Power Amplifier Circuit Diagram: Class AB and Class D Amplifier Stages Explained

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A power amplifier circuit diagram shows the signal path from an input stage through voltage amplification to an output stage, where transistors or MOSFETs deliver current into a speaker load with minimal distortion.

A power amplifier converts a low-power audio signal (instrument level or line level, typically in the milliwatt range) into sufficient power to drive a loudspeaker load (typically 4–16 Ω). The circuit diagram must show the complete signal chain, the biasing network that establishes the operating point of each stage, the power supply rails, and the thermal management approach.

Class AB amplifiers dominate analogue audio applications. In a Class AB output stage, two complementary transistors — typically an NPN and a PNP bipolar junction transistor (BJT) pair, or an N-channel and P-channel MOSFET pair — are connected in a push-pull configuration. Each transistor conducts for slightly more than 180 degrees of the audio signal cycle (hence 'AB' — between the Class A full cycle and Class B half cycle). The overlap is controlled by a bias network (often a Vbe multiplier circuit, also called a bias spreader) that sets a small quiescent current through both devices simultaneously. This quiescent current eliminates the crossover distortion that would occur if each device switched cleanly at the zero crossing. A Class AB amplifier's typical efficiency is 50–70% — the remainder is dissipated as heat in the output transistors and requires heatsinking.

Class D amplifiers operate on a fundamentally different principle: the output transistors act as switches, alternating between fully on and fully off. The audio signal modulates the duty cycle of a high-frequency pulse-width modulated (PWM) carrier (typically 200 kHz to over 1 MHz). An output low-pass filter (inductor and capacitor) reconstructs the audio signal from the PWM waveform before it reaches the loudspeaker. Because the output transistors spend negligible time in the linear region, Class D efficiency is typically 85–95%, producing far less heat and enabling much smaller physical packaging for a given output power.

Both topologies require adequate power supply decoupling, protection circuitry (DC offset protection, thermal protection, overcurrent protection), and correct grounding of the signal input relative to the output ground to avoid ground loop hum.

How to wire power amplifier circuit diagram

Define the performance requirements Specify the required output power, speaker impedance (4 Ω, 8 Ω, or 16 Ω), supply voltage available, acceptable distortion level (THD+N), signal-to-noise ratio, frequency response, and whether the application favours size and efficiency (Class D) or absolute audio quality (Class AB).
Select the amplifier topology For a high-fidelity home audio application with good heatsinking space, Class AB is a proven choice. For a compact battery-powered application, automotive amplifier, or where heat dissipation is constrained, Class D is preferred. Confirm that a suitable integrated circuit (IC) or discrete transistor pair is available for the chosen topology and output power level.
Design the input and voltage amplification stage The input differential pair (or long-tail pair) provides the initial voltage gain and establishes the feedback reference node. In a discrete design, choose transistors with low noise figure (expressed in nV/√Hz) for the input pair. Connect the non-inverting input to the signal source through a DC-blocking capacitor, and the inverting input to the negative feedback network from the output.
Design the output stage biasing network For Class AB: choose complementary output transistors rated for the peak supply voltage and peak collector (or drain) current. Size the Vbe multiplier to produce the correct quiescent current (typically 25–100 mA for a home audio stage) at operating temperature. Thermally bond the Vbe multiplier transistor to the output stage heatsink.
Add protection circuitry Include a DC offset protection relay or electronic crowbar that disconnects the speaker if a DC fault voltage appears at the output. Add a thermal protection circuit that mutes or shuts down the amplifier if the heatsink temperature exceeds the safe limit. Include input current limiting (emitter resistors in BJT stages) to define the safe operating area.
Design the power supply A Class AB amplifier requires a regulated or well-filtered dual-rail supply (e.g., ±35 V). Size the filter capacitors for the required ripple current and ripple voltage. A Class D amplifier typically accepts an unregulated supply. Decouple the supply locally at the output stage with bulk electrolytic capacitors and bypass with film or ceramic capacitors to handle high-frequency switching transients.
Verify with simulation before construction Simulate the circuit (using SPICE or equivalent) to verify quiescent operating points, AC frequency response, stability margins (gain margin and phase margin), and transient behaviour under load. A minimum gain margin of 10 dB and phase margin of 45 degrees is a common target. Verify stability with full capacitive load (to simulate long speaker cable).

Specifications

Class AB typical efficiency	50–70%
Class D typical efficiency	85–95%
Class A typical efficiency	Up to 25–30%
Typical THD+N (Class AB, well designed)	0.001–0.1% at rated power
Typical Class D PWM carrier frequency	200 kHz – 1 MHz
Audio frequency range	20 Hz – 20 kHz
Typical quiescent current (Class AB home audio)	25–100 mA
Common loudspeaker impedance	4 Ω, 8 Ω, or 16 Ω

Safety warnings

Power amplifier circuits operate from high-voltage DC supply rails (typically ±35 V to ±80 V in high-power designs). These voltages can deliver lethal current and cause serious burns. Always discharge supply filter capacitors before working on the circuit — large electrolytic capacitors can store significant charge even with the mains disconnected. Use a discharge resistor, not a screwdriver short.
Mains-connected power supplies within an amplifier chassis are a shock hazard. Any work on the mains section must be performed by a competent person and with mains power disconnected and verified absent.
Output transistors and heatsinks reach high temperatures during operation. Do not touch heatsinks on a running amplifier. Allow adequate cooling time before working on the output stage.
A DC fault (failed output transistor going to one supply rail) can destroy the loudspeaker. Always include a DC protection circuit when driving valuable loudspeakers from a discrete amplifier design.
High-power Class D amplifiers generate significant switching electromagnetic interference (EMI). The output filter and PCB layout must be designed to meet applicable EMC standards (e.g., IEC 55022/CISPR 22, FCC Part 15) before the product is sold or distributed.

