In this paper, a power amplifier system with a bi-purpose auxiliary amplifier is suggested to provide improved efficiency and linearity. This auxiliary amplifier acts as an IMD3 canceller, as well as an efficiency enhancer of the power amplifier. From the design equations of power amplifier systems with main and auxiliary amplifiers, the conditions required to cancel out the IMD3 and to expand the input range to achieve high efficiency are suggested. The IMD3 cancelling is adjusted by controlling a transition parameter of the auxiliary amplifier, and the efficiency enhancement is achieved by the early saturation of the main amplifier at a lower input power, while the auxiliary amplifier supplies the additional power. For the purpose of verification, a power amplifier of 25dBm at 2.35GHz was simulated and implemented with two class-F power amplifiers. The measurement results show a maximum IMD3 improvement of 24dB and peak efficiency of 75%. This work is expected to be useful for the design of efficient power amplifiers with reasonable linearity.