High-efficiency MESFET linear amplifier operating at 4 GHz

being developed are: 0.5W of output power with a two-carrier intermodulation ratio (C/I) of 40 dB, 8% efficiency and 20-dB gain over the 3.7 to 4.2-GHz frequency range. Presently, only one of the two power amplifiers composing the power stage of the multistage amplifier has been completed and will be described. The power amplifier provides approximately an output power of 300mW with a C/I of 40 dB, an efficiency of 14% and a gain of 13 dB over the 3.7 to 4.2-GHz frequency range. The MESFETs are five-cell devices featuring a gatelength of 1.2 pm and a total gate width of 3000 pm'. The saturated output power at 4.2 GHz is about 1.1W with an associated gain of 10 dB and a small-signal gain of 13 dB. The output tuning for lowest intermodulation distortion differs from the tuning for either maximum saturated output power or for maximum gain; thus a systematic characterization of the active device in terms of constant C/I and constant output power contours was needed. This characterization, the key for the design of highperformance broadband linear amplifiers, was obtained with the aid of a computer-controlled tuner and a spectrum analyzer both connected at the output of the device under test. An example of these data is shown in Figure 1. For any value of C/I there exists a value of load impedance (closest to the center of the contours) that provides the maximum output power. Similar data, obtained at various frequencies and at various RF input powers, made it possible to define the load impedance as a function of frequency for best intermodulation performance. The chosen load impedance for optimum intermodulation performance varies from lO+jO 0 at 3.7 GHz to 10+j2.5 0 at 4.2 GHz. The output circuit, shown in Figure 2, was designed as a double h/4 transformer. It was computer optimized so that its impedance follows closely the optimum load impedance over the operating bandwidth. The circuit was then realized with microstrip lines on a 25-mil thick A1203 substrate. The input tuning circuit matches the low input impedance of the device, approximately 3 a, to the 50 0 of the generator. The The major design goals of a multistage MESFET amplifier