Audio Amplifier Circuit Diagram: Class AB and Class D Explained

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Understand audio amplifier circuit topology from input gain stages through Class AB and Class D output stages, with practical component selection and testing guidance.

An audio amplifier circuit increases the power of an audio signal so that it can drive a loudspeaker. Every practical amplifier consists of three functional stages: the input stage (voltage gain and input impedance matching), the driver stage (current gain to prepare the output devices), and the output stage (power delivery to the load).

Class AB amplifiers are the dominant topology in linear audio applications. The output stage uses a complementary pair of bipolar transistors (NPN and PNP) or MOSFETs biased slightly into conduction simultaneously — a small quiescent current (typically 20–80 mA per output pair) that eliminates the crossover distortion of a pure Class B design while keeping efficiency significantly higher than Class A. Efficiency for Class AB typically ranges from 50–70% at full power. The quiescent current is set by a Vbe multiplier or bias network whose temperature coefficient must track the output transistors to prevent thermal runaway.

Class D amplifiers use pulse-width modulation (PWM) to switch the output transistors fully on or fully off at a carrier frequency typically between 200 kHz and 1 MHz. Because the transistors spend almost no time in the linear region, efficiency reaches 85–98%, making Class D the standard choice for battery-powered and high-power applications where heat dissipation is a constraint. The output requires a low-pass filter (typically an LC filter with a cutoff around 20–50 kHz) to reconstruct the analogue waveform before the speaker load.

Both topologies employ negative feedback — routing a fraction of the output signal back to the inverting input of the gain stage — to reduce distortion and improve frequency response. The feedback network (a resistor divider in most op-amp-based designs) sets the closed-loop voltage gain according to the formula: Gain = 1 + (Rf / Rin).

Power supply rejection, input impedance, output impedance, signal-to-noise ratio, and total harmonic distortion (THD) are the key performance parameters to evaluate when comparing or designing amplifier circuits.

How to wire audio amplifier circuit diagram

Define the design requirements Establish output power (watts into the speaker load impedance), supply voltage, speaker impedance (typically 4 Ω or 8 Ω), frequency response (20 Hz–20 kHz for audio), maximum THD target, and efficiency requirements. These parameters drive every subsequent component choice.
Select the amplifier topology Choose Class AB for fixed installations, studio monitors, or where audio quality is paramount. Choose Class D for automotive, portable speakers, or any application where battery life or heat dissipation is a constraint. Hybrid designs (Class H) vary the supply rail to improve efficiency while retaining linear output devices.
Design the input gain stage The input stage sets the overall voltage gain and input impedance. A non-inverting op-amp configuration with Rin = 10 kΩ and Rf = 47 kΩ gives a gain of approximately 5.7 and presents 10 kΩ input impedance to the signal source. Include a low-pass RC filter at the input (e.g., 10 kΩ series + 100 pF to ground) to limit RF interference.
Design the driver and output stage (Class AB) Select complementary output transistors rated for at least twice the supply voltage and well above the expected peak current (peak current = √(2 × P_out / R_load)). Set quiescent current using the Vbe multiplier. Mount bias transistors in direct thermal contact with the output devices on the heatsink, not on the PCB.
Calculate and specify the power supply Supply voltage determines maximum output power: P_out ≈ (Vs²) / (2 × R_load) in theory (practical efficiency reduces this). For 50 W into 8 Ω, the supply should be approximately ±28–35 V. Specify a toroidal transformer with secondary current rating of at least 1.5× the expected average load current.
Design the power supply filtering Use large electrolytic capacitors (typically 4 700–22 000 µF per rail) to reduce power supply ripple. Ripple voltage at the supply rail appears in the output as hum. Lower ripple improves signal-to-noise ratio. Add 100 nF ceramic bypass capacitors in parallel with the electrolytics at each op-amp power pin.
Test and measure performance Use an oscilloscope to check for oscillation, clipping, and crossover distortion. Measure quiescent current (should match design target). Perform a frequency sweep from 20 Hz to 20 kHz at a safe power level and measure the output amplitude. Test output power into a resistive load — never into a reactive load without current limiting during initial testing.

Specifications

Class AB efficiency (typical)	50–70% at full rated power
Class D efficiency (typical)	85–98% at full rated power
Class AB quiescent bias current (typical per output pair)	20–80 mA
Class D switching frequency (typical)	200 kHz – 1 MHz
Class D output LC filter cutoff (typical)	20–50 kHz
Non-inverting voltage gain formula	Gain = 1 + (Rf / Rin)
Audio frequency range (full bandwidth)	20 Hz – 20 kHz
Typical THD+N target (high-quality Class AB)	< 0.1% at rated power, 1 kHz

Safety warnings

Mains-connected power supplies operate at hazardous voltages (typically 120 V or 230 V AC). All mains wiring must comply with IEC 60364, BS 7671, NEC (NFPA 70), or the relevant national standard. Use correctly rated fuses, earth / ground conductors, and insulated mains-side components.
High-voltage DC rails (±35 V and above) store significant energy in filter capacitors. After disconnecting mains power, wait for capacitors to discharge (at least 30 seconds) and verify rail voltages are below 5 V before touching any rail-connected components.
Output transistors in Class AB designs can reach junction temperatures above 100°C under sustained load. Always verify the heatsink thermal resistance is sufficient and that thermal compound is applied to all device-to-heatsink interfaces. Never operate without the heatsink fitted.
Class D switching edges can radiate significant electromagnetic interference. Ensure adequate shielding, short loop areas on the output filter, and compliance with applicable EMC regulations (e.g., FCC Part 15, CISPR 32) before use in a commercial product.
Prevent accidental short circuits on the output terminals during testing — a shorted output can destroy output transistors in milliseconds. Use a series resistive load during initial testing, never a direct short or a reactive-only load.

