A theoretical analysis and experimental confirmation of the optimally loaded and overdriven RF power amplifier

Although the conventional Class B approach to RF amplifier design yields high output power and reasonable collector efficiency (78.5 percent at maximum output power), neither the power nor the efficiency are optimum, and both are dependent on RF drive level. This paper presents an analysis of appropriately selected collector voltage and current waveforms which determine the load impedance at the fundamental and harmonically related frequencies; these conditions define the ClassB "optimum efficiency" case with 100 percent collector efficiency and 1.27 times the conventional Class B value of output power. If the RF drive level is increased, and the collector voltage and current waveforms are appropriately selected so that the amplifier is overdriven, a different load impedance is determined; these conditions define the "optimum power" case with 1.46 times the conventional Class B value of output power and 88 percent collector efficiency. The "optimum power" case has the added advantage that the output power and collector efficiency are essentially constant over a predetermined range of drive level. Finally, the theory is verified by the construction and testing of a UHF power amplifier having a power output of 46 watts and an over-all dc to RF conversion efficiency of 65 percent with a 1 dB for 10.5- dB insensitivity of output power to RF drive.     

The positively tapered traveling-wave tube

A new and unexpected concept of TWT circuit modification which results in high basic beam and depressed collector efficiencies is described. The test results which are presented disclose that 50 percent depressed collector efficiency has been achieved over a 10 per cent bandwidth, and that maxima of 42 and 52 percent, respectively, of beam and depressed collector efficiency have been measured at spot frequencies. The tube which produced these results contained a single continuous helix tapered at the output end so that the phase velocity of the fundamental circuit wave was increased.     

计算机设计的降压收集极的实验验证

本文提出了利用文献[1]的轴对称多级降压收集极计算模型和通用程序进行设计的方法,以及叙述了用该法设计的收集极在高效率C波段行波管中的使用情况。     

Theory and performance of depressed trochoidal collectors for improving traveling-wave tube efficiency

This paper discusses the theory and performance of depressed trochoidal collectors which are used to improve the efficiency of S-band traveling-wave amplifiers. These crossed-field multistage collectors sort the electrons in the spent beam into velocity classes, and collect each class at an appropriately low potential. The sorting and collecting actions depend on the tendency of electrons to flow to the lower-potential anode in a split-anode cylindrical magnetron. The magnetic field required in the collector region to produce sorting action is evaluated, and the design parameters of the collector structure are determined from an analysis of electron ballistics inside the collector tunnel. In the trochoidal-collector tube, spent electrons are sorted into eight energy levels and collected by eight electrodes biased at different potentials. This tube provides information on the fraction of electrons collected at a voltage equal to, or greater than, the equivalent voltage lost by the electrons in the process of RF interaction, and on the collection voltages required for spent electrons at different levels of RF drive. Complete performance curves for the assembly are included for both an unmodulated beam and an RF driven beam. When the tube produces a power output of 16 watts and a saturated gain of 35 db at 2360 Mc, the collector efficiency is 42 per cent, the over-all efficiency 38 per cent, and the electronic efficiency 16 per cent. These results are achieved without adverse effects on the RF performance as a result of beam instabilities and regenerations caused by space-charge blocking of electron flow or by motion of electrons returning to the helix region.     

High-Power CW Klystron for Fusion Experiments

A 3.7-GHz 700-kW klystron in continuous-wave (CW) operation has been developed to upgrade the lower hybrid RF plasma heating power in a tokamak up to 10 MW. The klystron is equipped with a diode gun, a five-cavity RF structure, two BeO-disk RF windows, and a large-size X-ray-shielded hypervapotron collector. The output power is recombined in a four-port junction which we also developed. The tube is designed to deliver 620-kW CW RF power with a mismatched load (VSWR = 1.4) and 700-kW CW with a matched load. Several prototypes have been built with successive design improvements. The major improvement was to change one single RF output into two RF outputs. The most recently built prototype meets all design specifications at 73.1 kV and 20.7 A, with an efficiency of 47% on a matched load and 40% with a 1.4 : 1 VSWR load, worst case phase. The power losses dissipated in the body have been measured as low as 17 kW, which corresponds to the RF heating and implies low beam interception. The measured temperatures of the output cavity noses and collector wall have been kept below 130degC and 200degC, respectively, which results in large thermal margin.     

