What Are Switching Amplifiers?

Switching amplifiers are power amplifier circuits that amplify signals by rapidly switching output transistors between fully on and fully off states rather than operating them in a linear active region. The dominant implementation is the Class D amplifier, in which a pulse-width modulated (PWM) signal drives a complementary pair of MOSFETs. Because each transistor is either saturated or cut off, it dissipates little power during steady-state operation, yielding efficiencies of 85 to 95 percent compared with 25 to 50 percent for linear Class AB amplifiers. A low-pass reconstruction filter at the output removes the high-frequency switching carrier and recovers the amplified version of the original signal.

The class designation follows the same amplifier classification system used for Class A, B, and AB topologies, where the letter indicates the portion of the signal cycle during which the output device conducts. Class D amplifiers conduct for the full cycle but only at the supply rail voltages, rather than at intermediate linear values, so their transistor dissipation is concentrated at switching transitions rather than distributed across the cycle.

Pulse-Width Modulation and Modulator Topologies

The input analog signal is converted to a PWM stream by comparing it against a high-frequency triangular or sawtooth carrier, typically at 200 kHz to 1 MHz for audio applications. When the input exceeds the carrier, the output is switched high; when the carrier exceeds the input, the output is switched low. The duty cycle of the resulting pulse train is directly proportional to the instantaneous amplitude of the input signal.

Several modulator topologies exist for Class D designs. Open-loop PWM modulation is simple but susceptible to power supply noise and output impedance effects. Closed-loop designs with negative feedback around the output stage suppress these errors, at the cost of added loop stability constraints. Self-oscillating modulators, where the switching frequency is set by the loop delay rather than an external oscillator, achieve very low total harmonic distortion. Analog Devices' technical article on Class D amplifier fundamentals describes how each modulation approach affects distortion, bandwidth, and power supply rejection.

Output Filter Design

The output low-pass filter, typically a second-order LC network, must pass the audio bandwidth while attenuating the switching carrier and its harmonics. Filter inductor and capacitor values are chosen so that the corner frequency falls between the upper audio limit (20 kHz) and the switching frequency, commonly in the range of 50 to 150 kHz. The filter interacts with the load impedance, which is important for loudspeaker applications where impedance varies with frequency. Filterless Class D topologies exist for driving resistive loads, relying on the load's own inductance and the short PCB trace lengths to adequately attenuate the high-frequency content without discrete LC components.

Electromagnetic interference (EMI) is a key design concern. The switching transitions generate harmonic energy extending well above the carrier frequency. Spread-spectrum techniques that vary the switching frequency slightly reduce peak emissions, and careful PCB layout minimizes radiated coupling from the high-current switching loop.

Efficiency and Thermal Management

The IEEE Xplore publication on Class D amplifier design for hearing aids demonstrates how the efficiency advantage of Class D becomes decisive in battery-powered applications: at milliwatt output levels, a linear amplifier might dissipate ten times its useful output as heat, while the Class D equivalent dissipates a small fraction. For high-power applications such as subwoofer amplifiers or motor drive stages producing hundreds of watts, the heat sink and package size reduction from Class D operation is similarly significant. MDPI's review of Class D audio power amplifier technology surveys modern techniques for further improving efficiency and reducing dead-time distortion.

Applications

Switching amplifiers have applications in a range of fields, including:

• Consumer audio amplifiers in soundbars, receivers, and portable speakers
• Hearing aids and wearable audio devices requiring minimal power consumption
• Automotive audio systems where efficiency reduces thermal load on the vehicle
• Industrial motor drivers and servo amplifiers using switching stages for torque control
• RF power amplifiers in transmitters using envelope elimination and restoration techniques