Anritsu HFE0104_RaabPart5 电路图.pdf

《Anritsu HFE0104_RaabPart5 电路图.pdf》由会员分享，可在线阅读，更多相关《Anritsu HFE0104_RaabPart5 电路图.pdf（6页珍藏版）》请在收音机爱好者资料库上搜索。

1、46High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS RF and Microwave Power Amplifier and Transmitter Technologies Part 5 By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington, Zoya B. Popovich, Nick Pothecary, John F. Sevic and Nathan O. Sokal The ever-increasing de

2、mands for more band- width,coupled with requirements for both high linearity and high efficiency create ever- increasing challenges in the design of power amplifiers and transmit- ters. A single W-CDMA signal, for example, taxes the capabilities of a Kahn-technique transmitter with a conven- tional

3、class-S modulator. More acute are the problems in base-station and satellite trans- mitters, where multiple carriers must be amplified simultaneously, resulting in peak- to-average ratios of 10 to 13 dB and band- widths of 30 to 100 MHz. A number of the previously discussed tech- niques can be appli

4、ed to this problem,including the Kahn EER with class-G modulator or split- band modulator, Chireix outphasing, and Doherty. This section presents some emerging technologies that may be applied to wideband, high efficiency amplification in the near future. RF Pulse-Width Modulation Variation of the d

5、uty ratio (pulse width) of a class-D RF PA 112 produces an amplitude- modulated carrier (Figure 59). The output envelope is proportional to the sine of the pulse width, hence the pulse width is varied in proportion to the inverse sine of the desired envelope. This can be accomplished in DSP, or by c

6、omparison of the desired envelope to a full-wave rectified sinusoid. The pulse timing conveys signal phase information as in the Kahn and other techniques. Radio-frequency pulse-width modulation (RF PWM) eliminates the series-pass losses associated with the class-S modulator in a Kahn-technique tran

7、smitter. More important- ly, the spurious products associated with PWM are located in the vicinity of the har- monics of the carrier 113 and therefore easi- ly removed. Consequently, RF PWM can accommodate a significant RF bandwidth with only a simple, low-loss output filter. Ideally, the efficiency

8、 is 100 percent. In practice, switching losses are increased over those in a class-D PA with a 50:50 duty ratio because drain current is nonzero during switching. Emerging techniques are examined in this final installment of our series on power amplifier technolo- gies, providing notes on new modula

9、tion methods and improvements in linearity and efficiency 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 Techn

10、iques, March 2002. 2002 IEEE. Reprinted with permission. Figure 59 RF pulse-width modulation. From January 2004 High Frequency Electronics Copyright Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库 48High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS Previous applications of RF

11、PWM have been limited to LF and MF transmitters (e.g., GWEN 114). However, the recent development of class-D PAs for UHF and microwave frequencies (Figure 60) offers some interesting possibilities. Delta-Sigma Modulation Delta-sigma modulation is an alternative technique for directly modulating the

12、carrier produced by a class-D RF PA (Figure 61) PA8,PA9. In contrast to the basical- ly analog operation of RF PWM, delta-sigma modulation drives the class-D PA at a fixed clock rate (hence fixed pulse width) that is generally higher than the carrier fre- quency (Figure 62). The polarity of the driv

13、e is toggled as necessary to create the desired output envelope from the average of the cycles in the PA. Phase is again conveyed in pulse timing. The delta-sigma modulator employs an algorithm such as that shown in Figure 63.The signal is digitized by a quantizer (typically a single-bit comparator)

14、 whose out- put is subtracted from the input signal through a digital feedback loop, which acts as a band-pass filter. Basically, the output signal in the pass band is forced to track the desired input signal. The quantizing noise (associated with the averaging process necessary to obtain the desire

15、d instantaneous output amplitude) is forced outside of the pass band. The degree of suppression of the quantization noise depends on the oversampling ratio; i.e., the ratio of the digital clock frequency to the RF bandwidth and is rela- tively independent of the RF center frequency. An exam- ple of

16、the resultant spectrum for a single 900-MHz carri- er and 3.6-GHz clock is shown in Figure 64. The quanti- zation noise is reduced over a bandwidth of 50 MHz, which is sufficient for the entire cellular band. Out-of- band noise increases gradually and must be removed by a band-pass filter with suffi

17、ciently steep skirts. As with RF PWM, the efficiency of a practical delta- sigma modulated class-D PA is reduced by switching loss- es associated with nonzero current at the times of switch- ing.The narrow-band output filter may also introduce significant loss. Carrier Pulse-Width Modulation Carrier

