: it now reads 35mV. Ten minutes later, it reads 150mV, the heatsink is burning hot, and suddenly your power supply fuse blows with a loud pop.

Common Confusions: Class AB vs. Class A, B, and D

It is easy to mix up amplifier classes when reading spec sheets. Here is how Class AB stacks up against the alternatives.

For a deeper dive into the mathematical derivation of these efficiency limits, the Analog Devices guide on amplifier classes provides excellent waveform breakdowns. Additionally, Texas Instruments' audio amplifier portfolio documentation highlights the practical trade-offs between their Class AB and Class D silicon offerings.