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

Abstract A brief history of the Heterostructure Field-Effect Transistor (HFET) is presented. Several interesting physical properties of the HFET are discussed and its inherently low-noise microwave characteristics are explained. The designs of the five new NRAO HFET balanced amplifiers, covering the 300–1200 MHz band, are described and the measured noise performance presented. These 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 bandwidth. The differences in noise performance of single-ended amplifiers with circulators and balanced amplifiers are discussed in the context of experiments involving a high-Q resonant cavity critically coupled to the amplifier.