This paper reviews the state-of-the-art of millimeterwave power amplifiers, focussing on broadband design techniques. An overview of the main solid-state technologies is provided, including Si, GaAs, GaN and other III-V materials, and both field-effect and bipolar transistors. The most popular broadband design techniques are introduced, before critically comparing through the most relevant design examples found in the scientific literature. Given the wide breadth of applications that are foreseen to exploit the millimeter-wave (mm-wave) spectrum, this contribution will represent a valuable guide for designers who need a single reference before adventuring in the challenging task of mm-wave power amplifier design. Index Terms-Broadband, millimeter wave, power amplifiers. I. INTRODUCTION T HE millimeter-wave (mm-wave) spectrum is attracting a great interest for applications such as 5G and future satellite communications that go well beyond the traditional niche of military and scientific use in terms of investments and potential revenues. The most attractive feature of mm-waves compared to the RF and microwave band is the huge spectrum availability that gives a great advantage in terms of capacity for telecommunication systems. Other advantages of mm-wave systems are the compact size of circuits, especially antennas, and the intrinsic easiness of frequency reuse thanks to the high free-space attenuations. There are also some advantages that are band-specific; for example, the very high attenuation in the 60 GHz band due to oxygen absorption that enables intrinsically secure communications. On the other hand, the very high frequency of operation poses significant challenges to system and circuit design. Similarly to what happens at lower frequency, one of the most critical circuit components is the power amplifier (PA). Its

Abstract:Modern mobile communication signals require power amplifiers able to maintain very high efficiency in a wide range of output power levels, which is a major issue for classical power amplifier architectures. Following the load-modulation approach, efficiency enhancement is achieved by dynamically changing the amplifier load impedance as a function of the input power. In this paper, a review of the widely-adopted Doherty power amplifier and of the other load-modulation efficiency enhancement techniques is presented. The main theoretical aspects behind each method are introduced, and the most relevant practical implementations available in recent literature are reported and discussed.

Thermal stress in microwave power devices is a major issue for space applications, with a detrimental impact on the operating lifetime of MMICs on board satellites. To limit this, derating rules are applied to the maximum operating junction temperature, which however limit the potential device performance when GaN/Si technology is employed. In this framework, classical power amplifier design paradigm must be reconsidered, moving to a thermal-aware design approach. To this aim, it is crucial to have access to highly reliable thermal models of the adopted devices. This work will show that, adopting a simplified but effective thermal model and proper design strategy, GaN/Si technology can be successfully adopted for space-compliant MMIC design up to Ka-band. In particular, the preliminary design of a 10 W MMIC working in Ka-band at 36 GHz will be presented based on the 100 nm gate-length GaN/Si HEMT process from OMMIC foundry.

This paper discusses the design of a wideband class AB-C Doherty power amplifier suitable for 5G applications. Theoretical analysis of the output matching network is presented, focusing on the impact of the non-ideally infinite output impedance of the auxiliary amplifier in back off, due to the device’s parasitic elements. By properly accounting for this effect, the designed output matching network was able to follow the desired impedance trajectories across the 2.8 GHz to 3.6 GHz range (fractional bandwidth = 25%), with a good trade-off between efficiency and bandwidth. The Doherty power amplifier was designed with two 10 W packaged GaN HEMTs. The measurement results showed that it provided 43 dBm to 44.2 dBm saturated output power and 8 dB to 13.5 dB linear power gain over the entire band. The achieved drain efficiency was between 62% and 76.5% at saturation and between 44% and 56% at 6 dB of output power back-off.