Cryogenic, low-noise, balanced amplifiers for the 300-1200 MHz band using heterostructure field-effect transistors

A brief history of the Heterostructure Field-Effect Transistor (HFET) is pr esented. Several interesting physical properties of the HFET are discussed and its inherently low-noise microwave characteristics are explained. The d esigns of the five new NRAO HFET balanced amplifiers, covering the 300-1200 MHz band, are described and the measured noise performance presented. Thes e amplifiers have noise temperatures ranging from 1.5-4 K with 17-28 dB of gain. The input match is better than -15 dB over the specified amplifier ba ndwidth. The differences in noise performance of single-ended amplifiers wi th circulators and balanced amplifiers are discussed in the context of expe riments involving a high-Q resonant cavity critically coupled to the amplif ier.