An ultra-low power (ULP) sub-threshold CMOS low noise amplifier (LNA) with tunable input matching for 2.4 GHz ISM band wireless sensor network applications is presented. A mathematical approach has been presented to find input matching network for any type of impedance transformation like complex to complex and complex to real. An Impedance transformation using parallel inductor sub-threshold LNA topology has been presented for IEEE 802.15.4a standard using a standard low cost 0.18μm RF CMOS process. The inductive source degeneration is avoided here to facilitate higher gain, better figure of merit with less area. Continuously tuned characteristics are obtained by using a switch, and the input matching circuit is equipped with varactor tuning techniques. It occupies the frequency range between 2.2 GHz and 2.55 GHz, and achieves 24dB gain, a 3.6dB noise figure with-5 dBm P1dB. This draws a supply voltage of 0.532 mA from 1.8 V. © 2020 IEEE.