Investigation And Design Of Digital Pulse Width Modulator In D-Class Power Amplifier

With the rapid development of modern microelectronic technology, digital audio devices are progressively replacing analogue ones. In order to amplify digital sound sources, high-precision D/A convertors have to be added between digital sources and conventional amplifi-ers. But that will greatly increase design costs and difficulty. Anew conversion scheme, using all digital methods, from digital sources directly to pulse width signals is discussed and ex-plored. It avoids the introduction of high-precision D/A convertors. The design process is as follows:First, based on the exploration of pulse width modulator structures, there are some disad-vantages in achieving digital pulse width modulation with conventional structures. So a new structure of digital pulse width modulator based on Sigma-delta modulation technology is ex-plored in this paper.Second, the basic structure of Sigma-delta modulator is modulated through the study of basic theory of Sigma-delta modulator and noise shaping. Two optimized high order modula-tor structures are discussed.Third, a 5-order 7-level Sigma-delta modulator is simulated and designed with the help of MATLAB. The overall parameters of the modulator are derived through the designed Noise Transfer Function (NTF). Then, we simulate the stability and input range of the modulator with full parameters. The results show that two structures of the designed 5-order 7-level Sigma-delta modulator have the same quantizer input limit range 1.8V, which satisfies digital audio signals limited in [-1,1]. For the modulated structure of optimized poles only, Signal Noise Ratio (SNR) can be achieved to 103.2dB while the structure of optimized poles and zeros both attains SNR of 128dB. The latter structure gets higher SNR at the cost of structural complexity.Fourth, in order to offer the oversampled signals to the Sigma-delta modulator, based on the explorations of oversampling technology and sample rate conversion technology, a 32 times upsampling convertor is designed through three different structures. Among these struc-tures, CIC Filters cascaded with Half Band Filters and Polyphase Structure Filters are imple-mented in FPGA. After comparing and analyzing the three structures, we find that the arche-type FIR filters which consume excessive system resources, and the other two structures both achieve the goal of 32 times upsampling for digital audio signals.A new kind of digital pulse width modulator based on Sigma-delta modulation technol-ogy is explored in this paper. The modulator offers a direct conversion solution from digital sound signals to pulse-width signals.