| Summary: | 碩士 === 國立中央大學 === 通訊工程研究所 === 93 === Digital class-D amplifiers have been widely used in both Hi-Fi and portable audio playback systems because of its high power efficiency. How to reach the high fidelity while maintaining low power dissipation is a major concern in designing a class-D amplifier. The switching frequency plays a key factor in the tradeoff between the fidelity and the power efficiency. Conventionally, all digital amplifiers are operated at a fixed switching frequency for convenience. In order to achieve the high fidelity performance, this switching frequency must be designed to be high enough so that a simple analog low-pass filter in the output stage can be used. In speech communication applications, a high switching frequency is not required in many operating situations such as in silence or with background noise. This thesis proposes a switching frequency modulation scheme, in which the switching frequency is adaptive modulated in accordance with the speech spectrum and the frame classification. The power dissipation is thus reduced while the high speech quality is maintained. The simulation results show that the speech quality of the proposed digital amplifier with a variable switching frequency achieves similar speech quality compared with the system with a fixed switching frequency operating at 256 kHz while the power dissipation is reduced by 23%.
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