This paper presents an energy-efficient transmitter for UWB impulse-radio. The proposed circuit uses a novel combined mixer and power amplifier to generate a Gaussian pulse with 1.25GHz bandwidth and center frequency of 7.87GHz. The combined MXR-PA includes a monolithic transformer to reach a maximum output voltage swing of 3.2V_pp. The transformer has been designed so as to maximize the transmitter power efficiency and at the same time to realize a fourth-order ladder filter, in order to reduce out-of band emissions. The transmitter has been implemented in a 0.13μm CMOS technology, occupies an area of 0.14mm² and dissipates 1.92mW for a 10-pulse burst with 1.2V power supply.