An audio 91-dB THD third-order fully-differential class-D amplifier

Class-D amplifiers exhibit high efficiency in spite of their simple implementation and, therefore, they are often used in portable devices with typical THD performance of the order of -65 dB. Presently, the possibility of using class-D amplifiers in applications requiring better THD (THD <; -85 dB) is being investigated, in consideration of their possible application in huge markets (like high-performance audio). For this reason, the THD performance of conventional class-D structures is not enough and new topologies have to be analyzed and implemented. In this paper, high-order class-D structures have been investigated to achieve the target THD performance. Among them, we selected a 3^rd-order fully-differential class-D amplifier, which has been implemented in a 0.18-μm CMOS technology, starting from a previously available 1^st-order class-D structure. The measurements on the realized 3^rd-order device show a THD ≈ -91 dB with -1 dB_FS input signal, about 30 dB better than the 1^st-order structure.