Experimental analysis of biased gap klystron

A commercially available multicavity klystron rated at 45 percent efficiency and equipped with external cavities was modified so that a dc potential could be applied across the interaction gaps. When the tube was carefully tuned, an efficiency of 60 percent was obtained; when an accelerating bias was applied across the penultimate gap of the tube, efficiencies of 66 percent were recorded without collector depression, and of over 70 percent with collector depression. In addition, applying bias to the tube made possible efficiencies in excess of 60 percent for a five to one range of power levels (for different cathode voltages). Furthermore, it was possible to apply bias to the tube in such a way that, for a constant drive level, the power output remained constant, the efficiency remained constant at over 50 percent, but the phase could be varied through more than 180°.     

Experimental verification of a computational procedure for the design of TWT-refocuser-MDC systems

A comparison of analytical and experimental results is presented for a high performance dual-mode helix TWT, equipped with multistage depressed collectors (MDC), and operated over conditions ranging from saturation to the linear regime. The computations are carried out with advanced multidimensional computer programs which model the electron beam as a series of disks or rings of charge and follow their trajectories from the RF input of the TWT, through the slow-wave structure, through the refocusing system, to their points of impact in the depressed collector. Twr performance, collector efficiency, and collector current distribution are computed and compared with measurements. Very good agreement is obtained between computed and measured TWT performance and collector efficiencies. The analytical techniques were subsequently applied to the design of a smaller MDC of nearly equal efficiency.     

Exact analysis of class E tuned power amplifier with only one inductor and one capacitor in load network

Presents an exact analysis of that amplifier, an accurate and simple design procedure, and experimental results which confirm the theoretical analysis. The following performance parameters are determined for optimum operation with any switch duty ratio: the current and voltage waveforms, the peak values of collector current and collector-to-emitter voltage, the output power, the power-output capability, and the component values of the load network. The output harmonic spectrum is given for several values of switch duty ratio. The transistor power losses and collector efficiency are also estimated. The measured collector efficiency was over 95 percent with 3 W output power at 1 MHz, using a BSX60 TO-39 transistor.     

一种温度不敏感的高线性功率放大器

设计了一种温度不灵敏的高线性度的射频功率放大器芯片,采用新颖的带温度反馈环路的有源片上自适应偏置电路,该电路降低了温度引起的放大器集电极直流电流分量的变化量,补偿了由温度变化而引起的性能偏差,进而有效提高了放大器的线性度。基于这个温度不灵敏的偏置结构采用InGaP/GaAs HBT工艺设计了一个工作在2110~2170 MHz频段的功率放大器。测试结果表明,该功放在工作频段内的增益大于等于35.3 dB;在中心频率2140 MHz处,1 dB功率压缩点大于33 dBm,功率附加效率在输出功率24.5 dBm时为18%;使用LTE_FDD调制信号,获得邻信道功率比为-47 dBc。在环境温度为-40℃、+25℃和+80℃条件下,功放的增益平坦度较好,增益变化量小于1.5 dB,输出级集电极电流基本不变,有效降低了功放对温度的敏感性。     

A new grid-controlled pulse TWT with depressed collector

A grid-controlled pulse TWT with depressed collector and PPM focusing system is reported in this paper. It operates at X-band and delivers peak output power of 1 kW with saturation gain of 47 dB. The duty cycle is 3%. The electron beam transmission is 95% with RF output at saturated condition. The efficiency is not less than 30% (excluding the heater power).     

一支新型降压收集极栅控脉冲行波管

正 本文介绍一支新研制的X波段、降压收集极、栅控脉冲功率行波管。该管采用大电流密度钪酸盐阴极和小型化钐钴PPM聚集。利用切断慢波线缩短了管长、提高了增益。采用新型散热措施,在3％的工作比下,输出功率达到千瓦级。用单栅,栅丝直径为0.025 mm,其电流截获率约为10％。采用单级降压收集极,降压度40％。电子注动态流通率约为95％,图1为降压收集极的实验曲线。试验管用的是活动收集极,其轴向位置可     

