Anritsu HFE0503_Raab 电路图.pdf

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1、22High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS R F 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 intro- duction and historical backg

2、round 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 usedthis will be provided with all four installments. Author affiliations and con- tact information are also pro

3、vided at the end of each part. 1. INTRODUCTION 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 requi

4、re- ments 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

5、HF through Ka band. Power amplifiers can be combined into transmitters in a similarly wide variety of architectures, including linear, Kahn, enve- lope tracking,outphasing,and Doherty. Linearity can be improved through techniques such as feedback, feedforward, and predistor- tion. Also discussed are

6、 some recent develop- ments that may find use in the near future. A power amplifier (PA) is a circuit for con- verting 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

7、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, mod- ulators, signal processors, linearizers, and power supplies. The clas

8、sic architecture employs progressively larger PAs to boost a low-level signal to the desired output power. However, a wide variety of different architec- tures in essence disassemble and then reassemble the signal to permit amplification with higher efficiency and linearity. Modern applications are

9、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 sys- tems to 1 MW in long-range broadcast trans- mitters.Almost every conceivable type of mod- ulation is being used

10、in one system or anoth- er. PAs and transmitters also find use in sys- tems such as radar, RF heating, plasmas, laser drivers, magnetic-resonance imaging, and miniature DC/DC converters. With this issue, we begin a four-part series of articles that offer a comprehensive overview of power amplifier t

11、echnologies. Part 1 covers the key topics of amplifier linearity, efficiency and available RF power devices 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 Na

12、than O. Sokal 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

13、 permission. From May 2003 High Frequency Electronics Copyright 2003 Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 24High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS No single technique for power amplification nor any single trans- mitter architecture is best for all applic

14、ations. Many of the basic tech- niques that are now coming into use were devised decades ago, but have only recently been made practical because of advances in RF-power devices and supporting circuitry such as digital signal processing (DSP). 2. HISTORICAL DEVELOPMENT The development of RF power amp

15、lifiers and transmitters can be divided into four eras: Spark, Arc, and Alternator In the early days of wireless com- munication (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 v

16、oltage, usually from the AC mains. A discharge (spark) through the gap then rings the capacitor, tun- ing inductor, and antenna, causing radiation of a damped sinusoid. Spark-gap transmitters were rela- tively inexpensive and capable of generating 500 W to 5 kW from LF to MF 1. The arc transmitter,l

17、argely attributed to Poulsen, was a contem- porary of the spark transmitter. The arc exhibits a negative-resistance characteristic which allows it to oper- ate as a CW oscillator (with some fuzziness). The arc is actually extin- guished and reignited once per RF cycle, aided by a magnetic field and

18、hydrogen ions from alcohol dripped into the arc chamber. Arc transmit- ters 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 oper- ation at LF, and a technique dev

19、el- oped by Alexanderson extended the operation to LF 3. The frequency was controlled by adjusting the rota- tion speed and up to 200 kW could be generated by a single alternator. One such transmitter (SAQ) remains operable at Grimeton, Sweden. Vacuum Tubes With the advent of the DeForest audion in

20、1907, the thermoionic vac- uum tube offered a means of elec- tronically generating and controlling RF signals. Tubes such as the RCA UV-204 (1920) allowed the transmis- sion of pure CW signals and facilitat- ed the transition to higher frequen- cies of operation. Younger readers may find it con- ven

21、ient to think of a vacuum tube as a glass-encapsulated high-voltage FET with heater. Many of the con- cepts for modern electronics, includ- ing class-A, -B, and -C power ampli- fiers, originated early in the vacuum- tube era. PAs of this era were charac- terized by operation from high volt- ages int

22、o high-impedance loads and by tuned output networks. The basic circuits remained relatively un- changed 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 rela- tiv

23、ely 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 sil- icon bipolar transistors such as the 2N6093 (75 W HF SSB) by RCA. Power MOSFETs for HF and VHF ap

24、peared in 1974 with the VMP-4 by Siliconix. GaAs MESFETs introduced in the late 1970s offered solid state power at the lower microwave fre- quencies. The introduction of solid-state RF-power devices brought the use of lower voltages, higher currents, and relatively low load resistances. Ferrite-load

25、ed transmission line transformers enabled HF and VHF PAs to operate over two decades of bandwidth without tuning. Because solid-state devices are temperature- sensitive, bias stabilization circuits were developed for linear PAs. It also became possible to implement a vari- ety of feedback and contro

26、l tech- niques through the variety of op- amps 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 out- puts as high as 600 W were available from a single packaged push-pull device (MRF157). The designer basi- c

27、ally 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 prolif

28、eration variety of new solid- state devices including HEMT, pHEMT, HFET, and HBT, using a variety of new materials such as InP, SiC, and GaN, and offering amplifica- tion at frequencies to 100 GHz or more. Many such devices can be oper- ated only from relatively low voltages. However, many current a

29、pplications need only relatively low power. The combination of digital signal process- ing and microprocessor control allows widespread use of complicated feed- back and predistortion techniques to improve efficiency and linearity. Many of the newer RF-power devices are available only on a made- to-

30、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. 3. LINEARITY The need for linearity is one of the principal driv

31、ers in the design of RadioFans.CN 收音机爱 好者资料库 26High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS modern power amplifiers. Linear amplification is required when the signal contains both amplitude and phase modulation. It can be accom- plished either by a chain of linear PAs or a co

32、mbination of nonlinear PAs. Nonlinearities distort the signal being amplified, resulting in splatter into adjacent channels and errors in detection. Signals such as CW, FM, classical FSK, and GMSK (used in GSM) have constant envelopes (amplitudes) and therefore do not require linear ampli- fication.

