RF and microwave power amplifiers and transmitters are used in a wide variety of applications including wireless communication, jamming, imaging, radar, and RF heating. This article provides an introduction and historical background for the subject, and begins the technical discussion with material on signals, linearity, efficiency, and RF-power devices. At the end, there is a convenient summary of the acronyms used—this will be provided with all four installments. Author affiliations and contact information are also provided at the end of each part.
The generation of significant power at RF and microwave frequencies is required not only in wireless communications, but also in applications such as jamming, imaging, RF heating, and miniature DC/DC converters. Each application has its own unique requirements for frequency, bandwidth, load, power, efficiency, linearity, and cost. RF power can be generated by a wide variety of techniques using a wide variety of devices. The basic techniques for RF power amplification via classes A, B, C, D, E, and F are reviewed and illustrated by examples from HF through Ka band. Power amplifiers can be combined into transmitters in a similarly wide variety of architectures, including linear, Kahn, envelope tracking, outphasing, and Doherty.
Linearity can be improved through techniques such as feedback, feedforward, and predistortion. Also discussed are some recent developments that may find use in the near future.
A power amplifier (PA) is a circuit for converting DC input power into a significant amount of RF/microwave output power. In most cases, a PA is not just a small-signal amplifier driven into saturation. There exists a great variety of different power amplifiers, and most employ techniques beyond simple linear amplification.
A transmitter contains one or more power amplifiers, as well as ancillary circuits such as signal generators, frequency converters, modulators, signal processors, linearizers, and power supplies. The classic architecture employs progressively larger PAs to boost a low-level signal to the desired output power.
However, a wide variety of different architectures in essence disassemble and then reassemble the signal to permit amplification with higher efficiency and linearity.
Modern applications are highly varied.
Frequencies from VLF through millimeter wave are used for communication, navigation, and broadcasting. Output powers vary from 10 mW in short-range unlicensed wireless systems to 1 MW in long-range broadcast transmitters.
Almost every conceivable type of modulation is being used in one system or another. PAs and transmitters also find use in systems such as radar, RF heating, plasmas, laser drivers, magnetic-resonance imaging, and miniature DC/DC converters.
HISTORICAL DEVELOPMENT
Spark, Arc, and Alternator
I
n the early days of wireless communication (from 1895 to the mid
1920s), RF power was generated by spark, arc, and alternator techniques.
The original RF-power device, the spark gap, charges a capacitor to a high voltage, usually from the AC mains. A discharge (spark) through the gap then rings the capacitor, tuning inductor, and antenna, causing radiation of a damped sinusoid.
Spark-gap transmitters were relatively inexpensive and capable of generating 500 W to 5 kW from LF to MF [1].
The arc transmitter, largely attributed to Poulsen, was a contemporary of the spark transmitter. The arc exhibits a negative-resistance characteristic which allows it to operate as a CW oscillator (with some fuzziness). The arc is actually extinguished and reignited once per RF cycle, aided by a magnetic field and hydrogen ions from alcohol dripped into the arc chamber. Arc transmitters were capable of generating as much as 1 MW at LF [2].
The alternator is basically an AC generator with a large number of poles. Early RF alternators by Tesla and Fessenden were capable of operation at LF, and a technique developed by Alexanderson extended the operation to LF [3]. The frequency was controlled by adjusting the rotation speed and up to 200 kW could begenerated by a single alternator. One
such transmitter (SAQ) remains operable at Grimeton, Sweden.
Vacuum Tubes
With the advent of the DeForest audion in 1907, the thermoionic vacuum tube offered a means of electronically generating and controlling RF signals. Tubes such as the RCA UV-204 (1920) allowed the transmission of pure CW signals and facilitated the transition to higher frequencies of operation.
Younger readers may find it convenient to think of a vacuum tube as a glass-encapsulated high-voltage FET with heater. Many of the concepts for modern electronics, including class-A, -B, and -C power amplifiers, originated early in the vacuumtube era. PAs of this era were characterized by operation from high voltages into high-impedance loads and by tuned output networks. The basic circuits remained relatively unchanged throughout most of the era.
Vacuum tube transmitters were dominant from the late 1920s through the mid 1970s. They remain in use today in some high power applications, where they offer a relatively inexpensive and rugged means of generating 10 kW or more of RF power.
Discrete Transistors
Discrete solid state RF-power devices began to appear at the end of the 1960s with the introduction of silicon bipolar transistors such as the 2N6093 (75 W HF SSB) by RCA. Power MOSFETs for HF and VHF appeared in 1974 with the VMP-4 by Siliconix. GaAs MESFETs introduced in the late 1970s offered solid state power at the lower microwave frequencies.
The introduction of solid-state RF-power devices brought the use of lower voltages, higher currents, and relatively low load resistances. Ferrite-loaded transmission line transformers enabled HF and VHFPAs to operate over two decades of bandwidth without tuning. Because solid-state devices are temperaturesensitive, bias stabilization circuits were developed for linear PAs. It also became possible to implement a variety of feedback and control techniques through the variety of opamps and ICs.
Solid-state RF-power devices were offered in packaged or chip form. A single package might include a number of small devices. Power outputs as high as 600 W were available from a single packaged push-pull device (MRF157). The designer basically selected the packaged device that best fit the requirements. How the transistors were made was regarded as a bit of sorcery that occurred in the semiconductor houses and was not a great concern to the ordinary circuit designer.
Custom/Integrated Transistors
The late 1980s and 1990s saw a proliferation variety of new solidstate devices including HEMT, pHEMT, HFET, and HBT, using a variety of new materials such as InP, SiC, and GaN, and offering amplification at frequencies to 100 GHz or more. Many such devices can be operated only from relatively low voltages. However, many current applications need only relatively low power. The combination of digital signal processing and microprocessor control allows widespread use of complicated feedback and predistortion techniques to improve efficiency and linearity. Many of the newer RF-power devices are available only on a madeto- order basis. Basically, the designer selects a semiconductor process and then specifies the size (e.g., gate periphery). This facilitates tailoring the device to a specific power level, as well as incorporating it into an RFIC or MMIC.
By Edgar Alberto Servita 18.856.338
CAF
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