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A 5-WATT, 37-GHz MONOLITHIC GRID AMPLIFIER

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A 5-WATT, 37-GHz MONOLITHIC GRID AMPLIFIERBlythe Deckman1, Donald S. Deakin, Jr.2, Emilio Sovero2, David Rutledge11Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 911252Rockwell Science Center, Thousand Oaks, CA 91358AbstractÜA 5-Watt Ka-band amplifier hasbeen demonstrated. The area of the grid am-plifier is 1 cm2, and there are 512 transistors.The small-signal gain of the grid is 8 dB at37.2 GHz, with 1.3 GHz bandwidth. At 5 Wattsoutput, the gain is 5 dB with 15% power-addedefficiency. An aluminum-nitride heat spreaderallows continuous operation with an estimatedgate temperature of 70◦C.IntroductionThe use of spatial power combiners as an efficientmethod to incorporate the outputs of many solid stateamplifiers has in recent years drawn significant researcheffort. Liu et al. demonstrated a 44 - GHz monolithicgrid amplifier that gave 670 mW with a power-added ef-ficiency of 4% [1]. Hacker et al. recently demonstrateda monolithic 1.44 cm2amplifier that gave 1.8 W with3.7-dB gain at 38.6 GHz [2]. Hybrid amplifiers havealso been successfully demonstrated with higher pow-ers [3, 4]. Recent advances in computer speed and fieldsimulators have facilitated the development of accuratemodeling techniques [5].Here we demonstrate a monolithic grid amplifierthat shows 8-dB small-signal gain, 1.3-GHz bandwidth,and delivers 5 W of power at 37.2 GHz with 5-dBgain. Devices used here are 80-µm by 0.18-µm GaAspHEMTs, capable of delivering 11 mW of p ower at39 GHz in load pull measurements. The grid uses 512such pHEMTs, giving a total gate width of 41 mm. Fig-ure 1 shows the basic grid amplifier layout and simpli-fied detail of the unit cell. The unit cell uses two tran-sistors connected as a differential pair. Modeling of theamplifier is after [5], exploiting the symmetry of thegrid to analyze the structure as a single unit cell. Fig-ure 2 shows a photograph of a corner of the fabricatedactive grid.Thermal ManagementPrevious grid amplifiers lacked a heat spreader, soFigure 1. Basic grid layout and simplified unit cell lay-out. Two devices operate in differential mode. The inputbeam enters from the left, and changes polarization as it isamplified. The output radiates to the right.they could only be biased for short periods of time. Thegrid amplifier in [1] gave 670 mW, with 4% power-addedefficiency, leaving 16 W of waste heat to be dissipated.The active devices were fabricated on a GaAs substrate635 µm thick, and the GaAs was mounted on a Duroidcarrier. Both GaAs and duroid are quite po or thermalconductors, so the grid could be biased for only 0.6seconds and required a 3-minute cooling time.In the current work, the devices were fabricated ona GaAs substrate thinned to 75 µm and mounted on analuminum-nitride heat spreader 2 mm thick (thermalconstant 170 W/cm2). The carrier was then mountedinto a Lucite frame, and water circulated around theedges to dissipate the heat.Using a finite-element heat simulator by Tanner Re-search, the thermal resistance of the bulk of the GaAssubstrate was predicted at approximately 1◦C/W. Heatsimulations were also performed for individual amplifi-cation cells giving a predicted temperature of the deviceInternational Microwave Symposium, Boston, Massachusetts, June, 2000Figure 2. Photograph of the grid amplifier. The period ofthe grid is 625 µm.junctions at 13◦C above the substrate. Measurementsof a fully biased grid (using a mercury thermometer)operating with the cooler running showed a bulk ther-mal resistance of 0.8◦C/W.Thermal measurements of a fully biased grid werealso made using a Thermacam PM290 infrared cam-era. Since GaAs is transparent to the infrared wave-lengths detected by the camera, the thermal imagemainly showed the heat profile of the thermal polymerused to attach the GaAs substrate to the heat spreader.The thermal emissivity of the system was calibratedby placing a piece of black electrical tape (with knownemissivity of 0.95) next to the grid on the carrier, andmeasuring the temperature of the tape. The camerawas then fo cused on an area of the grid amplifier freefrom lens reflections, and the measurement emissivityadjusted until the measurement temperature matchedthe recorded temperature of the black tape. Thermalmeasurements of the grid amplifier gave peak temper-atures of 60◦C, a result consistent with the simulatedpeak temperature of 55◦C. The resolution of the lensused for the thermal measurement is 100µm, so the in-dividual device gates are not resolvable.Bias LinesOptimal power performance of the grid’s transistorsrequires a gate-source voltage of about −0.3 V. Drops inthe source bias lines, then, cause significantly differentgate-source bias voltages for devices near the center ofthe grid. Consequently, two straight gold traces werechosen to carry the bias current, and the bias supplieswere connected to both ends of the grid amplifier-chip.Source bias lines were fabricated 3 µm thick and 25 µmwide. The shunt reactance introduced by the bias linesis tuned out by appropriately p ositioning the polarizers.Since the gates of the transistors draw negligible DCcurrent, the gate voltage is supplied to the transistorsvia the same gate trace that is used to detect the inci-dent beam. The gate voltage is passed from one devicein a cell to the other through a 2-kΩ resistor, providingRF isolation from the virtual ground in the center ofthe unit cell.StabilityFigure 3 shows that the uncompensated devices usedin this work are potentially unstable across most of theband of interest. A typical amplifier design (using mi-crostrip, for example) that uses a potentially unstabledevice usually proceeds by selecting source and loadreflection coefficients that avoid the unstable operat-ing conditions of the device. The stability of the con-structed amplifier, then, depends on the precision withwhich the source and load reflection coefficients can becontrolled. Grid amplifiers, however, require dynamictuning of the input and output by adjusting the posi-tions of the polarizers. Such tuning adjustments cancause the source and load reflection coefficients to varywidely, easily falling into unstable regions. To protectthe amplifier from oscillations while tuning, resistivefeedback of 1,245 Ω is used to stabilize the devices overthe entire band of interest. A DC blocking capacitor isused to protect the gate bias. Figure 3 also shows theFigure 3. Uncompensated, compensated, and loaded sta-bility factors.


A 5-WATT, 37-GHz MONOLITHIC GRID AMPLIFIER

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