The design of a 4 Gc/s nitrogen-cooled non-degenerate parametric amplifier suitable for installation at the focus of a dish aerial in a communication-satellite-system earth-station is described. Two singlediode amplifiers, each of 15 dB gain and 70 Mc/s bandwidth to the −3 dB points, are cascaded to provide a minimum gain of 30 dB and a −3 dB staggered tuned bandwidth of 60 Mc/s with 1 dB ripple in the passband. The noise temperature of the amplifier including a cooled circulator when cooled to 77°K is 35 ± 5°K (with contributions from the waveguide connexions and external circuitry increasing the overall noise temperature to 46 ± 5°K)