A NIR CMOS preamplifier with DC photocurrent rejection for pulsed light source

A micro-power transimpedance amplifier is presented to acquire modulated optical signal for heart-rate measurement. The circuit is composed of a transimpedance amplifier (TIA), a sample-and-hold (S/H) circuit, an error amplifier and a current-sink transistor, configured in a feedback loop to reject DC photocurrent generated from photodiode, which varies from person to person. To reduce power consumption, the LED is pulsed and thus the input to the TIA is a modulated current pulse. The proposed circuit is designed to accurately reject the average of the original continuous time signal, rather than that of the pulsed signal. This is achieved by inserting an S/H stage between the TIA and error amplifier. Simulation results show that the circuit works properly from a 2-V power supply and has a rejection range from 40 μA to 95 μA.