This paper proposes a Broadband Power Amplifier (BPA) for L and S bands applications based on ATF13786 transistor, using GaAs process. To improve the broadband performance, an approximate transformation of previously designed lumped elements into transmission lines, and a multi-section quarter wave impedance transformer are used. With neatly design of the broadband networks and bias circuit, a maximum gain of 14.89 dB is achieved across 1.1 GHz  3 GHz. At 2 GHz, the simulated large signal results demonstrate that the designed BPA achieves a saturated output power of 17 dBm, with 1-dB compression point at 4 dBm input power level, and a PAE of 20 %. For the whole bandwidth, the input return loss and output return loss are below than-10 dB. The maximum value achieved by the reverse transmission is-20 dB over the operation frequency ranges. Considering the broad frequency coverage, the performance of the proposed design compares favorably with the state-of-art.