33、 Full-carrier amplitude mod- ulation is best produced by high level amplitude modulation of the final RF PA. Classic signals that require lin- ear amplification include single side- band (SSB) and vestigal-sideband (NTSC) television. Modern signals that require linear amplification include shaped-pu

34、lse data modula- tion and multiple carriers. Shaped Data Pulses Classic FSK and PSK use abrupt frequency or phase transitions, or equivalently rectangular data pulses. The resultant RF signals have con- stant amplitude and can therefore be amplified by nonlinear PAs with good efficiency. However, th

35、e resultant sinc-function spectrum spreads sig- nal energy over a fairly wide band- width. This was satisfactory for rela- tively low data rates and a relatively uncrowded spectrum. Modern digital signals such as QPSK or QAM are typically generat- ed by modulating both I and Q sub- carriers. The req

36、uirements for both high data rates and efficient utiliza- tion of the increasingly crowded spec- trum necessitates the use of shaped data pulses. The most widely used method is based upon a raised-cosine channel spectrum, which has zero intersymbol interference during detection and can be made arbit

37、rari- ly close to rectangular 4. A raised- cosine channel spectrum is achieved by using a square-root raised-cosine (SRRC) filter in both the transmitter and receiver. The resultant SRRC data pulses (Figure 1) are shaped somewhat like sinc functions which are truncated after several cycles. At any g

38、iven time, several different data pulses contribute to the I and Q mod- ulation waveforms. The resultant modulated carrier (Figure 2) has simultaneous amplitude and phase modulation with a peak-to-average ratio of 3 to 6 dB. Multiple Carriers and OFDM Applications such as cellular base stations,sate

39、llite repeaters,and multi-beam “active-phased-array” transmitters require the simultane- ous amplification of multiple signals. Depending on the application, the signals can have different ampli- tudes, different modulations, and irregular frequency spacing. In a number of applications including HF

40、modems, digital audio broadcasting,and high-definition television, it is more convenient to use a large number of carriers with low data rates than a single carrier with a high data rate. The motiva- tions include simplification of the modulation/demodulation hardware, equalization, and dealing with

41、 multi- path propagation. Such Orthogonal Frequency Division Multiplex (OFDM) techniques 5 employ carri- ers with the same amplitude and modulation, separated in frequency so that modulation products from one carrier are zero at the frequencies of the other carriers. Regardless of the characteristic

42、s of the individual carriers, the resul- tant composite signal (Figure 2) has simultaneous amplitude and phase modulation.The peak-to-average ratio is typically in the range of 8 to 13 dB. Nonlinearity Nonlinearities cause imperfect reproduction of the amplified signal, resulting in distortion and s

43、platter. Amplitude nonlinearity causes the instantaneous output amplitude or Figure 1 SRRC data pulses.Figure 2 RF waveforms for SRRC and multicarrier signals. RadioFans.CN 收音机爱 好者资料库 28High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS envelope to differ in shape from the correspo

44、nding input. Such nonlinear- ities are the variable gain or satura- tion in a transistor or amplifier. Amplitude-to-phase conversion is a phase shift associated with the signal amplitude and causes the introduc- tion of unwanted phase modulation into the output signal. Amplitude-to- phase conversion

45、 is often associated with voltage-dependent capacitances in the transistors. While imperfect frequency response also distorts a signal, it is a linear process and therefore does not generate out-of- band signals. Amplitude nonlinearity and amplitude-to-phase conversion are described by transfer func

46、tions that act upon the instantaneous signal voltage or envelope. However, memo- ry effects can also occur in high- power PAs because of thermal effects and charge storage. Thermal effects are somewhat more noticeable in III- V semiconductors because of lower thermal conductivity, while charge- stor

47、age effects are more prevalent in overdriven BJT PAs. Measurement of Linearity Linearity is characterized, mea- sured, and specified by various tech- niques depending upon the specific signal and application. The linearity of RF PAs is typically characterized by C/I,NPR,ACPR,and EVM (defined below).

48、 The traditional measure of lineari- ty is the carrier-to- intermodulation (C/I) ratio. The PA is driven with two or more carriers (tones) of equal a m p l i t u d e s . Nonlinearities cause the produc- tion of intermodu- lation products at frequencies corre- sponding to sums and differences of mult

49、iples of the carrier frequencies 6. The amplitude of the third-order or maximum intermodulation distor- tion (IMD) product is compared to that of the carriers to obtain the C/I. A typical linear PA has a C/I of 30 dB or better. Noise-Power Ratio (NPR) is a tra- ditional method of measuring the lin- earity of PAs for broadband and noise-like signals. The PA is driven with Gaussian noise with a notch in one segment of its spectrum. Nonlinearities cause power to appear in the notch. NPR is the ratio of the notch power to the total signal power. Adjacent Channel Power Ratio (ACPR) characte