18、 pulse-width modulation was first used in a UHF rescue radio at Cincinnati Electronics in the early 1970s. Basically, pulse-width modulation as in a class-S modulator gates the RF drive (hence RF drain current) on and off in bursts, as shown in Figure 65.The width of each burst is proportional to th

19、e instantaneous envelope of the Figure 60 Current-switching PA for 1 GHz (courtesy UCSD). Figure 61 Prototype class-D PA for delta-sigma mod- ulation (courtesy UCSD). Figure 62 Delta-sigma modulation. RadioFans.CN 收音机爱 好者资料库 January 200449 desired output.The amplitude-modulated output signal is reco

20、vered by a band-pass filter that removes the side- bands associated with the PWM switching frequency. The PWM signal can be generated by a comparator as in a class-S modulator or by delta-sigma techniques. As with RF PWM and delta-sigma modulation, the series-pass losses and bandwidth limitations of

21、 the high- level modulator are eliminated. The switching frequency in carrier PWM is not limited by capabilities of power- switching devices and can therefore easily be tens of MHz, allowing large RF bandwidths. A second advantage is that carrier PWM can be applied to almost any type of RF PA. A dis

22、advantage is that a narrow-band output fil- ter with steep skirts is required to remove the switching- frequency sidebands, and such filters tend to have losses of 1 to 2 dB at microwave frequencies. Nonetheless, the losses in the filter may be more than offset by the improvement in efficiency for s

23、ignals with high peak-to- average ratios. Power Recovery A number of RF processes result in significant RF power dissipated in “dump” resistors. Examples include power reflected from a mismatched load and dumped by a circulator and the difference between two inputs of hybrid combiner dumped to the b

24、alancing resistor. The notion of recovering and reusing wasted RF power was originally applied to the harmonics (18 percent of the output power) of an untuned LF class-D PA 117. More recently, power recovery has been applied to out- phasing PAs with hybrid combiners 118, 119. The instantaneous effic

25、iency of such a system depends upon both the efficiency of the PA and that of the recovery sys- tem. Since the two PAs operate at full power regardless of the system output, inefficiency in the PA has a significant impact upon the system efficiency at the lower outputs. Nonetheless, a significant im

26、provement over convention- al hybrid-coupled outphasing is possible, and the PAs are presented with resistive loads that allow them to operate optimally. Typically, 50 percent of the dumped power can be recovered. The power-recovery technology can also be used to implement miniature DC-DC converters

27、. Basically, a high-efficiency RF-power amplifier (e.g., class-E) converts DC to RF and a high-efficiency rectifier circuit converts the RF to DC at the desired voltage. Implementation at microwave frequencies reduces the size of the tuning and filtering components, resulting in a very small physica

28、l size and high power density. In a prototype that operates at C band 120, the class-E PA uses a single MESFET to produce 120 mW with a PAE of 86 percent. The diode rec- tifier consists of a directional coupler with two Schottky Figure 65 Carrier pulse-width modulation. Figure 63 Delta-sigma modulat

29、or. Figure 64 Spectrum of delta-sigma modulation. 30 40 50 60 70 80 90 100 Power Spectral Density (dBm/Hz) 600 800 1000 1200 Frequency (MHz) RadioFans.CN 收音机爱 好者资料库 50High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS diodes connected at the coupled and through ports and has a 98-p

30、ercent conversion efficiency and an overall effi- ciency (including mismatch loss) of 83 percent. For a typ- ical DC output of 3 V, the DC-DC conversion efficiency is 64 percent. Switched PAs with Transmission-Line Combiners RF-power amplifiers cannot simply be connected in series or parallel and sw

31、itched on and off to make a trans- mitter module that adapts to variable peak envelope power. Attempting to do so generally produces either lit- tle effect or erratic variations in load impedance, some- times leading to unstable operation and destruction of the transistors. Systems of microwave PAs

32、that are toggled on and off are therefore connected through networks of quarter-wavelength transmission lines. The Doherty transmitter (discussed in part 4 of this series) is a classic example of this sort of technique. An alternative topology (Figure 66) uses shorting switches and quarter-wavelengt

33、h lines to to decouple off- state PAs 121, 122.The inactive PA is powered-down (by switching off its supply voltage), after which its output is shorted to ground. The quarter-wavelength line produces an open circuit at the opposite end where the outputs from multiple PAs are connected together to th

