The present invention is related to low power low noise amplifiers (LNA) including on/off switching capability, which are preferably used for impulse radio (IR) ultra-wideband (UWB) receivers. In the invention a very low power and high-gain Common-Gate Capacitive Cross Coupling Cascaded LNA is proposed, to provide an optimum gain for large ON times. The invention provides also a Common-Source Cascoded LNA for shorter received pulse widths, with the aim of implementing a fast ON/OFF switching. An input signal is applied to a first amplifying stage and the switching means are coupled to a second amplifying stage so that they do not interfere with the matching network of the first stage. The invention is also related to a method of amplifying a radio frequency signal using a plurality of low noise amplifying elements.