New-Tech Europe | July 2018

Designing a Broadband, Highly Efficient, Gallium Nitride, RF Power Amplifier (RFPA) Using NI AWR Design Environment Software Platform

J. Brunning, and R. Rayit, SARAS Technology

Introduction Demand for linear RF power amplifiers (RFPAs) covering the frequency range 1.5 – 2.8 GHz is driving new design methods for broadband, linear, and highly efficient RFPAs operating in output back-off mode (OBO). Improving efficiency in power amplifiers has long been a challenge for designers, in part due to poor control of harmonic loading impedances. The difficulty of measuring waveforms at microwave frequencies makes it hard to determine if optimum waveshaping has been achieved. Broadband design adds an additional challenge when a harmonic of a lower operating frequency lies in the intended operating band. These inherent difficulties can be compounded by imprecise design techniques, leading to multiple time- consuming and expensive iterations.

In this article, a first-pass design flow is described that uses NI AWR Design Environment, specifically Microwave Office circuit design software, as well as a measurement technique for the input and output impedances of the matching networks prior to RFPA “turn-on.” Several approaches to the problems inherent in PA design are presented with an aim of minimizing uncertainty and achieving first-time success. The effectiveness of this approach is demonstrated using a commercially available discrete 10 W gallium nitride (GaN) on silicon (SiC) packaged high- electron-mobility transistor (HEMT) using a 0.25 µm process (Qorvo T2G6000528) and a 20 mil RO4350B printed circuit board (PCB) dielectric. The fabricated RFPA achieved a peak power of >+40 dBm and a peak drain efficiency of >54 percent over its operating bandwidth. In back

off-mode the RFPA achieved an un- corrected linearity of 30 dBc and drain efficiency ≥34 percent when driven with a coded orthogonal frequency- division multiplexing (COFDM) 2.5 MHz, 9.5 dB peak-to-average power ratio (PAPR) modulated signal in the 2.0 – 2.5 GHz band. RFPA Design Flow Device Selection The initial design of the RFPA began with a thorough device/ technology selection process, the purpose of which was to select a best candidate device against a specific set of criteria prior to the time-consuming tasks of load/ source pull and network synthesis. Several candidate devices seemed acceptable on the basis of claimed frequency and power. In addition to the more common parameters such as Vds, gain, operating frequency

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