Design Of Low-power Filter-free Class-D Audio Power Amplifier

Compared with the traditional analog-audio power amplifier (class-A, class-B and class-AB), the class-D type which is based on switching mode will be greatly developed in the fields of high-power and portable audio and video devices in the future. Because the class-D audio power amplifier has several excellent characteristics such as high efficiency, low distortion, low power, small size and other advantages.This paper designed a single-channel filter-free class-D audio power amplifier, using the technology of pulse width modulation (PWM), feedback loop design, fully differential input and full-bridge output. Base on the principle and the structure of the class-D audio power amplifier, we design and simulate several blocks of the amplifier including input circuits, ramp generator, PWM, output circuits, and so on. The input stage use full-differential common-mode-feedback structure to increase the common-mode signal rejection, and to reduce the influences of RF and noise interference on the amplifiers. We use the architecture of optimum PWM modulator with filterless output, which can reduce quiescent power consumption, save system cost and simplify system design. The output stage use full-bridge output mode (BTL), which utilize high-speed drive circuit to drive the output of power transistors, and set the rational deadtime under the requirement of THD .This chip is based on Hejian 0.35μm 1p2m CMOS process, using Cadence platform for circuit design, simulation and layout. The test results have showed that the performance of the chips is good and meet the requirements well. The chip operates from 2.5V to 5.5V. The sampling frequency is 250KHz. It can provide a power of 2.5W to a 4? load under the terms that the harmonic distortion is less than 10%, the efficiency is as high as 82%, the quiescent current of 2.8mA, and there is no switching noise when turn on or turn off.