Abstract: A transformer based power amplifier introducing leakage inductance to extend a working bandwidth thereof and corresponding design methodology are provided. An ultra-wideband transformer comprises a primary coil, a secondary coil mutual coupling with the primary coil, a primary tuning capacitor coupled with the primary coil in parallel and a secondary tuning capacitor coupled with the secondary coil in parallel. By means of setting a self-inductance of the primary coil, a self-inductance of the secondary coil, a mutual coupling factor between the primary coil and the secondary coil, a capacitance value of the primary tuning capacitor, and a capacitance value of the secondary tuning capacitor, a 3 dB bandwidth of the transformer covers a first mutual resonated frequency and a second mutual resonated frequency formed by the transformer.