Anritsu HFE0703_RaabPart2 电路图.pdf

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1、22High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS RF and Microwave Power Amplifier and Transmitter Technologies Part 2 By Frederick H. Raab, Peter Asbeck, Steve Cripps, Peter B. Kenington, Zoya B. Popovich, Nick Pothecary, John F. Sevic and Nathan O. Sokal P art 1 of this series

2、 introduced basic concepts, discussed the characteristics of sig- nals to be amplified, and gave background infor- mation on RF power devices. Part 2 reviews the basic techniques, rat- ings, and implementation methods for power amplifiers operating at HF through microwave frequencies. 6a. BASIC TECH

3、NIQUES FOR RF POWER AMPLIFICATION RF power amplifiers are commonly desig- nated as classes A, B, C, D, E, and F 19. All but class A employ various nonlinear, switch- ing, and wave-shaping techniques. Classes of operation differ not in only the method of operation and efficiency, but also in their po

4、wer-output capability. The power-output capability (“transistor utilization factor”) is defined as output power per transistor nor- malized for peak drain voltage and current of 1 V and 1 A, respectively. The basic topologies (Figures 7, 8 and 9) are single-ended, trans- former-coupled,and complemen

5、tary.The drain voltage and current waveforms of select- ed ideal PAs are shown in Figure 10. Class A In class A, the quiescent current is large enough that the transistor remains at all times in the active region and acts as a cur- rent source,controlled by the drive. Consequently, the drain voltage

6、 and current waveforms are (ideally) both sinusoidal. The power output of an ideal class-A PA is Po= Vom2/ 2R(5) where output voltage Vomon load R cannot exceed supply voltage VDD. The DC-power input is constant and the efficiency of an ideal PA is 50 percent at PEP. Consequently, the instantaneous

7、efficiency is proportional to the power output and the average efficiency is inversely proportional to the peak-to-average ratio (e.g., 5 percent for x = 10 dB). The uti- lization factor is 1/8. For amplification of amplitude-modulated signals, the quiescent current can be varied in proportion to th

8、e instantaneous signal enve- lope.While the efficiency at PEP is unchanged, the efficiency for lower ampli- Our multi-part series on power amplifier tech- nologies and applications continues with a review of amplifier configurations, classes of operation, device characterization and example applicat

9、ions 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 permissi

10、on. Figure 7 A single-ended power amplifier. 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 tudes is considerably improved. In an FET PA,the implementation requires lit

11、tle more than variation of the gate-bias voltage. The amplification process in class A is inherently linear, hence increas- ing the quiescent current or decreas- ing the signal level monotonically decreases IMD and harmonic levels. Since both positive and negative excursions of the drive affect the

12、drain current, it has the highest gain of any PA. The absence of harmonics in the amplification process allows class A to be used at frequencies close to the maximum capability (fmax) of the transistor. However, the efficiency is low. Class-A PAs are therefore typ- ically used in applications requir

13、ing low power, high linearity, high gain, broadband operation, or high-fre- quency operation. The efficiency of real class-A PAs is degraded by the on-state resistance or saturation voltage of the transis- tor. It is also degraded by the pres- ence of load reactance, which in essence requires the PA

14、 to generate more output voltage or current to deliver the same power to the load. Class B The gate bias in a class-B PA is set at the threshold of conduction so that (ideally) the quiescent drain cur- rent is zero. As a result, the transis- tor is active half of the time and the drain current is a

15、half sinusoid. Since the amplitude of the drain cur- rent is proportional to drive ampli- tude and the shape of the drain-cur- rent waveform is fixed, class-B pro- vides linear amplification. The power output of a class-B PA is controlled by the drive level and varies as given by eq. (5). The DC- in

16、put current is, however, proportion- al to the drain current which is in turn proportional to the RF-output current. Consequently, the instanta- neous efficiency of a class-B PA varies with the output voltage and for an ideal PA reaches /4 (78.5 per- cent) at PEP. For low-level signals, class B is s

17、ignificantly more efficient than class A, and its average efficien- cy can be several times that of class A at high peak-to-average ratios (e.g., 28 vs. 5 percent for = 10 dB). The utilization factor is the same 0.125 of class A. In practice, the quiescent current is on the order of 10 percent of th

