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We have proposed two methods of enhancing efficiency of an envelope tracking power amplifier (ET PA) from an interlock operation. The first is the utilization of sinking current. The sinking current is a critical efficiency reduction factor since it is a wasted power. To reduce the sinking current, the gate bias of the power amplifier (PA) is increased so that the sinking current is delivered to the PA and is utilized for amplification. The other one is the RF input shaping method. The input signal of ET PAs is a modulated RF signal, and the signal does not guarantee fully saturated operation of the PA at all power levels due to g m nonlinearity of a device. To obtain the maximum efficiency for all of the envelope voltage, we have found the optimum RF input conditions and applied it to the input of the PA. To verify the methods, the proposed ET PA is implemented using a Cree CGH40045 GaN HEMT. For a long-term evolution 5-MHz signal with 6.5-dB peak-to-average power ratio, the PA delivers power-added efficiency of 58.76% with 40.14-dBm output power at 889 MHz.