A proposed monolithic microwave integrated circuit (MMIC) power amplifier has been designed to operate at frequencies around 28 GHz. According to a computer simulation of performance, this amplifier would operate at a power-added efficiency of more than 60 percent, which is more than 20 percent above that of the prior state-of-the-art power amplifiers in this frequency range.
The design calls for a two-stage amplifier (see Figure 1), each stage containing a pair of pseudomorphic high-electron-mobility transistors (PHEMTs) in a push/pull configuration. The PHEMTs would be biased for class-B or -C operation (conduction angle ≤180° at the fundamental frequency). The push/pull configuration would be achieved by connecting the PHEMTs to each other and to the input and output terminals by embedding them in a coplanar waveguide network of microstrip conductor lines and slots (see Figure 2). The network would include input and output balun transformers in the form of slot/line hybrid junctions.
Efficiency greater than the theoretical maximum of π/4 for classical class-B operation would be achieved by a wave-forming design concept directed toward minimizing the time integral of (drain voltage × drain current) for a given output level at the fundamental frequency. This concept involves (1) exploiting the specific transfer characteristics of the high-performance PHEMTs, to obtain approximately square-wave drain-voltage waveforms, and (2) suppressing the harmonic contents of the square waves on the way to the output terminal to shape the final output waveform into a fundamental-frequency sinusoid. For this purpose, the waveguide network would include wave-forming subnetworks in which the harmonics would be terminated in reactive impedances.
This work was done by James M. Schellenberg of Schellenberg Associates for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.