A Low-Noise Gallium Arsenide Field Effect Transistor Amplifier for 4-GHz Radio

Gallium arsenide Field Effect Transistors (GaAs F E T s ) are effecting a revolution in both the design philosophy and the performance capability of new microwave systems. In addition, these devices can often provide an economical means for significantly upgrading the performance of existing systems. Such is the case with the 4 GHz radio system, where an RF preamplifier with a maximum noise figure of 2 dB is achieved with GaAs F E T s . In this application, each common multichannel amplifier permits the output power of typically five transmitters to be dropped 4 dB, from 5 watts to 2 watts, while still maintaining the system thermal noise objective for 1500 channels. This significantly increases the life of the transmitter amplifier triodes, thus improving the overall system reliability. 479 AMPLIFIER MODULE O-- INPUT Q--D>--O OUTPUT O Fig. 1--Single-ended amplifier with provision for unpowered transmission. II. GENERAL DESIGN CONSIDERATIONS The use of the GaAs FET amplifier as an RF preamplifier for FM systems requires low intermodulation as well as a low noise figure. In addition, since the amplifier is common to several channels (including the protection channel), reliability is of utmost importance. The two most serious failure mechanisms envisioned are: (t) transistor failure and (ii) power supply failure. With either type of failure, the GaAs FET amplifier inherently exhibits an unacceptable transmission loss (>20 dB) for radio applications. Use of a balanced amplifier with two transistors coupled with input and output 3 dB hybrid couplers would reduce the gain by only 6 dB for failure of a single transistor.