A 3.54 nJ/bit receiver front-end for 3-5 GHz impulse radio UWB is proposed in this paper. Noncoherent receiver architecture enables the implementation of a low cost, low power and low complexity receiver. Current-reuse technique is employed in the LNA to reduce the power consumption while achieving high gain. The designed LNA can achieve 40 dB voltage gain and the receiver front-end can achieve a detection sensitivity of -90 dBm. The total power consumption of the receiver front-end is 23.6 mW at 1.8 V supply. With a power-on duration of 150 ns (equivalent to 15% for 1 MHz data rate transmission), an energy efficiency of 3.54 nJ/bit is achieved.