34、e load. This technique may be more easy to implement (especial- ly for multiple PAs) than Doherty because a short is more readily realized than an open. If PA #1 is the only PA active, its load is simply R0. If both #1 and #2 are active, the combination produces an effective load impedance of 2Ro at

35、 the load ends of the lines. Inversion of this impedance through the lines places loads of R0/2 on the RF PAs. Consequently, the peak power output for two active PAs is four times that with a single PA. As in discrete envelope tracking, the RF PAs operate as linear amplifiers. The number of PAs that

36、 are active is the minimum needed to produce the current output power.The peak power is thus kept relatively close to the saturated output, eliminating most of the effects of operating in back-off. The efficiency can therefore reach PEP efficiency at a number of different output levels, as shown in

37、Figure 67. The advantage of this technique is the ease in design associated with relying on short circuits rather than open circuits to isolate the off-state PAs. A possible disadvan- tage is operating individual PAs from multiple load impedances without retuning and a limited number of power steps

38、available (e.g., 9/9, 4/9, 1/9 for a three-PA sys- tem). Electronic Tuning The performance of virtually all power amplifiers is degraded by load- impedance mismatch. Mismatched loads not only reduce efficiency, but also create higher stresses on the transistors. Because high-efficiency PAs generally

39、 require a specific set of harmonic impedances, their use is often restricted to narrow-band applications with well-defined loads. Electronic tuning allows frequency agility, matching of unknown and variable loads, and amplitude modulation. Components for electronic tuning include pin-diode switches

40、, MEMS switches, MEMS capacitors, semicon- ductor capacitors, ceramic capacitors (e.g., BST), and bias- controlled inductors. To date, electronic tuning has been applied mainly to small-signal circuits such as voltage- controlled oscillators. Recently demonstrated, however, are two electronically tu

41、ned power amplifiers. One oper- ates in class E, produces 20 W with an efficiency of 60 to 70 percent, and can be tuned from 19 to 32 MHz (1.7:1 range) through the use of voltage-variable capacitors Figure 66 Switched PAs with quarter-wavelength transmission line combiner. Figure 67 Instantaneous ef

42、ficiency of switched PAs. 52High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS 123, 124.The second (Figure 68) operates in class D, pro- duces 100 W with an efficiency of 60 to 70 percent, and can be tuned from 5 to 21 MHz (4.25:1 range) through the use of electronically tunable in

43、ductors and capacitors 125. Load Modulation The output of a power amplifier can be controlled by varying the drive, gate bias, DC supply voltage, or load impedance. “Load modulation” uses an electronically tuned output filter (Figure 69) to vary load impedance and thereby the instantaneous amplitude

44、 of the output signal.The modulation bandwidth can be quite wide, as it is limited only by the bias feeds to the tuning components. A key aspect of load modulation is a diligent choice of the impedance locus so that it provides both good dynam- ic range and good efficiency. For ideal saturated PAs o

45、f classes A, B, C, and F, the optimum locus is the pure resis- tance line on the Smith chart that runs from the nominal load to an infinite load. For ideal class-E PAs with series inductance and shunt susceptance for optimum operation with the nominal load, the optimum locus is the unity- efficiency

46、 line running from the nominal load upward and rightward at an angle of 65 126. For real PAs, the opti- mum locus is found by examination of load-pull contours. The simple T filter has a single electronically variable element, but provides an approximately optimum locus for class E over the top 12 d

47、B of the dynamic range. The experimental 20-W, 30-MHz 124, 126 shown in Figure 70 achieves a 41-dB range of amplitude variation. The measured instantaneous-efficiency curve (Figure 71) cor- responds to a factor of 2.1 improvement in the average efficiency for a Rayleigh-envelope signal with a 10-dB

48、peak-to-average ratio. A load-modulated PA for communications follows the electronically tuned filter with a passive filter to remove the harmonics associated with the nonlinear elements. Predistortion compensates for the incidental phase mod- ulation inherent in dynamic tuning of the filter. Variat

49、ion of the drive level can be used to conserve drive power and to extend the dynamic range. Figure 70 Load-modulated class-E PA (courtesy GMRR). Figure 71 Instantaneous efficiency of load modula- tion compared to class-B linear amplification. Figure 68 Electronically tunable class-D PA (courtesy GMRR). Figure 69 Load modulation by electronic tuning. 54High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS References 112. Figures 8 and 9 in Part 2 of tis series,High Frequency Electronics, July 2003. 113. F. H. Raab, “Radio frequency p