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An extremely sensitive Ka band radiometer, operating at a temperature of approximately 15 Kelvin, is being developed for the EC funded FARADAY radio astronomy project. To maximize the sensitivity of the radiometer lattice-matched indium phosphide HEMT technology has been used: all of the active components of the radiometer, with the exception of the detectors, have been manufactured on a single wafer process. The radiometer, which uses the band 26 to 36 GHz, will be used to make radio maps of the sky from a 32 metre diameter radio telescope. Several new MMICs have been designed, including a 10 GHz bandwidth low noise amplifier with average noise temperature less than 20 K, and a 180° phase switch using low noise HEMTs as the switching elements. Design principles are described, together with a comparison of modelled and measured results.