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Class A vs AB vs D Amplifiers: Key Differences Explained

An amplifier's class describes how its output transistors are operated. Class A keeps current flowing through the transistor over the whole cycle, so distortion is lowest, but it consumes power even with no signal, and its efficiency is 25% or 50% even in theory. Class AB is Class B, in which two transistors share the positive and negative half-cycles, with a small idle current added to suppress crossover distortion; its efficiency approaches Class B's theoretical maximum of about 78.5%. Class D switches the transistors on and off and filters pulses such as PWM back into the original signal, achieving high efficiency of around 90% at high output.

AspectClass AClass ABClass D
Operating principleCurrent always flows through the transistor (linear region) and increases or decreases with the signalA push-pull pair of two transistors shares the positive and negative half-cycles. A small idle current is kept flowingThe transistors switch on and off; the signal is converted into pulses such as PWM, amplified, and restored by an LC low-pass filter
Conduction angle (period during which one transistor conducts)360° (full cycle)More than 180° and less than 360° (Class B is 180°)Used fully on or off, so it is not classified by conduction angle
Theoretical maximum efficiency25% (with a series resistive load). 50% with transformer or choke couplingSlightly below Class B's approximately 78.5% (π/4)100% with ideal switches. In practice, around 90% at high output
Efficiency at low outputVery low. Uses the same power even with no signalDrops with output, but much better than Class AEasy to keep high (at very low output, switching losses take a larger share)
DistortionLowest. No crossover distortionLow. Crossover distortion is suppressed by the idle current and negative feedbackDistortion arises from dead time, imperfect switching and supply variation. It can be reduced with negative feedback and similar techniques
HeatHighest. Maximum when there is no signalModerate. A heat sink is often neededLow. The heat sink can be made small or omitted
Noise and EMINo switching noiseNo switching noiseSwitching at hundreds of kHz to several MHz produces EMI. Care is needed with the output filter and wiring
Circuit and componentsSimple. Can be built with a single transistorA pair of output transistors and a bias circuitA pulse modulator, a gate driver circuit and an output filter (inductor and capacitor)
Typical usesSmall-signal amplification, preamplifiers, driver stages that drive the following stage, some high-end audioAudio power amplifiers, headphone amplifiers, op-amp output stagesSpeaker drive in smartphones and TVs, car audio, portable speakers, high-power amplifiers

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Class A
When the output is small and low distortion is the top priority. Small-signal amplification, preamplifiers and driver stages that drive the following stage.
Class AB
When you want both low distortion and good efficiency. Audio power amplifiers, headphone amplifiers, and output stages of op-amps and buffers.
Class D
When you want long battery life, less heat or a smaller heat sink, or high output power. Speakers in smartphones and TVs, car audio.

FAQ

Why does Class A have two maximum efficiencies, 25% and 50%?

Because the load is connected differently. If the load resistor is connected in series with the transistor, DC current also flows through the load and wastes power, so the theoretical maximum is 25%. If the load is coupled through a transformer or choke, the DC power is not wasted in the load and the output voltage can swing up to twice the supply voltage, so the maximum is 50%.

What is crossover distortion?

In a Class B push-pull stage, when the signal passes near 0 there is a region where neither transistor is on, which distorts the output waveform. With bipolar transistors it occurs because no current flows until about 0.6 to 0.7 V is applied between base and emitter. Class AB removes this region by keeping a small idle current flowing.

Does the D in Class D stand for digital?

No. The name simply follows Class A, B and C and does not mean digital. Many Class D amplifiers work by comparing an analog input signal with a triangle wave in a comparator to convert it into PWM pulses.

Why is Class D so efficient?

Because the output transistors are used fully on or fully off. When on, the voltage across them is nearly zero, and when off, the current is nearly zero, so little power is turned into heat in the transistors. The remaining losses include conduction losses due to on-resistance, losses during switching transitions and the power needed to drive the gates.

Check your understanding

Compared with a class A amplifier, which is a characteristic of a class B push-pull amplifier?

  1. ALow efficiency but very low distortion
  2. BIt always conducts a large bias current
  3. CHigh power efficiency (theoretical maximum about 78.5%), but prone to crossover distortion
Show answer and explanation

Answer: C. High power efficiency (theoretical maximum about 78.5%), but prone to crossover distortion

Class A always draws a large bias current, giving low distortion but a theoretical maximum efficiency of only 25% with a resistive load (50% transformer-coupled), and it heats up even without a signal. Class B draws almost no current at idle, reaching about 78.5% (π/4), but suffers crossover distortion at the handover. In practice, class AB with a small bias current and the even more efficient class D are used.

Which distortion tends to occur in a class B push-pull amplifier in which NPN and PNP transistors handle the positive and negative half-cycles?

  1. AQuantization error
  2. BAliasing
  3. CCrossover distortion
Show answer and explanation

Answer: C. Crossover distortion

In a class-B push-pull stage, near zero input neither transistor reaches its ~0.6 V VBE, so there is a dead zone where the waveform distorts at the handover—crossover distortion, most noticeable at low volume. Biasing the bases about 1.2 V apart with diodes or a VBE multiplier lets a small idle current flow (class AB) and reduces it. Most audio amplifier output stages are class AB.

Which value is closest to the maximum power efficiency of an ideal class B push-pull amplifier?

  1. AAbout 100%
  2. BAbout 78.5%
  3. CAbout 25%
Show answer and explanation

Answer: B. About 78.5%

In class B, each transistor conducts for half a cycle and almost no current flows without a signal. At full sine-wave output the theoretical maximum efficiency is π/4 ≈ 78.5%, higher than class A, which always draws bias current (25% with a resistive load, 50% transformer-coupled). Where still higher efficiency is needed, class D uses the transistors as switches (over 90%) and is now standard in smartphones and flat-panel TVs.

  1. In an amplifier, what is the DC voltage and current state of the transistor with no input signal called?
  2. Which method is commonly used by microcontrollers to adjust LED brightness or motor speed?

Practice Analog and electronic circuits questions

Key terms explained

Crossover distortion
Distortion in a class-B push-pull amplifier near 0 V, where neither transistor conducts during the handover between positive and negative half-cycles. Class-AB biasing with a small idle current reduces it.
Pulse-width modulation (PWM)
A method that adjusts average voltage or power by varying the high time of an output while keeping the period constant. Used to dim LEDs and control motor speed.
Negative feedback
Returning part of the output to the input with opposite polarity. Gain drops, but it becomes stable, and distortion and bandwidth improve.

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