Effect of the input second harmonic control on saturated amplifier is investigated. With only nonlinear input capacitor, the proper phase relationship between fundamental and second harmonic currents for the half-sinusoidal voltage waveform can be achieved at the expense of the enlarged conduction angle. However, with both input and output nonlinear capacitors, the output capacitor, which generates a lot of second harmonic with small higher harmonics, can be employed for the half-sinusoidal voltage shaping. Thus, the input second harmonic can be terminated to reduce the conduction angle for high efficiency. The effect of the input second harmonic control is validated using simplified and real device models. Based on the strategy for the harmonic control, a high-efficiency saturated amplifier is implemented at 3.5 GHz, delivering a power-added efficiency of 78.5% at a saturated output power of 38.2 dBm.