5.1 INTRODUCTION

Solid-state devices have largely superseded vacuum tubes in logic and other low-power circuits and even in some very high power applications such as power supplies and power converters below 1 MHz. The only exception seems to be cathode-ray tubes (CRTs), which are less costly than large plasma displays. In radar transmitters, the transition from high-power klystrons, traveling-wave tubes (TWTs), crossed-field amplifiers (CFAs), and magnetrons to solid-state has been more gradual because the power output of individual solid-state devices is quite limited. However, compared with tubes, solid-state devices offer many advantages:

1. No hot cathodes are required; therefore, there is no warmup delay, no wasted heater power, and virtually no limit on operating life.

2. Device operation occurs at much lower voltages; therefore, power supply voltages are on the order of volts rather than kilovolts. This avoids the need for large spacings, oil filling, or encapsulation, thus saving size and weight and leading to higher reliability of the power supplies as well as of the microwave power amplifiers themselves.

3. Transmitters designed with solid-state devices exhibit improved mean time between failures (MTBF) in comparison with tube-type transmitters. Module MTBFs greater than 100,000 h have been measured.

4. No pulse modulator is required. Solid-state microwave devices for radar generally operate Class-C, which is self-pulsing as the RF drive is turned on and off.

5. Graceful degradation of system performance occurs when modules fail. This results because a large number of solid-state devices must be combined to provide the power...