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Design an X-band PA Using a Surface-Mount Transistor

Dec. 7, 2015
This application note demonstrates that a discrete GaN transistor in a surface-mount package can be used to design a 5-W PA at X-band frequencies.

Gallium-nitride (GaN) discrete transistors for microwave applications are available from a number of suppliers. Most of the GaN transistors for X-band applications are offered as either bare die or in a ceramic package. However, offering these devices in a surface-mount-technology (SMT) package can simplify handling and assembly while reducing product costs. In the five-page white paper, “5W X-Band GaN Power Amplifier Using a Commercially Available Discrete Plastic Packaged SMT Transistor,” Plextek RFI demonstrates the design of a 5-W, X-band PA. The single-stage design is realized with a GaN transistor in an SMT package.

The PA is designed with Qorvo’s TGF2977-SM GaN transistor. This device is housed in a 3-×-3-mm, SMT package, which incorporates a solid copper base. An 8-mil-thick Rogers 4003 laminate with 1-oz. copper cladding is used to build the PA.  

A plot of the transistor’s maximum available gain is presented, demonstrating that the device has a maximum gain of approximately 14 dB at 9.4 GHz. Although the device demonstrates unconditional stability across the X-band frequency range, design effort is required to ensure stability below 4.8 GHz and above 12.5 GHz. In addition, the results from large-signal load-pull measurements are discussed. At 9.4 GHz, output power of +37.5 dBm at the 3-dB compression point is achieved.

The PA’s biasing networks consist of radial stubs, which behave as short circuits at the midband frequency. A damping circuit is employed at the PA’s input to ensure low-frequency stability. Additionally, both the input and output matching networks are implemented as distributed two-section lowpass structures.  

The final PA assembly is mounted onto a heatsink. Small-signal S-parameters were measured at package-base temperatures of -33°C, +25°C, and +85°C. These measurements demonstrate a small-signal gain of approximately 11 dB across the frequency range, which varies by approximately ±1.5 dB over temperature. The PA’s output power at the 3-dB compression point was +37.1 dBm at midband while varying by only ±0.2 dB over temperature. The document also provides the PA’s efficiency performance.

Plextek RFI, London Road, Great Chesterford, Saffron Walden, CB10 1NY, UK; +44 (0) 1799 533200.

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