SUMMARY

2-59 Q-73. At frequencies above 100 megahertz, the intrinsic (i) region causes a pin diode to act as what? Q-74. The pin diode is primarily used for what purpose? Microwave Transistors Transistors, like vacuum tubes, have had a very limited application in the microwave range. Many of the same problems encountered with vacuum tubes, such as transit-time effects, also limit the upper frequency range of transistors. However, research in the area of microwave transistors, and especially MICROWAVE INTEGRATED CIRCUITS (ICs), is proceeding rapidly. GALLIUM-ARSENIDE FET AMPLIFIERS have been developed which provide low-noise amplification up to about 30 dB in the 7- to 18-gigahertz range. The power output of many of these amplifiers is relatively low, approximately 20 to 200 milliwatts, but that is satisfactory for many microwave applications. Research has extended both the frequency range and the power output of gallium-arsenide FET amplifiers to frequencies as high as 26.5 gigahertz and power levels in excess of 1 watt in multistage amplifiers. SILICON BIPOLAR-TRANSISTOR AMPLIFIERS in integrated circuit form have been developed that provide up to 40 watts peak power in the 1- to 1.5-gigahertz range. Other types of microwave transistor amplifiers combined into multistage modules are capable of providing power outputs approaching 100 watts. Microwave transistor amplifiers, because of their stability, light weight, and long life, are rapidly replacing microwave tubes in the first stages of high-powered radar and communications transmitters. In the future new systems will be almost completely solid state. SUMMARY The information that follows summarizes the important points presented in this chapter. The use of microwave frequencies forced the development of special tubes to offset the limitations caused by interelectrode capacitance, lead inductance, and electron transit-time effects in conventional tubes. Microwave tubes, such as the klystron and twt, take advantage of transit-time effects through the use of VELOCITY MODULATION to amplify and generate microwave energy. The KLYSTRON is a velocity-modulated tube which may be used as an amplifier or oscillator. The klystron, when used as an amplifier, requires at least two resonant cavities, the buncher and the catcher. A diagram of a basic klystron is shown at the right.