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The design details and measurement results for a cooled 1-GHz-bandwidth L-band high electron mobility transistor (HEMT) amplifier are presented. No facilities-were available for measuring low-temperature S parameters, but the HEMT noise parameters were measured at a physical temperature of 12 K. The absence of S-parameter information precluded the design of a feedback amplifier, so a balanced configuration was adopted. This has the advantage of providing a good input match even though the amplifiers in the two arms of the balanced circuit are poorly matched. However, there are disadvantages. The loss of the input hybrid degrades the noise temperature and coupling errors in the hybrids, and differences between the amplifiers reduce the gain and result in a noise contribution from the input load. In the amplifier described, these effects degrade the noise temperature by less than 1 K. The amplifier uses commercially available packaged HEMT devices. At a physical temperature of 12 K the amplifier achieves noise temperatures between 3 and 6 K over the 1 to 2 GHz band. The associated gain is approximately 20 dB.