結合D類放大器之數位式音訊放大器在近年來被廣泛的研究，如何達到高傳真（High fidelity）為其重點，而為了達到高傳真之效果，D類放大器的切換頻率需固定在一高切換頻率，使音訊放大器的輸出失真能儘量降至最低。傳統上，與全數位式音訊放大器相關的系統架構均採固定切換頻率方式。但在語音通訊的應用上，有許多情況如靜音、高背景雜訊時，是不需要如此高的切換頻率。由於功耗消耗與切換頻率成正比，在這些情況下，高切換頻率造成了不必要的功率消耗。為了減少功率消耗，延長可攜式通訊裝置電池的使用時間，本論文提出一種切換頻率調變機制，根據輸入語音訊號的頻率成份及音框分類結果，動態調整D類放大器的切換頻率，以達到降低功率消耗之效果，同時提供高品質之語音播放。經由模擬實驗結果證實，與固定切換頻率於256kHz時相比較，本系統輸出之語音訊號品質與其相近，同時平均而言可降低23%的功率消耗，證明本系統之輸出可維持在一定的品質之上，而同時達到節省功率消耗的目的。 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%.