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This paper presents a fully differential low noise amplifier (LNA) and I/Q down-conversion mixers for 3-5 GHz multiband OFDM (MB-OFDM) UWB receivers. The LNA adopts capacitive cross-coupling common-gate (CG) topology to achieve wideband input matching and low noise figure (NF). Tuning technique is used in the double-balanced Gilbert mixer to realize flat conversion gain in the whole 3-5 GHz UWB band. Degeneration inductors are used in the mixer to improve linearity. Designed in 0.18 mum CMOS technology, the LNA achieves a NF of 3.2-3.5 dB and a S11 of less than -13 dB. The LNA and mixer show a total conversion gain of 23.5~25.5 dB, a NF of less than 5dB, an input IP3 of-4.3 dBm and an input IP2 of 47 dBm, while consuming only 10.1 mA from a 1.8 V power supply. The chip area is 1.38 x 0.95 mm2.