Class D power amplifiers, one of the most critical devices for application in sound systems, face severe challenges due to the increasing requirement of smartphones, digital television, digital sound, and other terminals. The audio power amplifier has developed from a transistor amplifier to a field-effect tube amplifier, and digital amplifiers have made significant progress in circuit technology, components, and ideological understanding. The stumbling blocks for a successful power amplifier are low power efficiency and a high distortion rate. Therefore, Class D audio amplifiers are becoming necessary for smartphones and terminals due to their power efficiency. However, the switching nature and intrinsic worst linearity of Class D amplifiers compared to linear amplifiers make it hard to dominate the market for high-quality speakers. The breakthrough arrived with the GaN device, which is appropriate for fast-switching and high-power-density power electronics switching elements compared with traditional Si devices, thus, reducing power electronic systems’ weight, power consumption, and cost. GaN devices allow Class D audio amplifiers to have high fidelity and efficiency. This paper analyzes and discusses the topological structure and characteristics and makes a judgment that Class D amplifiers based on GaN amplifiers are the future development direction of audio amplifiers.

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The wire connection under harsh environmental conditions (high temperature or high radiation) results in many limitations. Laser power transmission (LPT) is regarded as a convenient conduit for power delivery wirelessly. LPT has many benefits, which include zero electromagnetic interference, immunization from temperature and radiation, and long transmission distance. Based on the LPT structure, an LPT powered actuator system is proposed. The steady-state characteristic of the system is analyzed, and the result shows the steady-state characteristic mainly depends on the load torque. A robust speed control method is proposed by feedback of the DC motor speed to determine the laser on/off state to adjust the speed. The result indicates that in the analyzed-controllable range, the speed is stably controlled.

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