18、e peak drain current and adjusted to minimize crossover distortion caused by transistor nonlinearities at low outputs. Class B is generally used in a push-pull configuration so that the two drain-currents add together to produce a sine-wave output. At HF and VHF, the transformer-coupled push-pull to

19、pology (Figure 8) is gen- erally used to allow broadband oper- ation with minimum filtering. The use of the complementary topology (Figure 9) has generally been limited to audio, LF, and MF applications by the lack of suitable p-channel tran- sistors. However, this topology is attractive for IC impl

20、ementation and has recently been investigated for low-power applications at frequen- cies to 1 GHz 20. Class C In the classical (true) class-C PA, the gate is biased below threshold so that the transistor is active for less than half of the RF cycle (Figure 10). Linearity is lost, but efficiency is

21、increased. The efficiency can be increased arbitrarily toward 100 per- cent by decreasing the conduction angle toward zero. Unfortunately, this causes the output power (utiliza- tion factor) to decrease toward zero and the drive power to increase toward infinity. A typical compromise is a conduction

22、 angle of 150 and an ideal efficiency of 85 percent. The output filter of a true class-C PA is a parallel-tuned type that Figure 8 Transformer-coupled push-pull PA. Figure 9 Complementary PA.Figure 10 Wavefrorms for ideal PAs. RadioFans.CN 收音机爱 好者资料库 26High Frequency Electronics High Frequency Desig

23、n RF POWER AMPLIFIERS bypasses the harmonic components of the drain current to ground with- out generating harmonic voltages. When driven into saturation, effi- ciency is stabilized and the output voltage locked to supply voltage, allowing linear high-level amplitude modulation. Classical class C is

24、 widely used in high-power vacuum-tube transmit- ters. It is, however, little used in solid-state PAs because it requires low drain resistances, making imple- mentation of parallel-tuned output filters difficult. With BJTs, it is also difficult to set up bias and drive to produce a true class-C coll

25、ector-cur- rent waveform. The use of a series- tuned output filter results in a mixed-mode class-C operation that is more like mistuned class E than true class C. Class D Class-D PAs use two or more tran- sistors as switches to generate a square drain-voltage waveform. A series-tuned output filter p

26、asses only the fundamental-frequency compo- nent to the load, resulting in power outputs of (8/2)VDD2/R and (2/2)VDD2/R for the transformer-cou- pled and complementary configura- tions, respectively. Current is drawn only through the transistor that is on, resulting in a 100-percent effi- ciency for

27、 an ideal PA.The utilization factor (1/2 = 0.159) is the highest of any PA (27 percent higher than that of class A or B). A unique aspect of class D (with infinitely fast switch- ing) is that efficiency is not degraded by the presence of reactance in the load. Practical class-D PAs suffer from losse

28、s due to saturation, switching speed, and drain capacitance. Finite switching speed causes the transis- tors to be in their active regions while conducting current. Drain capaci- tances must be charged and dis- charged once per RF cycle. The asso- ciated power loss is proportional to VDD3/2 21 and i

29、ncreases directly with frequency. Class-D PAs with power outputs of 100 W to 1 kW are readily imple- mented at HF, but are seldom used above lower VHF because of losses associated with the drain capaci- tance. Recently, however, experimen- tal class-D PAs have been tested with frequencies of operati

30、on as high as 1 GHz 22. Class E Class E employs a single transis- tor operated as a switch. The drain- voltage waveform is the result of the sum of the DC and RF currents charging the drain-shunt capaci- tance. In optimum class E, the drain voltage drops to zero and has zero slope just as the transi

31、stor turns on. The result is an ideal efficiency of 100 percent, elimination of the losses associated with charging the drain capacitance in class D, reduction of switching losses, and good tolerance of component variation. Optimum class-E operation requires a drain shunt susceptance 0.1836/R and a

32、drain series reac- tance 1.15R and delivers a power out- put of 0.577VDD2/R for an ideal PA 23. The utilization factor is 0.098. Variations in load impedance and shunt susceptance cause the PA to deviate from optimum operation 24, 25, but the degradations in perfor- mance are generally no worse than

33、 those for class A and B. The capability for efficient opera- tion in the presence of significant drain capacitance makes class E use- ful in a number of applications. One example is high-efficiency HF PAs with power levels to 1 kW based upon low-cost MOSFETs intended for switching rather than RF us