Performance characteristics of an MSDC IOT amplifier

The design and performance of an inductive output tube (IOT) based amplifier incorporating multistage depressed collector (MSDC) technology is summarized in this paper. The amplifier performance goal was to provide an enhanced efficiency rf amplifier for the UHF digital television broadcast market. The operational frequency of the amplifier is 470 to 820 MHz. The design emphasizes simplicity to minimize upgrade requirements for existing digital television transmitter designs and insure a low cost IOT. The enhanced efficiency IOT is based on an existing standard collector IOT capable of providing peak envelope powers of 60 kW. The collector design incorporates three elements with one element serving as a depression stage. The collector design curtails the reflection of spent beam electrons back to the IOT interaction gap and minimizes the generation of secondary electrons at critical collector surface locations. Testing emphasized the 8-VSB modulation scheme adopted by the ATSC for digital television broadcasting. Performance results indicate that the IOT electrical efficiency increased from 34% to 51% with the addition of MSDC collector when the amplifier is operated at 15 kW average rf power using the 8-VSB modulation scheme     

多级降压收集极行波管高压电源的设计

为了提高真空管雷达发射机的效率、缩小整机的体积和重量,常使用多级降压收集极行波管作为功率射频放大器.为了保证多级降压收集极行波管高增益、高效率和良好线性等性能发挥的更好,就要在行波管每个电极上加合适的电压,确保行波管内建立稳定的电场.因此,根据行波管各电极的特性合理地选择各收集极的电压以及合理的设计高压电源显得尤其重要.文中结合多级降压收集极行波管的工作特性介绍了4种高压电源的设计方法及其特点.     

A novel electrostatic-focusing depressed collector for linear beam tubes

In a theoretical study, various types of retarding electrostatic fields are compared with respect to their usefulness in a depressed collector. Without space charge, fields having focusing properties lead to higher collector efficiencies ηcollthan defocusing fields. For a special case, the hyperbolic field, ηcollis given as a function of the angular beam spread, beam diameter, and initial beam energy. ηcollincreases with 1) decreasing angular beam spread, 2) increasing ratio of collector size to beam diameter, 3) increasing initial beam energy, and 4) the number of collector stages. Experimentally, a 3-stage collector of the hyperbolic field type was studied in connection with a PPM-focused high-power Dual Mode TWT, having a pulse-up ratio of 8 dB. At 50-, 75,, and 100- percent depression, the tube efficiency was 26 percent in the low-power mode and 34 percent in the high-power mode, corresponding to collector efficiencies of 76 and 71 percent, respectively.     

微波功率管特征参数测试

由于大功率全固态电子设备在体积、性能和可靠性方面的优势,微波功率管将逐步取代行波管在雷达发射机中的核心地位。随着微波功率管的大量应用,正确的测试和评价功率管性能已经显得迫切。微波功率管的特性参数包括直流和射频两部分,直流参数包括:反向击穿电压、反向漏电流和直流增益;射频参数包括:输出功率、增益、顶降、集电极效率、反射系数、1 dB过激励输出功率和抗适配等。文中对微波功率管的特征参数先从理论上阐述,然后结合DC和RF测试平台详细描述了各个特征参数的测试,得出测试结果,并对测试结果进行分析。     

A cyclotron-wave rectifier for S-band and X-band

A device for converting microwave power into either dc power or low-frequency ac power has been investigated both theoretically and experimentally. Rotational energy is stored in an electron beam by a Cuccia coupler, then converted to longitudinal energy by interaction with a space-dependent dc magnetic field, and finally recovered as electric energy by a depressed collector. A simple kinematic analysis demonstrates that the Cuccia coupler can convert large amounts of microwave power into electron beam rotation. Limits on the electric field strength and asynchronism between signal frequency and cyclotron frequency are established for both classical and relativistic coupler operation. Efficiency analyses of the process of conversion from orbital energy to dc electric energy, both classical and relativistic, indicate that the efficiency exceeds 95 percent for a particular range of operating conditions. As an ac power supply, the device responds to the modulating frequency of the signal. Theory predicts near-negligible harmonic distortion as well as flatness of frequency response from dc to about 1.0 MHz modulating frequency. Four tubes and a prototype (with "artificial" coupler) were tested experimentally. The first three tubes exhibited efficiencies up to 22 percent, being hindered by reflection of electrons into the coupler. Certain design changes were tested on the prototype, where efficiencies from 36 percent to 75 percent were obtained. Incorporating these design changes into the fourth tube yielded measured efficiencies up to 34 percent, or when corrected to disregard unusually large cavity losses, up to 59 percent. Experimental tests of the tube as an ac converter yielded excellent frequency response and about 20 percent second-harmonic distortion. It is concluded that the theoretical foundation of efficiency predictions has thus far been based on too optimistic assumptions.     

