A Low-EMI 3-W Audio Class-D Amplifier Compatible With AM/FM Radio

A 3-W audio class-D amplifier compatible with AM/FM radio while requiring minimal external filtering is presented. The amplifier uses driver slew rate control to reduce electromagnetic interference (EMI) in the FM band and digital modulation to spread the PWM common-mode signal and place a null at the tuned AM channel. Two original feedback techniques are employed to linearize the H-bridge. One cancels signal-dependent propagation delays in the H-bridge to improve harmonic distortion and eliminate high-frequency noise folding into the audio band. The second technique boosts the power supply rejection ratio to simplify and reduce the cost of supply filters. The amplifier system, implemented in a standard 110-nm CMOS process, operates with a supply voltage ranging from 2.7 to 6.6 V and achieves an SNR of 95 dB and 85% efficiency.