Conventional linear power amplifiers (Class A, AB, or B) are notorious for their inefficiency, dissipating a significant portion of generated power as heat while offering relatively low distortion. However, there exists a solution that combines low distortion, efficient power amplification, and good efficiency: Class D amplifiers. These employ a switching technique akin to that of switching regulators, where the input signal is chopped and converted into a Pulse Width Modulation (PWM) signal. In this process, MOSFET transistors, one p-type (Q1) and one n-type (Q2), alternate between on and off states under the control of a gate driver, switching the output between VDD and ground. During transistor off-states, minimal power is consumed, with some loss occurring during the switch transitions. Class D amplifiers boast efficiencies exceeding 80%, far surpassing their linear counterparts.
The PWM gate drive signal is generated by comparing a triangle wave with the audio input signal, producing a PWM output with a switching frequency typically ranging from 200 kHz to 2 MHz, surpassing the highest frequency of the input signal by a factor of at least five. This PWM signal is then fed to the MOSFET switches. An LC low-pass filter situated between the transistors and the speaker smoothens the PWM pulses, reconstructing the high-power version of the input signal. Capacitor C0 UT eliminates any DC component from the output. The following shows class d amplifier circuit diagram.
The following graphs shows the signal processing hinting the power saving solution in class-d power amplifier.
To achieve a true AC output signal without necessitating positive and negative power supplies, a full H-bridge circuit arrangement is employed. This configuration, depicted in circuit diagram below, utilizes four MOSFETs connected in an H-bridge setup, with the speaker connected between the outputs of the half-bridge. Known as a bridge-tied load, this setup lacks a ground connection for the speaker, enabling bidirectional current flow.
Class D amplifiers, commonly utilized in audio power amplifiers ranging from 10 to 400 watts, exhibit higher harmonic distortion compared to linear Class AB amplifiers but remain within acceptable limits. For projects requiring audio amplification, Class D operational amplifiers are the preferred choice. Notable manufacturers of Class D integrated circuits include International Rectifier, Maxim Integrated, and Texas Instruments.
