A VSAT Packaged HPA for Ground Terminal and Digital Radio Applications
Two broadband satellite networks are in the final stages of development and plan a service launch for North America in early 2004. Systems like these are expected to provide wideband, high speed communications for Internet and multimedia applications across North America and Europe for years to come. High power ground terminals are required at each customer site to communicate with these satellite networks, and designers are searching for ways to reduce costs and increase efficiencies in on-going development programs.
Low cost packaged high power amplifiers (HPA) at Ka-band are needed for emerging satellite ground terminal and millimeter-wave digital radio applications. Expensive hermetic packages will not meet the price point goals for these markets. The industry has instead searched for a simple, low cost packaged alternative that can be dropped into the RF module with minimal impact to assembly flow. The challenge is to provide a low cost MMIC with acceptable performance that can function in a cost-effective, Ka-band package environment. Such a single MMIC package solution will allow subsystem integrators to replace multi-chip modules now in use, thereby decreasing product complexity and cost.
A compact 4 W HPA in a low cost, high performance package has been developed to meet this need. The TGA4905 is ideally suited for these applications as a transmit HPA for either broadband satellite ground terminal or digital radio applications that require high linear power.
This product was created using TriQuint's mature 0.25 µm pHEMT production process on 50 µm substrate technology. The saturated output power is 36 dBm with an associated gain of 22 dB. These results set a new standard within the industry for high CW-power and gain for a single MMIC and packaged part at Ka-band to date.
Figure 1 shows a block diagram of a typical transmit amplifier chain for either ground terminal or digital radio applications. TriQuint's device line-up provides a complete solution for transmit gain and output power requirements for a variety of 30 GHz applications. The configuration utilizing the TGA4905 HPA as the power device provides 4 W Psat at 30 GHz. A higher power product, the TGA4915, is also available that provides > 7 W Psat at 30 GHz. Total transmit path gain (following the upconverting mixer) of > 55 dB can be achieved when utilizing both the pre-driver and driver with the output PA. A transmit path gain > 40 dB is achieved when utilizing only the driver MMIC with the output PA. Both the driver and pre-driver MMIC can be supplied as die or in an SMT package.
The TGA4510 and TGA4902 30 GHz drivers and the TGA4905 and TGA4915 30 GHz power amplifiers have already been designed into Ka-band VSAT ground terminals and millimeter-wave radio products by leading systems and subsystems manufacturers. The technology behind the TGA4905 has evolved through years of research and development and custom product designs in the high performance millimeter-wave IC market. More than 10 years of reliability history exists on the 0.25 µm pHEMT process with over 59 million device test hours recorded. TriQuint's mature, high yield 0.25 µm power pHEMT process produces high frequency device performance typically associated with 0.15 µm gate devices.
Compact High Power MMIC Solution
The 30 GHz high power product development has kept the MMIC physical size and IO connections constant while launching progressively better performing products using this footprint. The latest chip in this product family is the TGA4505 - the MMIC inside TriQuint's TGA4905 packaged part. The TGA4505 was designed using TriQuint's standard 0.25 mm gate pHEMT process with 50 mm substrate technology. The TGA4505's compact MMIC design was achieved by adopting a proper matching topology for a reduction of chip size, shunt capacitors over vias and 50 mm substrate technology with slot vias that allow for compact FET layouts.
Extensive EM-simulations were performed on critical areas of a chip to fully assess coupling effects between components. A chip microphotograph of the TGA4505 is shown in Figure 2 . The actual chip size is 12.88 mm2.
The TGA4505 circuit consists of four stages and is biased at Vdq = 6 V and Idq = 2.1A. The output stage-matching network was designed to match the impedance of output stage FETs to the optimum power load, which was obtained from load-pull measurements. The TGA4505 design combines 16 FET cells in the output stage-matching network to achieve high output power and uses shunt capacitors over vias and micro-strip lines to achieve proper impedance transformation. Each inter-stage matching network was designed so that stages 1, 2 and 3 would all have enough power to drive the next stage FETs. The gain flatness was also considered in designing input and inter-stage matching networks.
Low Cost Package Solution
A low cost, non-hermetic package was developed for the TGA4505 MMIC. The package construction is based on Rogers 4350™ laminate board material bonded to a metal heat spreader.
This package consists of a thin, two-sided circuit board with a cutout cavity in which the MMIC and 100 pF bias capacitors are assembled. Package component assembly uses industry standard equipment and processes. Electrical interconnect of the RF components is achieved with standard wire bond assembly techniques. All components are attached to a metal heat-sink carrier plate. The package can be mechanically attached to the next level assembly using four screws, or conductive epoxy. The TGA4905 package provides an advantage because its ground makes direct contact with the carrier plate (heat spreader). Since the MMIC ground is the same as the carrier plate, there is little added inductance while also providing for better high frequency response compared to surface-mount package or flip-chip parts.
Figure 3 shows the typical small signal response (Pin = -20 dBm) of the packaged TGA4905 at Vdq = 6 V and Idq = 2.1A. The gain (S21) is nominally 22 dB and input/output return losses are better than 7 dB at 30 GHz.
Results of power measurements shown in Figure 4 display nominal output power (Psat) of 36.0 dBm at 30 GHz for the TGA4905. Figure 5 shows the IMD3 performance of the TGA4905, 40 dBc at 20 dBm Pout.
With more than 10 years of R&D behind the VSAT high power amplifiers, the TGA4905 30 GHz HPA is ideally suited for satellite ground terminal and digital radio applications. As designers look to the future of broadband satellite networks, they need high power, low cost packaged parts for the Ka-band to replace bulkier, multi-chip modules that were a mainstay of earlier RF design. The TGA4905 is a simple, low cost alternative that delivers outstanding performance, less complexity and lower cost. The TGA4905 is currently shipping in pre-production volumes, with expected ramp to full production in late 2003.
Product details and a data sheet may be obtained at http://www.triquint.com/company/divisions/millimeter_wave/docs/tga4905/tga4905.pdf. For samples and additional sales information, contact TriQuint Semiconductor at (972) 994-8465, fax: (972) 994-8504 or e-mail: email@example.com.
TriQuint Semiconductor Texas, L.P. Richardson, TX (972) 994-8465, www.triquint.com. Circle No. 300