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?
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.
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.