Guidelines for Improving the RF Immunity of Audio Amplifiers

RF immunity, or RF susceptibility, is quickly becoming as important a design consideration as PSRR, THD+N, and SNR in the audio portions of cellular phones, MP3 players and notebook computers. Bluetooth is proliferating as a wireless-serialcable replacement for headsets and microphones in mobile applications. Wireless LAN (WLAN), using the IEEE 802.11b/g protocol, is practically standard in PC and laptop computers. The TDMA multiplexing scheme found in GSM, PCS and DECT technologies remains a considerable RF nuisance. Today’s dense RF environment raises concerns regarding an electronic circuit’s susceptibility to RF and RF’s impact on the integrity of the overall system. The audio amplifier is a system block that can be susceptible to RF. An audio amplifier can demodulate the RF carrier and reproduce the modulated signal and its harmonic components at the output. Some of the frequencies fall into the audio baseband, producing unwanted audible noises at the system’s speaker or headphone output. To avoid this problem, a system designer must fully understand the limitations of the selected amplifier IC and its respective PCB layout. This article will guide designers to optimize the RF immunity performance of an audio amplifier at the board level.