Tools needed

SPICE circuit simulator (for design verification)
Dual-rail bench power supply with current limiting
Digital oscilloscope (for waveform observation and distortion check)
Digital multimeter
Dummy load resistor (non-inductive, rated for amplifier output power)
Soldering iron and solder
PCB etching or prototyping facilities (veroboard / perfboard)
Infrared thermometer or thermal camera (for heatsink verification)

Common mistakes

Failing to thermally couple the Vbe multiplier transistor to the output stage heatsink, causing quiescent current to increase dramatically with temperature and leading to thermal runaway.
Omitting emitter resistors on parallel output transistors, allowing one transistor to hog current and overheat while the others remain cool.
Grounding the signal input ground and output power ground at different points, creating a ground loop that introduces mains hum into the signal path.
Under-sizing the supply filter capacitors, resulting in audible supply rail ripple at high output power levels.
Building the output stage without a DC protection relay and connecting it to loudspeakers, risking expensive speaker damage from a DC fault on first power-up.

Troubleshooting

Audible crossover distortion (harsh, gritty sound at low volumes): Cause: The Class AB quiescent current is too low, insufficient to maintain both output transistors conducting through the crossover region, reverting to effective Class B operation. Fix: Measure quiescent current by monitoring the voltage across the emitter resistors. Adjust the Vbe multiplier trimmer (if present) to increase quiescent current to the specified value. If there is no trimmer, recalculate the Vbe multiplier resistor ratio and replace with appropriate values.
Amplifier outputs are hot immediately on power-up with no signal: Cause: Quiescent current is set far too high (thermal runaway in progress), or an output transistor has failed short-circuit, pulling the output rail high. Fix: Switch off immediately. Measure output transistor junction resistance with a multimeter in diode test mode after discharging the supply capacitors. A shorted transistor will read near-zero in both directions. Replace failed devices. Re-check Vbe multiplier thermal coupling and quiescent current setting before re-powering.
Audible hum at 50 Hz or 60 Hz from the speaker: Cause: Insufficient supply ripple rejection (under-sized filter capacitors or a failed rectifier diode), or a ground loop between the signal input and the amplifier chassis/power ground. Fix: Measure the supply rail voltage with an oscilloscope while the amplifier is running — ripple should be well below 100 mV. If ripple is excessive, add capacitance in parallel with the filter capacitors. If ripple is low, trace the signal ground path and establish a single-point star ground connection for the input circuit.

Frequently asked questions

What is the difference between Class A, AB, and D power amplifiers?

Class A: the output transistor conducts for the full 360-degree signal cycle, giving the lowest distortion but the lowest efficiency (typically 25%). Class AB: two complementary output transistors each conduct for slightly more than 180 degrees, giving low distortion at moderate efficiency (50–70%). Class D: output transistors switch fully on and off at high frequency with PWM modulation, giving high efficiency (85–95%) with low distortion in modern implementations.

What is crossover distortion in a Class AB amplifier?

Crossover distortion occurs near the zero crossing of the audio waveform in push-pull output stages. When the signal transitions between the positive (NPN/N-channel) and negative (PNP/P-channel) transistor, if neither transistor is conducting at that instant (Class B operation), the output signal has a notch or discontinuity. Class AB biasing maintains a small quiescent current through both transistors, keeping them on the threshold of conduction and eliminating the crossover notch.

What is a Vbe multiplier and what does it do in a power amplifier?

A Vbe multiplier is a biasing circuit typically consisting of one transistor and two resistors, connected across the output stage bias network. It produces a voltage drop that tracks the transistor's base-emitter voltage (approximately 0.6 V per junction) multiplied by a factor set by the resistor ratio. Its job is to provide the correct forward bias voltage to the output transistors at their operating temperature, setting the Class AB quiescent current. It is usually thermally coupled to the output stage heatsink to compensate for thermal drift.

Does a Class D amplifier produce more noise or distortion than a Class AB?

Early Class D designs had measurably higher distortion at high frequencies, but modern Class D amplifiers using feedback from the output filter achieve total harmonic distortion plus noise (THD+N) figures comparable to well-designed Class AB designs. Switching noise from the PWM carrier is attenuated by the output filter, though it must still be managed carefully in EMC-sensitive applications.

What size heatsink does a Class AB power amplifier output stage need?

The required heatsink thermal resistance depends on the maximum power dissipation of the output transistors, the maximum ambient temperature, and the transistor's maximum junction temperature. For a Class AB stage, maximum dissipation occurs at approximately 40% of maximum output power. Calculate: P_dissipated = P_supply × (π/(4) – (P_output_peak/P_supply)), then size the heatsink so that Tjunction = Tambient + (P_dissipated × θ_total) remains below the transistor's rated Tjmax.

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