A 900 MHz RF energy harvesting system is proposed for a far-field wireless power transfer application. The topology of a single-stage CMOS rectifier loaded with an integrated boost DC-DC converter is implemented in a 40 nm CMOS technology. The co-design of a cross-coupled CMOS rectifier and an impedance matching network is described to optimize RF-DC conversion efficiency for the target input power. Two tuning techniques are employed to improve the power conversion efficiency of the entire system over a wide input power range, which include using an input-regulated DC-DC converter to control the rectifier with an optimal output voltage, and tuning an impedance matching network to compensate the input impedance variation of the rectifier. The measurement results show the proposed system can achieve an end-to-end power conversion efficiency higher than 30% over a 22dB input power range (-10 to 12 dBm) with a peak value of 47% at 0 dBm for a 1.5 V output voltage. The maximum output voltage of the system is 2.45V with a sensitivity of -15 dBm.