University College Dublin Achieves Highly Efficient Wideband Sequential Power Amplifier Using NI AWR Software
University College Dublin (UCD) is Ireland’s largest university, with over 1,300 faculty and 30,000 students. UCD is well known as a center of excellence in microwave CAD research. Led by Prof. Thomas J. Brazil and Dr. Anding Zhu, UCD’s RF & Microwave Research Group has more than 30 years of experience in advanced device modeling, high-frequency simulation algorithms, and behavioral modeling, as well as power amplifier design.
The Design Challenge
This project challenged students to design and prototype a wideband sequential power amplifier (PA). Sequential PAs offer similar efficiency benefits to Doherty PAs, but without the inherent narrowband restrictions. As part of the design challenge, a high-efficiency, broadband, 45 W peaking PA was required. A broadband Lange coupler for use as the output power combiner was required as well. Both of these designs were to be used in conjunction with a preexisting 10 W main amplifier.
In order to get the highly efficient and wideband performance desired from the combined amplifier, it was critical that each of the PA components perform well across the targeted bandwidth of 1.45 GHz to 2.4 GHz. Additional requirements included:
- The peaking PA needed to be a simple design, yet provide flat gain and output power, as well as high efficiency across the band.
- High isolation between the input and isolated ports was vital for the Lange coupler in order to prevent power leaking from the peaking amplifier into the main amplifier, causing unwanted load modulation.
- The coupling ratio needed to be constant across the band to maintain consistent performance.
- The optimal coupling ratio for use in the PA needed to be identified in order to maximize the back-off efficiency without compromising peak output power.
NI AWR Design Environment™ software, inclusive of Microwave Office circuit design software and AXIEM 3D planar electromagnetic (EM) analysis software, was used for the design of the PA and the Lange coupler.
For the PA design, the load pull script within the NI AWR software identified the input and output fundamental impedance trajectories, which provided both high efficiency and output power across the design band for the peaking PA. Matching networks were then designed to realize these trajectories while rejecting signals at the harmonics. Finally, AXIEM was used to EM-verify and confirm that the final design layouts would meet spec.
Once the peaking amplifier was designed, the optimum coupling ratio needed to be found. This was done using idealized coupler models within Microwave Office. From there, the resultant Lange coupler could then be synthesized in order to meet the required coupling ratio. In particular, the MLANGE2 and LMAN elements were used, with their parameters simply tuned until the coupler S-parameters met the design criteria.
Why NI AWR Design Environment
The key reasons that UCD selected NI AWR software for this work were the ease with which simulations could be set up, the fast simulation speeds, and the excellent convergence properties, as well as the availability of key models for the transistors used in the design. Ease of use and a straightforward user interface were the most compelling features, while the excellent technical support was also highly beneficial. Last but not least, the designer found that the NI AWR software converged more readily when compared to alternative tools and in particular for when optimization of nonlinear circuits was being undertaken.
To continue to read the full story, click to download the PDF document below...