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A veratile amplitude controller is presented, which is intended for phased array applications or QAMmodulators operating directly at the desired radio frequency. Due to the small geometry and the 0.2 pm GaAs/GaALAs HEMTtechnology, a bandwidth from 15 to 60 GHz could be achieved. Coplanar waveguides and lumped elements were used as passive circuit elements. The main topology consists of a differential amplifier and a coplanar balun. The control voltages of the differential amplifier are applied in a push-pull scheme. The balun operates as 1800-power combiner and subtracts both RF-signals. In this way, the comnon mode is rejected and the phase of the out signal changes by 1800, when the amplitude crosses zero.