A 2.1-Channel Class-D Amplifier Exploited Coupling Virtual-Audio-Image to Enhance Stereo

This brief presents a novel 2.1-channel stereoenhanced class-D audio amplifier to improve the quality of stereophonic sound in portable devices. By a way of cross coupling, the input audio signal from one channel can be amplified at the same time, with different gains in the two channels. The ratio of gains in the two channels can be adjusted by changing the resistance and then making the position of the virtual-audio-image change, which improves the stereo effect. The bass signals from different channels, which frequency range between 30 and 300 Hz, are superimposed and then amplified in the independent bass channel by using the Sallen-Key bandpass filter. A practical implementation in the SMIC 0.18-μm CMOS process has been done to validate the theoretical results. When a 200-mVPP sinusoidal signal at 1 kHz is applied in one channel and a dc signal applied in the other channel, the amplifier demonstrates a gain difference of -9.4 dB. The simulation and experiment results show that the amplifier can deliver 2.5 W × 2 into 4-Ω loads at 3.6-V supply in the left or right channel, with the total harmonic distortion plus noise (THD+N) of less than 0.1%. The gain in the bass channel is up to 30 dB at 3.6-V power supply, with the THD+N of about 0.05%.