Class AB Push-Pull: The Workhorse Output Stage

Class AB is the default analog power stage. A push-pull pair, each handling opposite half-cycles, biased slightly on so the handover between them never fully switches off. Efficiency reaches 50–78 % theoretically; the engineering battle is at the crossover.
> At a glance: 8 minute guide · part 6 of 10 in the amplifiers complete guide track · includes a worked example and a quick-reference table.

## Push-pull basics

The upper device sources current into the load on positive half-cycles; the lower sinks on negative. Each idles at a small quiescent current enough that neither enters hard cutoff during the handover, the trick that separates AB from B and its notorious crossover notch.

| Aspect | Class B | Class AB | Class A |
| — | — | — | — |
| Conduction | 180° each | Slightly >180° | 360° |
| Idle current | ≈ 0 | Small, critical | Maximum |
| Crossover distortion | Severe | Engineered out | None |
| Efficiency (theory) | 78.5 % max | Approaches 78.5 % | ≤ 50 % |
| Thermal care | Bias tracking | Essential | Continuous |

## Bias, temperature and the Vbe multiplier

Output device Vbe falls ~2 mV/°C; uncorrected, warm output stages over-bias and thermal-run. A Vbe multiplier transistor bolted to the same heatsink tracks and re-trims bias the classic stable AB output bias servo.

## Realistic distortion behaviour

Crossover residue shrinks with optimal quiescent current then returns as gm doubling at over-bias. Factory service manuals specify bias in milliamps for a reason. Measurement across temperature is how the design earns “hi-fi”.

## How to apply this in your build

Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. Set quiescent current to the design value at operating temperature
2. Mount the bias sensor on the output heatsink
3. Verify stability into reactive loads
4. Measure distortion at low output where crossover lives

### Worked example

An amplifier measured 0. 5 % THD at 1 W with bias 20 % low the crossover notch dominated. Setting the specified 40 mA and letting the multiplier track brought it under 0. 02 %. Run the numbers yourself with the Electrical Power Calculator and the result should agree to within rounding.

> Practical note from the bench. Servicing wisdom: measure output-stage idle current before touching anything else in an aging amp half of “aging sound” is drifted bias.

## Pitfalls that cost real hardware

– Biasing “by ear” instead of measured current
– Mounting the Vbe multiplier away from the output devices
– Substituting output transistors without re-checking bias

## Key takeaways

– Push-pull basics the foundation of this guide; revisit it if any measurement here surprises you.
– Bias, temperature and the Vbe multiplier the foundation of this guide. Revisit it if any measurement here surprises you.
– Realistic distortion behaviour the foundation of this guide; revisit it if any measurement here surprises you.

## Prerequisites and preparation

Before starting: set quiescent current to the design value at operating temperature and mount the bias sensor on the output heatsink. Keep the [Electrical Power Calculator](/tools/electrical-power) open every number in the worked example is reproducible. Total time including the bench steps: about 6–8 minutes.

## Who benefits most

Hobbyists meeting this topic for the first time, students who want the version with real numbers instead of abstract symbols. Returning engineers refreshing a corner of the craft. The mistake list alone justifies the visit every entry in it was learned the expensive way.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | Push-pull basics |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Pitfalls that cost real hardware |

## How this fits the amplifiers complete guide track

This guide is one stop in the structured learning path. Start from the [amplifiers complete guide](/tutorial/amplifiers-complete-guide) pillar page for the full map, or continue with [Class A comparison](/tutorial/class-a-amplifier-design) and [Class D technology](/tutorial/class-d-amplifier-technology). For the arithmetic, open the [Electrical Power Calculator](/tools/electrical-power).

## Frequently asked questions

Why does my Class AB amp distort at low volume?
Crossover distortion is a fixed residue largest relative to small signals. Correct bias is the cure.

Can I raise bias toward Class A?
Somewhere between lies “Class AAB” until heatsink physics vetoes. Thermal maths decides.

Is there a calculator for this?
Yes the [Electrical Power Calculator](/tools/electrical-power) tool runs the formulas from this guide instantly, client-side, with no signup.

## What to read after this

– The complete amplifiers & audio guide: [Amplifiers & Audio complete guide](/tutorial/amplifiers-complete-guide)
– Read next: [inverting op-amp: gain, virtual ground and design](/tutorial/inverting-op-amp-configuration)
– Also in this track: [non-inverting op-amp: high-impedance gain stage](/tutorial/non-inverting-op-amp-configuration)
– Continue with: [the op-amp voltage follower: small circuit, big leverage](/tutorial/op-amp-voltage-follower)
– Calculate as you go: [op-amp gain calculator](/tools/op-amp-gain) · [power dissipation tool](/tools/electrical-power) · [gain to dB converter](/tools/opamp-gain)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.

## Field notes

Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.

The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.
## Formulas and checks from this guide

Verification checklist for this track: measure bias at idle before signal testing, check heatsink temperature under load. Confirm gain across frequency rather than at 1 kHz alone. Amplifiers forgive nothing at their edges.

Bookmark this page against your next build in the track. The checklist above is the same one used across 19 guides in this series.

## Field lessons worth keeping

Thermal drift in bias. Check the bias tracker is bolted to the heatsink and re-set per the service values.

Across the emitter resistors at idle, no signal. Compare each output device.

## The habit that makes this stick

A note on thermal reality, which ends most arguments in this track: dissipation is set at idle, verified under load, and never forgiven by a bigger heatsink alone.

Working through Push-pull basicsand Bias, temperature and the Vbe multiplier with that habit in mind takes minutes, and it is the difference between reading about this topic and owning it.