A compact Ka-band power amplifier has been built with GaAs field-effect transistors (FETs) and microstrip conductors on a quartz substrate. Prior to the development of this amplifier, Ka-band solid-state amplifiers with useful output power levels (of the order of 1 W or more) were not readily available, and it was common practice to use traveling-wave-tube amplifiers (TWTAs), which are capable of higher power levels but are also relatively bulky and expensive. TWTAs are not well suited for applications in which compactness is paramount. The present amplifier is better suited for such applications, which include phased-array antennas and mobile communication systems.
The figure illustrates the basic functional blocks of the radio-frequency part of the amplifier circuit. There are (1) five driving stages, each containing a JS8864 GaAs FET, (2) two driving stages, each containing a JS8879 GaAs FET, and (3) a power stage containing two JS8879 FETs. Impedance-matching microstrip circuits between the stages are etched in gold film on the quartz substrate. At the output end, the amplifier can be matched to a load via an antenna-carrier plate that contains a tunable coaxial cavity. The load can be a microstrip antenna mounted flush on the antenna carrier plate, or else an external load that is reached via a coaxial connector mounted on the plate.
The amplifier provides 30.5 dB of small-signal gain over a frequency band about 760 MHz wide at a middle frequency of 29.6 GHz. The output power saturates at 28.5 dBm (≈ 0.7 W), and the onset of saturation is characterized by a 1-dB-compression output power of 25.5 dBm (≈ 0.35 W).
The development of this amplifier involved refinement of prior designs and design techniques within constraints and specifications more stringent than those governing the design of solid-state amplifiers that operate at lower frequencies. In addition to providing for a useful output power level at Ka band, the design of this amplifier accommodates the planned addition of a digitally controlled radio-frequency circuit that would produce phase shifts in increments of 22.5°. Development of the amplifier was stopped before the phase-shifting circuit could be incorporated; completion of this aspect of development would be prerequisite to phased-array applications.
Some of the technological advances embodied in this amplifier may have been surpassed by other, more recent advances in solid-state Ka-band technology. Noteworthy examples include advances in monolithic microwave integrated circuits and high- electron-mobility transistors. Nevertheless, the present amplifier can serve as a prototype for design alternatives with commercial potential.
This work was done by Patrick Fink of Johnson Space Center and Roland Shaw, David Kilpatrick, and Sheryl Andrews of Shason Microwave Corp. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.