COOLED HFET AMPLIFIER DEVELOPMENT
Centimeter-wave radio astronomy receivers (under 50 GHz) now almost universally use cooled HFET (heterostructure field-effect transistor) amplifiers as the low-noise input amplifier. The NRAO has worked on the development of these amplifiers for many years and is largely responsible for their wide acceptance by the radio astronomy community. HFET amplifiers are reliable, stable, and can be made up to an octave in bandwidth without significant penalty in noise. At high frequencies (above about 10 GHz), where waveguide is the preferred transmission medium, the fundamental bandwidth limit is set primarily by the waveguide bandwidth itself, or about 30% of the center frequency.
At the lowest frequencies, commercially available HFETs made on gallium arsenide (GaAs) have sufficiently low noise to provide nearly optimum performance. NRAO has designed and built a series of such amplifiers whose performance is summarized in an amplifier performance table. These include balanced amplifiers in the range 300 - 1200 MHz which have superior input impedance and better immunity from interference effects than single-ended amplifiers.
At frequencies above about 10 GHz. transistors made on indium phosphide (InP) have superior performance to GaAs HFETS. Using devices manufactured by Hughes, NRAO has recently designed and built amplifiers covering 18 to 116 GHz. This compares favorably to the performance of SIS mixers in this frequency range, as shown in an amplifier noise plot. A new 86 GHz receiver for the VLBA has been built using these amplifiers and is in operational use. An experimental version of the standard 75-110 GHz amplifier covers 68-116 GHz with some degradation in performance at the highest frequency end, partially due to a drop in gain which results in some noise due to the mixer; the noise temperature of this amplifier is shown in this prototype noise plot.
The use of HFETs instead of SIS mixers provides much wider instantaneous bandwidth and requires only a 2-stage refrigerator to cool the amplifier to 15K instead of a Joule-Thompson refrigerator reaching 4K. This results in a greatly simplified and less expensive design, as well as better immunity to overload from interference. We expect most future receivers in the frequency range below 100 GHz will use the new InP HFET amplifiers, including those for the Atacama Large Millimeter Array.
The Central Development Lab (CDL) has temporarily improved the performance of receivers using older GaAs designs by replacing the first stage transistor with an InP device. New amplifiers centered at 22, 30, 40, 60, and 90 GHz have been designed and tested for the Microwave Anisotropy Probe satellite. Some of the new designs will then be used directly in NRAO receivers. Other new designs using InP devices are expected to be developed in 1998.
Since there is little or no commercial interest in cryogenically cooled amplifiers, NRAO makes these available to other institutions on a cost reimbursement basis when time and facilities permit manufacture of extra units. Interested parties should contact the Director of the CDL for further information.
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