In recent years there has been an increasing demand of portable electronic devices, from mobile phones to personal computers, from MP3 players to hearing-aid systems which have to be smart, light and low-power consuming. As far as the power dissipation is concerned, the audio power results one of the most critical. For this reason, a lot of research has been carried out to and out solutions that allow to reach very high power efficiency, without reducing the audio quality. In fact, even if traditional linear amplifier classes (e.g. A, B, AB) can reach high performance in terms of audio quality, with low distortion, they have severe limitations in terms of effciency, dissipated heat and weight (in particular for high power levels).