34、e 26. Another example is the switching- mode operation at frequencies as high as K band 27. The class-DE PA 28 similarly uses dead-space between the times when its two tran- sistors are on to allow the load net- work to charge/discharge the drain capacitances. Class F Class F boosts both efficiency

35、and output by using harmonic resonators in the output network to shape the drain waveforms. The voltage wave- form includes one or more odd har- monics and approximates a square wave, while the current includes even harmonics and approximates a half sine wave. Alternately (“inverse class F”), the vo

36、ltage can approximate a half sine wave and the current a square wave. As the number of har- monics increases, the efficiency of an ideal PA increases from the 50 per- cent (class A) toward unity (class D) and the utilization factor increases from 1/8 (class A) toward 1/2 (class D) 29. The required h

37、armonics can in principle be produced by current- source operation of the transistor. However, in practice the transistor is driven into saturation during part of the RF cycle and the harmonics are produced by a self-regulating mecha- nism similar to that of saturating class C. Use of a harmonic vol

38、tage requires creating a high impedance (3 to 10 times the load impedance) at the drain, while use of a harmonic current requires a low impedance (1/3 to 1/10 of the load impedance). While class F requires a more com- plex output filter than other PAs, the impedances must be correct at only a few sp

39、ecific frequencies. Lumped-ele- ment traps are used at lower fre- quencies and transmission lines are used at microwave frequencies. Typically, a shorting stub is placed a quarter or half-wavelength away from the drain. Since the stubs for different harmonics interact and the open or short must be c

40、reated at a “virtual drain” ahead of the drain capacitance and bond-wire induc- tance, implementation of suitable networks is a bit of an art. Nonetheless, class-F PAs are success- fully implemented from MF through Ka band. A variety of modes of operation in- between class C, E, and F are possi- Rad

41、ioFans.CN 收音机爱 好者资料库 28High Frequency Electronics High Frequency Design RF POWER AMPLIFIERS ble. The maximum achievable effi- ciency 30 depends upon the number of harmonics, (0.5, 0.707, 0.8165, 0.8656, 0.9045 for 1 through 5 har- monics, respectively). The utilization factor depends upon the harmon

42、ic impedances and is highest for ideal class-F operation. 6b. LOAD-PULL CHARACTERIZATION RF-power transistors are charac- terized by breakdown voltages and saturated drain currents. The combi- nation of the resultant maximum drain voltage and maximum drain current dictates a range of load impedances

43、 into which useful power can be delivered, as well as an impedance for delivery of the maxi- mum power. The load impedance for maximum power results in drain voltage and current excursions from near zero to nearly the maximum rated values. The load impedances correspond- ing to delivery of a given a

44、mount of RF power with a specified maximum drain voltage lie along parallel-resis- tance lines on the Smith chart. The impedances for a specified maximum current analogously follow a series- resistance line. For an ideal PA, the resultant constant-power contour is football-shaped as shown in Figure

45、11. In a real PA, the ideal drain is embedded behind the drain capaci- tance and bond-wire/package induc- tance. Transformation of the ideal drain impedance through these ele- ments causes the constant-power contours to become rotated and dis- torted 31. With the addition of sec- ond-order effects,t

46、he contours become elliptical. A set of power con- tours for a given PA somewhat resembles a set of contours for a con- jugate match. However, a true conju- gate match produces circular con- tours. With a power amplifier, the process is more correctly viewed as loading to produce a desired power out

47、put. As shown in the example of Figure 12, the power and efficiency contours are not necessarily aligned, nor do maximum power and maxi- mum efficiency necessarily occur for the same load impedance. Sets of such “load-pull” contours are widely used to facilitate design trade-offs. Load-pull analyses

48、 are generally iterative in nature, as changing one parameter may produce a new set of contours.A variety of different parameters can be plotted during a load-pull analysis, including not only power and efficiency, but also distor- tion and stability.Harmonic impedances as well as drive impedances a

49、re also sometimes var- ied. A load-pull system consists essen- tially of a test fixture, provided with biasing capabilities, and a pair of low- loss, accurately resettable tuners, usually of precision mechanical con- struction. A load-pull characteriza- tion procedure consists essentially of measuring the power of a device, to a given specification (e.g., the 1-dB compression point) as a function of impedance. Data are measured at a large number of impedances and plotted on a Smith chart. Such plots are, of course, critically dependent on the accurate calibration of the tuners, both