Tools needed

Dual-trace oscilloscope (bandwidth ≥ 20 MHz)
Digital multimeter
Variable bench power supply (for initial testing before adding the mains supply)
Signal generator or audio source
Resistive dummy load (e.g., wirewound power resistors matched to speaker impedance)
Soldering station with temperature control
Thermal paste applicator
Desoldering pump or braid

Common mistakes

Mounting the bias transistor (Vbe multiplier) on the PCB rather than thermally coupled to the output devices — this causes the bias voltage to not track output transistor temperature, leading to thermal runaway or permanently elevated distortion.
Omitting the 100 nF bypass capacitor at each op-amp power pin, causing high-frequency oscillation that manifests as unexpected high-frequency hash on the output or IC destruction.
Using carbon composition or low-quality resistors in the feedback network — their noise and temperature coefficient degrade signal-to-noise ratio and gain stability. Use 1% metal film resistors throughout the signal path.
Grounding the input signal shield and the output ground at different points in the chassis, creating a ground loop that introduces 50 Hz or 60 Hz hum into the signal path. Use a star-ground topology with all grounds returning to a single common point.
Setting quiescent current higher than specified to reduce distortion, without verifying that the heatsink can handle the additional standby dissipation — overheating the output stage at idle eventually causes failure.

Troubleshooting

Persistent hum or buzz on the output: Cause: Ground loop, insufficient power supply filtering, or mains frequency pickup on input wiring Fix: Check that the signal input ground and power supply ground share a single star ground point. Measure ripple voltage on the supply rails with the oscilloscope — should be below 50 mV for a well-designed supply. Shield the input wiring and confirm the chassis is properly earthed.
High-frequency oscillation (squealing or instability): Cause: Insufficient phase margin in the feedback loop, missing bypass capacitors on op-amp supply pins, or capacitive loading on the output Fix: Check that 100 nF bypass capacitors are correctly placed at each IC supply pin. Add a small series resistor (10–33 Ω) between the amplifier output and the feedback network. If using a capacitive load (cables, filter capacitors), add a Zobel network (typically 10 Ω + 100 nF in series) across the output.
Output transistors fail or run excessively hot: Cause: Thermal runaway from incorrect bias temperature compensation, insufficient heatsink, or output short circuit Fix: Measure quiescent current (voltage across emitter resistors divided by resistance). Compare to design specification. Verify the bias transistor is thermally bonded to the output transistors. Check heatsink thermal resistance against power dissipation. If the amplifier shows any sign of oscillation, resolve that first — oscillation dissipates enormous power in the output stage.

Frequently asked questions

What is the difference between Class AB and Class D amplifiers?

Class AB operates in the linear region with output transistors partially biased on at all times, giving excellent audio quality at 50–70% efficiency. Class D uses PWM switching at high frequency, achieving 85–98% efficiency with negligible heat but requiring an output LC filter. Class D is preferred for high power and portable use; Class AB for audiophile and studio applications.

What is crossover distortion and how does Class AB prevent it?

Crossover distortion occurs in Class B designs when the NPN and PNP output transistors each switch off near the zero crossing of the waveform, creating a notch. Class AB prevents this by biasing both devices into slight conduction at rest so there is no dead zone around zero — the transition between the two halves of the output stage is continuous.

What is thermal runaway and how is it prevented?

Thermal runaway occurs when rising junction temperature in bipolar output transistors reduces their Vbe, increasing quiescent current, which raises temperature further — a positive feedback loop leading to device destruction. It is prevented by mounting a temperature-sensing element (diode or transistor) thermally coupled to the output devices to reduce bias current as temperature rises.

How do I calculate the voltage gain of an amplifier using an op-amp?

For a non-inverting configuration, voltage gain is 1 + (Rf / Rin), where Rf is the feedback resistor and Rin is the input resistor to the inverting input. For an inverting configuration, gain magnitude is Rf / Rin. For example, Rf = 47 kΩ and Rin = 4.7 kΩ gives a non-inverting gain of 11 (approximately 20.8 dB).

Why does an audio amplifier need a split or dual power supply?

A split supply (e.g., ±15 V or ±35 V) allows the output stage to swing symmetrically above and below ground, eliminating the large DC-blocking output capacitor that would be needed with a single-rail supply. Removing the output capacitor improves bass frequency response and reduces phase distortion at low frequencies.

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