Based On ¦²-¡÷ Modulation Of Closed-loop Class-d Power Amplifier Design

Because of its high efficiency, class D power amplifier is applied to audio system more frequently. The traditional class D amplifier is composed of three parts: modulation circuit, driving circuit and power amplification circuit. However, in view of structure, class D amplifier is a switch amplifier, so the nonlinear distortion inevitably appears at the output signal. Further more, class D is a open loop system, it can't suppress the nonlinearity and noise introduced by intermediate stage. For above disadvantages of class D, its application in high fidelity audio system is restricted. Generally, 90dB SNDR is required by high fidelity audio system and this thesis will follow this requirement.As we know, Σ-Δ modulation technique has the advantage of suppressing the low frequency quantization noise, so it is frequently applied in the modulation circuit of class D. But in open loop system, Σ-Δ modulation can only assure the high SNDR of input signal not the output signal because of the noise and nonlinearity introduced by power intermediate stages. However, close loop system can suppress the noise and nonlinearity introduced by intermediate stages at the premise of high loop gain, and Σ-Δ modulation just has this characteristic at low frequency.In this thesis, Σ-Δ modulation and close loop techniques are integrated and a new structure of class D is brought up. With this new structure, not only is high efficiency obtained, but also the high SNDR. At 5V supply voltage, its maxim SNDR is increased to 91dB, efficiency 80%, and maxim output power 0.57W.This thesis is divided into four parts. In chapter 1, the significance of research on class D amplifier is briefly introduced, the research level of class D is analyzed, different techniques are compared. On base of the concluded experience from chapter 1, a close loop class D structure based on Σ-Δ modulation is brought up, which is modeled and simulated at system level with Matlab, and structure and parameters of modulation circuit are decided. Transistor level circuit design is done and simulation is run with designed circuit. The simulation results indicate that not only is SNDR improved with new structure, but also the high efficiency is obtained, the target is reached. In chapter 4, a summary of whole thesis is presented, and plan of following work is brought up.