High Frequency Design From May 2003 High Frequency Electronics Copyright © 2003 Summit Technical Media, LLC RF POWER AMPLIFIERS RF and Microwave Power Amplifier and Transmitter Technologies — Part 1 By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington, Zoya B. Popovic, Nick Pothecary, John F. Sevic and Nathan O. Sokal F and microwave lope tracking, outphasing, and Doherty. With this issue, we begin a power amplifiers Linearity can be improved through techniques four-part series of articles Rand transmitters such as feedback, feedforward, and predistor- that offer a comprehensive are used in a wide variety tion. Also discussed are some recent develop- overview of power amplifier of applications including ments that may find use in the near future. technologies. Part 1 covers wireless communication, A power amplifier (PA) is a circuit for con- the key topics of amplifier jamming, imaging, radar, verting DC input power into a significant linearity, efficiency and and RF heating. This amount of RF/microwave output power. In available RF power devices article provides an intro- most cases, a PA is not just a small-signal duction and historical amplifier driven into saturation. There exists background for the subject, and begins the a great variety of different power amplifiers, technical discussion with material on signals, and most employ techniques beyond simple linearity, efficiency, and RF-power devices. At linear amplification. the end, there is a convenient summary of the A transmitter contains one or more power acronyms used—this will be provided with all amplifiers, as well as ancillary circuits such as four installments. Author affiliations and con- signal generators, frequency converters, mod- tact information are also provided at the end ulators, signal processors, linearizers, and of each part. power supplies. The classic architecture employs progressively larger PAs to boost a 1. INTRODUCTION low-level signal to the desired output power. The generation of significant power at RF However, a wide variety of different architec- and microwave frequencies is required not tures in essence disassemble and then only in wireless communications, but also in reassemble the signal to permit amplification applications such as jamming, imaging, RF with higher efficiency and linearity. heating, and miniature DC/DC converters. Modern applications are highly varied. Each application has its own unique require- Frequencies from VLF through millimeter ments for frequency, bandwidth, load, power, wave are used for communication, navigation, efficiency, linearity, and cost. RF power can be and broadcasting. Output powers vary from 10 generated by a wide variety of techniques mW in short-range unlicensed wireless sys- using a wide variety of devices. The basic tems to 1 MW in long-range broadcast trans- techniques for RF power amplification via mitters. Almost every conceivable type of mod- classes A, B, C, D, E, and F are reviewed and ulation is being used in one system or anoth- illustrated by examples from HF through Ka er. PAs and transmitters also find use in sys- band. Power amplifiers can be combined into tems such as radar, RF heating, plasmas, laser transmitters in a similarly wide variety of drivers, magnetic-resonance imaging, and architectures, including linear, Kahn, enve- miniature DC/DC converters. This series of articles is an expanded version of the paper, “Power Amplifiers and Transmitters for RF and Microwave” by the same authors, which appeared in the the 50th anniversary issue of the IEEE Transactions on Microwave Theory and Techniques, March 2002. © 2002 IEEE. Reprinted with permission. 22 High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS No single technique for power generated by a single alternator. One PAs to operate over two decades of amplification nor any single trans- such transmitter (SAQ) remains bandwidth without tuning. Because mitter architecture is best for all operable at Grimeton, Sweden. solid-state devices are temperature- applications. Many of the basic tech- sensitive, bias stabilization circuits niques that are now coming into use Vacuum Tubes were developed for linear PAs. It also were devised decades ago, but have With the advent of the DeForest became possible to implement a vari- only recently been made practical audion in 1907, the thermoionic vac- ety of feedback and control tech- because of advances in RF-power uum tube offered a means of elec- niques through the variety of op- devices and supporting circuitry such tronically generating and controlling amps and ICs. as digital signal processing (DSP). RF signals. Tubes such as the RCA Solid-state RF-power devices UV-204 (1920) allowed the transmis- were offered in packaged or chip 2. HISTORICAL DEVELOPMENT sion of pure CW signals and facilitat- form. A single package might include The development of RF power ed the transition to higher frequen- a number of small devices. Power out- amplifiers and transmitters can be cies of operation. puts