Class-G headphone driver in 65nm CMOS technology

Abstract

Modern cellular phones incorporate hands-free operation, MP3 music playback and DMB reception. The users may wish to use these features for many hours and a low efficiency amplifier could deplete the battery in a short time. There are two classes of power amplifiers usually used for this application: Class-D and Class-AB. Class-D architectures provide the benefit of power efficiency at the cost of slightly reduced performance and a level of switching noise, which might, in some cases, interfere with RF functions such as mobile phone, GPS or FM radio reception. Class AB architecture has the benefit of higher audio quality and doesn't produce any switching noise, but its power efficiency is much lower. Notwithstanding their lower efficiency, the great majority of the headphone drivers in commerce operate in Class AB. A Class G is a high-efficiency analog amplifier without EMI problems that tries to bring together the best of Class AB and Class D. It uses multiple voltage rails and switches to the appropriate voltage as required by the instantaneous output voltage level. In this way, it never uses the high supply voltage when a low voltage output is required. The core of a Class G amplifier is the switching circuitry, which should enable a smooth handover of the load driving between the low voltage supply and the higher one. Furthermore, the switching strategy should satisfy two key points: first, the distortion due to the switching operation must be minimized to maintain high audio quality; second, the switching point must be as close as possible to the low voltage supply to maximize efficiency.