An investigation into the effect of carrier generators on power supply noise in PWM Class D amplifiers

Class D amplifiers are ubiquitous as audio amplifiers due to their significantly higher power-efficiency compared to their linear counterparts (such as Class AB) due to the digital-like switching mode operation of the Class D output stage. However, one drawback of Class D amplifiers is the susceptibility to supply noise, qualified and quantified by Power Supply Rejection Ratio (PSRR) and Power Supply Induced Intermodulation Distortion (PS-IMD). In this paper, PSRR and PS-IMD of Single-ended, 2-state bridge-tied load (BTL) and 3-state BTL Class D amplifiers based on various carrier generators are investigated and compared. We show that the design of the carrier generator is critical and should be designed according to the structure of the Class D amplifier - we show in this paper that the same Carrier Generator may result in a high >70dB PSRR in a 3-state BTL Class D amplifier but an unacceptable 1dB PSRR in a 2-state BTL Class D amplifier. The investigation and comparison provide Class D amplifier designers useful insight on design and optimization of PSRR and PS-IMD.