as high as 600 W were available divided into four eras: Younger readers may find it con- from a single packaged push-pull venient to think of a vacuum tube as device (MRF157). The designer basi- Spark, Arc, and Alternator a glass-encapsulated high-voltage cally selected the packaged device In the early days of wireless com- FET with heater. Many of the con- that best fit the requirements. How munication (from 1895 to the mid cepts for modern electronics, includ- the transistors were made was 1920s), RF power was generated by ing class-A, -B, and -C power ampli- regarded as a bit of sorcery that spark, arc, and alternator techniques. fiers, originated early in the vacuum- occurred in the semiconductor houses The original RF-power device, the tube era. PAs of this era were charac- and was not a great concern to the spark gap, charges a capacitor to a terized by operation from high volt- ordinary circuit designer. high voltage, usually from the AC ages into high-impedance loads and mains. A discharge (spark) through by tuned output networks. The basic Custom/Integrated Transistors the gap then rings the capacitor, tun- circuits remained relatively un- The late 1980s and 1990s saw a ing inductor, and antenna, causing changed throughout most of the era. proliferation variety of new solid- radiation of a damped sinusoid. Vacuum tube transmitters were state devices including HEMT, Spark-gap transmitters were rela- dominant from the late 1920s pHEMT, HFET, and HBT, using a tively inexpensive and capable of through the mid 1970s. They remain variety of new materials such as InP, generating 500 W to 5 kW from LF to in use today in some high power SiC, and GaN, and offering amplifica- MF [1]. applications, where they offer a rela- tion at frequencies to 100 GHz or The arc transmitter, largely tively inexpensive and rugged means more. Many such devices can be oper- attributed to Poulsen, was a contem- of generating 10 kW or more of RF ated only from relatively low voltages. porary of the spark transmitter. The power. However, many current applications arc exhibits a negative-resistance need only relatively low power. The characteristic which allows it to oper- Discrete Transistors combination of digital signal process- ate as a CW oscillator (with some Discrete solid state RF-power ing and microprocessor control allows fuzziness). The arc is actually extin- devices began to appear at the end of widespread use of complicated feed- guished and reignited once per RF the 1960s with the introduction of sil- back and predistortion techniques to cycle, aided by a magnetic field and icon bipolar transistors such as the improve efficiency and linearity. hydrogen ions from alcohol dripped 2N6093 (75 W HF SSB) by RCA. Many of the newer RF-power into the arc chamber. Arc transmit- Power MOSFETs for HF and VHF devices are available only on a made- ters were capable of generating as appeared in 1974 with the VMP-4 by to-order basis. Basically, the designer much as 1 MW at LF [2]. Siliconix. GaAs MESFETs introduced selects a semiconductor process and The alternator is basically an AC in the late 1970s offered solid state then specifies the size (e.g., gate generator with a large number of power at the lower microwave fre- periphery). This facilitates tailoring poles. Early RF alternators by Tesla quencies. the device to a specific power level, as and Fessenden were capable of oper- The introduction of solid-state well as incorporating it into an RFIC ation at LF, and a technique devel- RF-power devices brought the use of or MMIC. oped by Alexanderson extended the lower voltages, higher currents, and operation to LF [3]. The frequency relatively low load resistances. 3. LINEARITY was controlled by adjusting the rota- Ferrite-loaded transmission line The need for linearity is one of the tion speed and up to 200 kW could be transformers enabled HF and VHF principal drivers in the design of 24 High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS Figure 1 · SRRC data pulses. Figure 2 · RF waveforms for SRRC and multicarrier signals. modern power amplifiers. Linear tively low data rates and a relatively Depending on the application, the amplification is required when the uncrowded spectrum. signals can have different ampli- signal contains both amplitude and Modern digital signals such as tudes, different modulations, and phase modulation. It can be accom- QPSK or QAM are typically generat- irregular frequency spacing. plished either by a chain of linear ed by modulating both I and Q sub- In a number of applications PAs or a combination of nonlinear carriers. The requirements for both including HF modems, digital audio PAs. Nonlinearities distort the signal high data rates and efficient utiliza- broadcasting, and high-definition being amplified, resulting in splatter tion of the increasingly crowded spec- television, it is more convenient to into adjacent channels and errors in trum necessitates the use of shaped use a large number of carriers with detection.