What Is a Class AB Power Amplifier?
A class AB power amplifier is a circuit topology that combines the low-distortion linearity of Class A with the higher efficiency of Class B by keeping both output transistors slightly conducting (biased) even when there is no input signal. In a real circuit, this biasing eliminates the dead-zone crossover distortion inherent in pure Class B push-pull stages, while avoiding the massive heat dissipation and 50% theoretical efficiency ceiling of pure Class A. Hobbyists and engineers commonly confuse Class AB with Class G/H (which are simply Class AB amplifiers with dynamically switching power supply rails for better efficiency) or Class D (which uses high-frequency PWM switching rather than linear transistor operation).
The Math: Bias Current and Crossover Distortion
To understand why Class AB works, you have to look at the quiescent bias current (Iq). In a pure Class B stage, the NPN transistor handles the positive half-cycle and the PNP handles the negative. When the signal crosses zero, both turn off momentarily. Think of it like a two-lane highway where one lane closes before the other opens, causing a traffic jam; in an audio circuit, this dead zone manifests as harsh, gritty crossover distortion.
Class AB fixes this by injecting a small standing current through both output devices. Let's calculate the required bias voltage for a discrete complementary push-pull stage using the popular ON Semi MJL21193 (NPN) and MJL21194 (PNP) power transistors.
Assume we want a quiescent current (Iq) of 50 mA to ensure thermal stability and eliminate crossover distortion.
- Each output transistor has a 0.22 Ω emitter resistor (Re) for current sharing and thermal compensation.
- Voltage drop across one Re at 50 mA: V = I × R = 0.050 A × 0.22 Ω = 11 mV.
- The base-emitter voltage (Vbe) for these specific BJTs at 50 mA is approximately 0.65 V (read from the datasheet transfer characteristics curve).
- The total bias voltage (Vbias) that the Vbe-multiplier transistor must provide across the two bases is:
Vbias = Vbe(NPN) + Vbe(PNP) + VRe(NPN) + VRe(PNP)
Vbias = 0.65 + 0.65 + 0.011 + 0.011 = 1.322 V.
If your Vbe-multiplier is set to 1.2 V, you fall back into Class B and get distortion. If you set it to 2.0 V, your Iq spikes, the transistors overheat, and you trigger thermal shutdown or melt the junction. Precision matters.
Where You Meet This in Practice
You will encounter Class AB topologies wherever linearity and low noise are prioritized over raw efficiency.
- Active Studio Monitors: Nearfield monitors use Class AB amplifiers to ensure flat frequency response and low intermodulation distortion (IMD) at low listening volumes, where Class D switching noise can be problematic.
- Guitar Amplifier Power Sections: While the preamp is Class A, the push-pull power section of a Marshall JCM800 or Fender Twin Reverb operates in Class AB to deliver clean headroom before clipping.
- RF Linear Amplifiers: Ham radio transceivers use Class AB biasing in their final LDMOS or bipolar RF stages to preserve the envelope of SSB (Single Sideband) signals without splattering into adjacent bands.
- High-Current LDOs: Low-dropout linear regulators with push-pull output stages (like discrete equivalents of the LT3080) use Class AB to source and sink current for fast transient response in digital logic rails.
Class AB vs. Class D: The Decision Matrix
Modern audio design often forces a choice between linear (Class AB) and switching (Class D). Here is how they stack up.
| Criteria | Class AB (Linear) | Class D (Switching) |
|---|---|---|
| Typical Efficiency | 50% - 65% | 85% - 95% |
| THD+N (Total Harmonic Distortion) | 0.001% - 0.05% | 0.01% - 0.1% (improving with GaN FETs) |
| Heatsink Requirement | Massive (e.g., 2.5°C/W for 50W) | Minimal or none |
| EMI / Switching Noise | None (purely linear) | High (requires LC output filters) |
| Best Application | Audiophile DACs, studio monitors, RF | Bluetooth speakers, subwoofers, PA systems |
Decision Tree: Which Amplifier IC or Topology Should You Choose?
When designing or repairing an amplifier, use this decision path to lock in your topology and part number.
- IF you are driving a subwoofer or battery-powered Bluetooth speaker THEN choose Class D. Pick the TI TPA3116D2.
- IF you need ultra-low noise for a headphone amp or phono preamp THEN choose Class A. Pick the OPA1612 or discrete JLH1969.
- IF you are building a high-fidelity stereo power amp for passive bookshelf speakers, need low EMI, and want a proven, forgiving DIY layout THEN choose Class AB.
The Default Pick for Class AB:
If your decision path terminates at Class AB for a DIY hi-fi audio build, the undisputed champion is the TI LM3886TF (the insulated package variant). According to the Texas Instruments LM3886 datasheet, it delivers 68W continuous into 4Ω with 0.03% THD. It includes built-in short-circuit protection, thermal shutdown, and a mute pin. Always use the TF suffix variant to avoid shorting the heatsink to the negative rail, and torque the mounting screw to 0.8 Nm with a dab of high-quality thermal paste. Power it from a dual-rail unregulated linear supply (e.g., ±28V DC from a 400VA toroidal transformer) for the best dynamic headroom.
Frequently Asked Questions (FAQ)
Why does my Class AB amplifier get hot even with no music playing?
That heat is the quiescent bias current (Iq) doing its job. Even with zero input signal, a Class AB amp draws continuous current through the output transistors to keep them in their linear region. A typical 50W Class AB amp might dissipate 10W to 15W of heat per channel at idle. Ensure your heatsink is rated for at least 1.5°C/W and has adequate airflow.
How do I measure crossover distortion on an oscilloscope?
Inject a 1 kHz sine wave at 1V peak-to-peak into the amplifier input and connect your oscilloscope probes across an 8Ω dummy load resistor. Zoom in on the zero-crossing point (the X-axis). In a poorly biased Class B or under-biased Class AB amp, you will see a distinct flat spot or notch exactly where the waveform crosses 0V. Adjust the bias potentiometer while watching the scope until that notch completely disappears and the sine wave becomes perfectly smooth.
Can I convert a Class B amplifier to Class AB?
Yes, by adding a Vbe-multiplier bias circuit or a pair of forward-biased diodes between the bases of the push-pull output transistors. You must measure the voltage drop across the output emitter resistors with a multimeter and adjust the bias potentiometer until you read the manufacturer's specified Iq voltage (usually between 10mV and 25mV).
What is the difference between Class AB and Class H?
Class H is essentially a Class AB amplifier wrapped in a dynamic power supply tracking circuit. When the audio signal is quiet, the Class H amp runs on low voltage rails (saving power). When a loud transient hits, the supply rails instantly switch to a higher voltage to prevent clipping. The output stage itself remains pure Class AB.
