This paper presents a fully integrated CMOS front-end matching network for ambient RF energy harvesting (RFEH) systems. The proposed method utilizes an on-chip step-up stacked transformer integrated to provide impedance transformation and passive voltage boosting. Besides, the on-chip transformer act as a balun which generates differential signal to drive a 5-stage Cross-Coupled Differential-Drive (CCDD) rectifier. Simulation results on a standard 65-nm CMOS process for harvesting ambient RF energy at 900MHz shows the proposed solution attains a peak sensitivity of -16.5 dBm at an output voltage of 1V.