AN 1128 L Band Amplifier Using the ATF-36077 Low Noise PHEMT

L Band Amplifier using theATF-36077 Low Noise PHEMTApplication Note 1128IntroductionThe Hewlett-Packard ATF-36077PHEMT device is described in alow noise amplifier that can beeasily optimized for any spotfrequency in the 900 through1800 MHz frequency range. TheATF-36077 is intended to be usedas a low noise amplifier in 12 GHzDBS applications providing 0.5 dBnoise figures. The device iscapable of noise figures below 0.4dB at frequencies below 2 GHz.Although the device has aninherent noise figure of around0.2 dB, actual circuit losses limitthe minimum noise figure that canbe obtained at about 0.3 dB.DesignAchieving the associated gain ofwhich the device is capable is difficultsince the device is notinherently stable at the desiredoperating frequency. It is notenough that the amplifier bestable at the operating frequency– it must be stable at allfrequencies. Any out-of-band oscillationwill make the amplifierunusable.The simplest technique to ensurebroad-band stability is to resistivelyload the drain. Resistiveloading produces a constant impedanceon the device over awide frequency range. In the caseof the ATF-36077, a 50 ohm resistorand a small amount of seriesinductance is used to provideshunt loading of the device. Theseries inductance also providessome impedance matching. Anadditional series resistor directlyoff the drain is also used to enhancestability. This circuittopology produces acceptablegain while ensuring a good outputmatch and retaining stability overas wide a bandwidth as possible.Obtaining the lowest possiblenoise figure from the device requiresthat the input matchingnetwork convert the nominal 50Table 1. ATF-36077 LNA Performanceohm source impedance to Γ opt.In the case of the ATF-36077 at1.3 GHz, a series inductor providesa good noise match.The source leads of the ATF-36077must be grounded with minimalsource inductance. The use of thecustomary bypassed source resistortechnique for biasing a typicalMESFET such as the ATF-10136will not work very well with aPHEMT device. Excessive sourceinductance will cause the deviceto oscillate at frequencies around20 to 22 GHz. The ATF-36077 willperform very well if platedthrough holes are placed directlyunder the source leads adjacent tothe package and if the printed cir-Frequency (MHz) Gain (dB) Noise Figure (dB)1100 19.5 0.531150 20.8 0.601200 22 0.441250 22.4 0.381300 22.6 0.331350 22 0.311400 21.2 0.281450 20.5 0.281500 19.5 0.301550 18.6 0.301600 17.5 0.40

2cuit board thickness is kept at0.031 inch or less.A schematic diagram and partslist are shown in Figure 1. Theschematic diagram suggests theuse of active biasing to preset thebias point. The device is biased ata nominal Vds of 1.5 Volts and anIds of 10 mA. Its operation will beexplained in the next section.INPUTZoC1C4L2R5R1ZoR7Figure 1. Schematic and Parts List for ATF-36077 LNAR6L1Q2Z1R8Passive biasing can be used but itwill require that a potentiometerbe connected to the output of thedc to dc converter U1 to set thegate voltage bias required fornominal drain current.PerformanceThe performance of the amplifieris shown in Table 1. Since the lossof the input microstripline canLL1/2Q1 R10 C2 OUTPUTC1 = 22 pF CHIP CAPACITOR C10C2 = 100 pF CHIP CAPACITORC11C3 = NOT USEDC4, C7 = 1000 pF CHIP CAPACITORC5, C6 = NOT USED, GROUND SOURCES DIRECTLYC9 = NOT USEDC8, C12, C13 = 0.1 µF CHIP CAPACITORC10, C11 = 10 µF CHIP CAPACITORL1 = 6T 0.007" DIA. WIRE, 0.050" ID, 0.14" LENGTHL2 = 0.18 TO 0.33 µH RFCLL1, LL2 = PLATED THROUGH HOLES TO BACKSIDE GROUNDPLANE DIRECTLY UNDER EACH LEADQ1 = HEWLETT-PACKARD ATF-36077 PHEMTR1 = 100 OHM CHIP RESISTORR2 = NOT USEDR3 = 50 OHM CHIP RESISTORR4 = NOT USEDR5, R7 = 10 k OHM CHIP RESISTORR6 = 1.3 k OHM CHIP RESISTORR8 = 2.7 k OHM CHIP RESISTOR (SETS DRAIN VOLTAGE)R9 = 270 OHM CHIP RESISTOR (SETS DRAIN CURRENT)R10 = 16 OHM CHIP RESISTOR (UP TO 27 OHMS OK)U1 = LINEAR TECHNOLOGY LTC1044CS8 VOLTAGE CONVERTERU2 = 5 VOLT REGULATOR TOKO TK11650UU3 = OPTIONAL MMIC AMP - NOT USEDZo = 50 OHM MICROSTRIPLINE, MINIMIZE LENGTH ON INPUT TO MINIMIZE LOSS AND EFFECTON NOISE FIGUREU1R9C7C12R3L23 VC8U2ZoC13V DDadd appreciable loss which addsdirectly to noise figure, thebreadboard amplifier was builtwithout any significant microstripetch in the input network. Theoptimum noise figure is about0.3 dB and this occurs over abouta 100 MHz bandwidth, centered at1400 MHz. The noise figure risesto 0.4 dB at 1600 MHz and 0.44 dBat 1200 MHz making the amplifierusable for broadband GPSapplications. The amplifier can betuned for other frequencies in the900 through 1800 MHz frequencyrange by either increasing ordecreasing the value of inductorL1.BiasingThe preferred method of biasingFETs is the use of an active biasnetwork as described in Figure 1.Although active biasing does addcost by requiring extra components,including a way ofgenerating a negative voltage, theadvantages generally outweigh thedisadvantages. Active biasing offersthe advantage that variationsin pinchoff voltage and Idss willnot necessitate a change in eitherthe source or drain resistor valuefor a given bias condition. The activebias network automaticallysets Vgs for the desired drain current.The typical active biasing schemefor FETs requires that the sourceleads be grounded and an additionalsupply be used to generatethe negative voltage required atthe gate for typical operation. Referringto Figure 1, resistors R6and R8 provide a regulated voltageat the base of Q2. The voltageis increased by 0.7 volts by virtueof the emitter-base junction of Q2and then applied to the drain ofQ1. Since R3 and R10 are in serieswith the drain, the voltage drop

