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This paper presents a novel dual band module which operates as amplifier or frequency doubler according to the input frequency. It satisfies the 802.11a/b/g frequency band of wireless LAN standard. A conventional dual band wireless LAN transmitter consists of the separating power amplifiers operating at each frequency band, but the proposed dual band module operates as an amplifier for the 802.11b/g signal and as a frequency doubler for the 802.11a signal according to signal frequency and bias. We have obtained stop band characteristics by using the microstrip defected ground structure (DGS) to suppress the fundamental frequency of the frequency doubler as well as the second harmonic of the amplifier. Second harmonic suppression is below -59dBc in the amplifier mode, fundamental suppression is below -35dBc in the frequency doubler mode. The designed module has 17.8dBm output at 2.4GHz and 10.1dBm power for 5.8GHz output, and shows 0.8dB and 2.8dB output power at 2.4GHz and 5.8GHz, respectively.