High-Efficiency WCDMA Envelope Tracking Base-Station Amplifier Implemented With GaAs HVHBTs

A record high-performance GaAs high-voltage HBT (HVHBT)-based WCDMA base-station power amplifier is presented, which uses an envelope tracking bias system to achieve high efficiency and linearity. A wideband envelope amplifier provides dynamic collector supply biasing to the RF stage. A digital pre-distortion technique is employed to satisfy the linearity specifications of WCDMA. The measured overall power-added efficiency reached 58% with a normalized root-mean-square (RMS) error of 2.9% and an adjacent channel leakage ratio (ACLR) of -49 dBc at 5-MHz offset at an average output power of 42 W and a gain of 10.3 dB for a single carrier WCDMA signal with 6.6-dB peak-to-average power ratio. A memory mitigation algorithm further improves the linearity, resulting in an ACLR of -70 dBc and a normalized RMS error of 0.3%. Measurements were made to quantify separately the efficiency contributions of the HVHBT-based RF stage, and of the envelope amplifier. The measurements show that the RF stage operates at collector efficiency above 85% over most of the instantaneous power range of the WCDMA signal. This remarkably high efficiency is the result of low ldquoon-resistancerdquo and low (and nearly voltage independent) output capacitance of the HVHBT.     

A VSWR-protected silicon bipolar RF power amplifier with soft-slope power control

This paper presents the design and measured performance of a 1.8-GHz power amplifier featuring load mismatch protection and soft-slope power control. Load-mismatch-induced breakdown can be avoided by attenuating the RF power to the final stage during overvoltage conditions. This was accomplished by means of a feedback control system, which detects the peak voltage at the output collector node and clamps its value to a given threshold by varying the circuit gain. The issue of output power control has been addressed as well. To this end, a temperature-compensated bias network is proposed, which allows a moderate power control slope (dB/V) to be achieved by varying the circuit quiescent current according to an exponential law. The nonlinear power amplifier was fabricated using a low-cost silicon bipolar process with a 6.4-V breakdown voltage. It delivers a 33.5-dBm saturated output power with 46% maximum power-added efficiency and 36-dB gain at a nominal 3.5-V supply voltage. The device is able to tolerate a 10:1 load standing-wave ratio up to a 5.1-V supply voltage. Power control slope is lower than 80 dB/V between -15 dBm and the saturated output power level.     

RF power performance of an LDMOSFET on high-resistivity SOI

This paper describes the RF power performance of an LDMOSFET technology on high-resistivity silicon-on-insulator wafers. The technology has an on-state breakdown voltage of greater than 10 V, and an off-state breakdown voltage of greater than 20 V. This device technology is shown to have excellent RF power characteristics at frequencies from 1.9 to 5.8 GHz. At 1.9 GHz, a peak power-added efficiency (PAE) of 63% was achieved with an output power of up to 520 mW from a single RF power cell. At 5.8 GHz, a peak PAE of 35% was achieved with an output power of up to 125 mW from a single RF power cell.     

Small-size 81- to 83.5-percent efficient 2- and 4-stage depressed collectors for octave-bandwidth high-performance TWT's

In a joint USAF-NASA Program, Lewis Research Center is carrying out an efficiency improvement program on traveling wave tubes (TWT) for use in electronic counter measures (ECM) by applying multistage depressed collector (MDC) and spent beam refocusing techniques developed at Lewis. In the analytic part of the effort, three-dimensional electron trajectories are computed throughout the TWT. Trajectory computation continues through the spent beam refocuser and the depressed collector. Collector efficiency, collector losses, and overall efficiency are identified and computed. On the experimental side, tube performance is evaluated first without the MDC; then, the spent beam is analyzed for symmetry, circularity, and velocity spread. Finally, the MDC is attached and its performance optimized and evaluated. The three-dimensional theory, for ideal tubes, predicts a MDC-efficiency, at mid-band, of 81 percent for a 2-stage MDC with symmetric, circular, and optimally refocused beams and 85.5 percent for a 4-stage MDC. Experimental results to date have yielded MDC efficiencies of a minimum of 81 and 83 percent for a 2- and 4-stage MDC, respectively, across a one-octave bandwidth of a 4.8 to 9.6 GHz 330-to-550-W TWT.