This paper presents the design of a one watt-level RF CMOS Power Amplifier (PA) based on power combining transformers PSCT in 0.13 µm technology using ADS 2011.10. The PA incorporates a parallel combination of four differential PA cores to generate high output power with acceptable efficiency and linearity. The first part the design for class-AB PA for WLAN applications is presented. The PA delivers an Output Power (Pout) of 30 dBm, Power Gain (Gp) of 30 dB and 40% PAE using 2.5 V supply. In the second part class-E PA is designed to provide an output power of 30 dBm, power gain of 30 dB, and 54% PAE at 2.45 GHz using 1.6 V supply. The layout of the transformers is designed and simulated with momentum RF EM simulator of ADS 2011.10 in order to realize a fully integrated power amplifier. The simulated efficiency of the designed transformer was 78% with minimum insertion losses (ILmin) of 0.87 dB.