Tiny implantable medical devices in millimeter size demand advanced wireless power solutions that operate at hundreds of megahertz and mainly use passive rectifiers for ac-dc power conversion. A conventional cross-connected (CC) rectifier can operate with high frequency and low input voltage but only achieves good efficiencies in a very narrow input power range, due to the shoot-Through and reverse currents. This work presents a CMOS passive rectifier with active bias tuning (ABT), allowing a widely extended input range with high power conversion efficiency (PCE). In addition, we compensate for the process, voltage, and temperature variations with the ABT scheme that leads to a robust design for very-high-frequency (VHF) operation. Meanwhile, we propose a peak V{\mathrm {OUT}} searching scheme to indicate the charging/discharging directions for the ABT. We obtain a bias-voltage balancing among stacked rectifier stages with a switched-capacitor network. The proposed rectifier is fabricated in a 65-nm CMOS process. Measurement results of three chips show that the proposed rectifier improves the PCE over a wide input range, with an average maximum PCE of 64.4%.