then the problem may be excessivesource inductance which iscausing a high frequency oscillation.If a device is oscillating atany frequency, even an out of theband frequency, it may be difficultto obtain rated performance.Make sure the plated throughholes are as close to the device aspossible, i.e., 0.02 to 0.03 inch maybe too long.If the problem is inband stability,then the solution is to add seriesresistance between the drain andoutput connector. Increase thevalue of R10. Additional gain reductioncan also be had bydecreasing the length of the shuntoutput inductor in series with R3.In some cases, higher than expectednoise figure can beattributed to noise from the dc todc converter or the PNP transistorused for active biasing. Use of a0.1 µF bypass capacitor in parallelwith C4 and at location C8 shouldhelp.The use of a lossy or low Q RFchoke for L2 can contribute to anincrease in noise figure. The useof a small molded RF choke for L2works well in prototypes. For surfacemount applications be sureto choose a high Q wire woundchoke such as those made byCoilCraft.In some instances, the enclosurecan cause undesirable feedbackacross the circuit board whichcan cause instabilities. This phenomenais true of any amplifierdesign. A cross sectional view ofthe housing can be viewed as apiece of waveguide whose dimensions,both width and height,determine the band of frequenciesthat it may pass with minimal attenuation.A combination of theamplifier response along with thehousing response could contributeto instabilities if notcontrolled. The use of low profilesurface mount components willminimize this effect.Making sure that the cover is nocloser to the printed circuit boardthan is necessary will minimizecoupling from the cover. It is preferredto have the cover at least0.3 to 0.5 inches above the circuitboard. The use of ECOSORBTMcan be used on the cover to minimizereflections if the cover hasto be in close proximity to theboard. The use of a metal dividerhanging down from the cover isalso another method of breakingup enclosure effects.ConclusionThe ATF-36077 can provide lessthan 0.4 dB noise figures over severalhundred MHz of bandwidth atL Band frequencies. A simple circuithas been shown that providesstable L Band operation.The single-element input matchingnetwork provides very goodperformance in this frequencyrange and offers the greatestbandwidth and least sensitivityto manufacturing tolerances. Theresistive loading in the outputnetwork provides the best broadbandstable performance bysacrificing some in-band gain.For technical assistance or the location ofyour nearest Hewlett-Packard sales office,distributor or representative call:Americas/Canada: 1-800-235-0312 or(408) 654-8675Far East/Australasia: Call your local HPsales office.Japan: (81 3) 3335-8152Europe: Call your local HP sales office.Data Subject to ChangeCopyright © 1997 Hewlett-Packard Co.Printed in U.S.A. 5966-0783E (11/97)