Eliminating the audible'buzz' in GSM phones: The fully differential amplifier can cancel RF rectification effects and silence cell-phone noise

摘要

Today's cell-phone PCBs are much more complex and internally "noisy" than ever before, due to added functionality and less board space. In many cases, the speaker is very close to the RF antenna, and the audio amplifier is closer to the RF power-amplifier (PA) section. That means the components on the board have to deal with higher levels of RF noise than in previous generations. In many cases, this noise causes RF rectification in the audio amplifier, which results in buzzing or clicking in the speaker. This annoyance has led semiconductor vendors to develop improved audio PAs that offer fully differential architectures with good RF common-mode noise rejection. We'll look at the causes of RF rectification and explain the best architectures for minimizing the problem-in particular Class-AB and Class-D technology with fully differential bridge-tied load (BTL) architectures providing better RF noise immunity compared to the standard master/slave BTL audio amplifiers.