A Switched Fast-Startup Ac-Coupled Amplifier for Low-Distortion Detection of Human Vital Signs

Microwave radar sensors have been used as wireless vital sign sensors for some time. After down-conversion to baseband, the radar output signals often require amplification before being digitized. AC-coupled amplifiers offer a simpler overall architecture compared to dc-coupled systems but suffer from long start times when used with frequencies in the range of human vital signs. Previous fast-start circuits have improved start time but introduce distortion at low frequencies. This paper proposes a new ac-coupled baseband amplifier architecture that uses switched MOSFETs to provide faster start time without impacting low frequency performance. The system theory shows its ability to improve start time, while experimental results using a benchtop and radar system demonstrate that the amplifier offers a simple and low-cost method of reducing start time without introducing distortion.