This paper presents the design of fully integrated transformer-based impedance matching for LTE power amplifier (PA) applications. The LTE standard requires high level of output peak-to-average power ratio; hence, it degrades the efficiency in back-off level. To enhance the efficiency and the output power, the power cell switching (PCS) technique is adopted by using a power-combining transformer. This technique allows configuring PAs in multiple power modes. Transformers are used to design the impedance matching, distribute and combiner power. A PA prototype with transformer-based impedance matching is implemented on the TSMC 65-nm CMOS process. The PA is composed of one driver, four parallel power cells and three transformers. The PA introduces a gain difference of only 2.5 dB but also provides a high PAE enhancement in power back-off. Thanks to the transformer-based power combiner, the proposed PA is able to cover a wide 3-dB bandwidth of 780 MHz.