Find the Key to Broadband PA Design

Find the Key to Broadband PA Design

This application note demonstrates a simulation-based approach for designing a broadband gallium-nitride (GaN) power amplifier (PA).

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A simulation-based methodology for broadband power-amplifier (PA) design can be accomplished using load-line, load-pull, and real-frequency synthesis techniques. Thus, by taking advantage of simulation software and nonlinear transistor models, the design process can be streamlined. In the application note, “A Simulation-Based Flow for Broadband GaN Power Amplifier Design,” National Instruments presents the design of a Class F PA using a gallium-nitride (GaN) high-electron-mobility transistor (HEMT). The design is achieved by utilizing a nonlinear model of the transistor with the NI AWR Design Environment.

A schematic was first created to bias and stabilize the transistor. Once the biasing and stability conditions were established, the initial load-line analysis and harmonic impedance tuning was performed. After determining the impedance of the fundamental frequency, the second- and third-harmonic impedances were tuned to a short circuit and an open circuit, respectively. The fundamental impedance of the input tuner was set to a conjugate match, thus providing maximum gain. Once all of the impedances were tuned, a final harmonic-balance (HB) simulation was performed to confirm the desired mode of operation.

The application note goes on to describe a load-pull impedance extraction method, which was performed at three different frequencies: 1.8, 2.0, and 2.2 GHz. Load-pull simulations were executed to generate contours for maximum power and then for maximum drain efficiency. The maximum power and efficiency contours at the fundamental frequency were both superimposed on a Smith Chart. By using this approach, a region of mutually acceptable power and efficiency could be determined. Load-pull simulations for the second- and third-harmonic frequencies were then performed.

The Amplifier Design Wizard (ADW) tool synthesized the broadband matching networks once all impedances were determined. Both the output and input matching networks were designed and subsequently exported to the Microwave Office software. Linear, HB, electromagnetic (EM), and dc simulations were performed to fine-tune the design. The actual PA was later built and tested without any bench tuning, demonstrating agreement with the simulation results.

National Instruments Corp., 11500 N Mopac Expwy., Austin, TX 78759-3504; (877) 388-1952; www.ni.com.

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