This year’s 6th annual RF/Microwave Power Amplifier (PA) Forum at European Microwave Week, organized by AWR Group, NI, provided attendees with a timely perspective on the state of RFPA design for semiconductor technologies and device modeling.
The full-day event began with a forward-looking presentation by keynote speaker Dr. Steve Cripps of Cardiff University on research and development in the area of load modulation and active/passive matching-circuit design. Dr. Cripps emphasized the role of reliable active device models and designer understanding of device behavior in order to achieve the desired PA design goals.
His talk discussed the fundamentals and performance possibilities of modifying the load impedance seen by a current generator (power transistor) through the use of a second current generator. The use of a second generator allows the load seen by the first generator to be modulated in magnitude and phase by selecting appropriate values for the second current source. This impedance-matching method is a paradigm shift for RF designers who think mainly about using passive reactive networks to perform impedance transformation, but has been employed for some time in active load-pull systems. Understanding this paradigm shift may be the answer to unlocking greater amplifier performance.
The load-modulating generator consumes power, which can be much larger than the power being generated by the main active device, depending on the level of load transformation. However, for PA design, it is possible to devise configurations whereby the load-modulating power is partially or fully recovered by the PA output. The Doherty PA (DPA) is perhaps the best-known example of this behavior. Dr. Cripps discussed the bandwidth limitations introduced by the use of passive networks, such as the frequency dependent quarter-wave transformer (impedance inverter) necessary in the DPA to provide the required direction of modulation with the device output current.
This bandwidth limitation was discussed in greater detail through a comparison of the frequency response of different impedance inverters (transformers) and the impact on overall amplifier bandwidth performance. To overcome these limitations, Cripps introduced a quadrature balanced amplifier, with an input derived control signal injected at the output terminated coupler port. This load-modulated balanced amplifier (LMBA) can be considered to be a reconfigurable PA with the ability to optimize performance over a wide bandwidth, enabling greater performance compared to passively matched designs.
In addition to the keynote, presentations from industry reinforced the importance of rigorous transistor modeling. Dr. Yueying Liu of Wolfspeed/Cree, Julien Poulain of OMMIC, and Eric LeClerc of UMS presented talks on their latest gallium nitride (GaN)/silicon carbide (SiC) processes, including details on device characterization, model validation, and the use of device models within Microwave Office circuit simulation software.
Multiple talks were also presented on design methodologies using both behavioral and compact models for GaN devices, which demonstrated different design approaches for a 10-W, Ka-band PA presented by Jonathan Leckey of ICONIC RF and a 10-W, 3.7-GHz Doherty PA using the AMCAD enhanced polyharmonic distortion (EPHD) model, presented by David Vye of AWR. Behavioral models were also featured in a talk by Wissam Saabe of AMCAD discussing the use of behavioral models in Visual System Simulator™ (VSS) system design software to capture the coupling effect between antenna arrays and nonlinear RF front ends and the impact on PA load impedance.
Vince Mallette of Focus Microwave presented a talk based on the work of Dr. Tushar Sharma et.al. on the second harmonic source impedance and its influence on high-performance PAs that tied together device characterization, harmonic load/source-pull measurement systems, and high-efficiency PA design. A very instructive, real-world case study presented by this year’s moderator, Dominic Fitzpatrick of AMETEK CTS discussed troubleshooting stability issues for a wideband PA.
This annual event continues to attract leading technologists and PA designers year after year and provides new design ideas and cutting-edge technologies of value to the audience (click here to register for presentation slides). As an added bonus, the event was recorded for future access/reference by microwave designers and each video presentation is now available on the https://www.awr.com/adf site for on-demand viewing.