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Two 3-5-GHz low-power ultra-wideband (UWB) low-noise amplifiers (LNAs) with out-band rejection function using 0.18- ??m CMOS technology are presented. Due to the Federal Communications Commission's stringent power-emission limitation at the transmitter, the received signal power in the UWB system is smaller than those of the close narrowband interferers such as the IEEE 802.11 a/b/g wireless local area network, and the 1.8-GHz digital cellular service/global system for mobile communications. Therefore, we proposed a wideband input network with out-band rejection capability to suppress the out-band properties for our first UWB LNA. Moreover, a feedback structure and dual-band notch filter with low-power active inductors will further attenuate the out-band interferers without deteriorating the input matching bandwidth in the second UWB LNA. The 55/48/45 dB maximum rejections at 1.8/2.4/5.2 GHz, a power gain of 15 dB, and 3.5-dB minimum noise figure can be measured while consuming a dc power of only 5 mW.