I. Introduction
For high-speed wireless data transmission, 5G millimeter-wave (mm-wave) systems tend to adopt large-scale active antenna arrays and beamforming techniques. This puts new requirements on the volume, bandwidth, output power, and energy efficiency of the RF front-end power amplifier (PA) [1], [2], [3]. The GaN monolithic microwave integrated circuit (MMIC) technology is found attractive in developing mm-wave PAs for 5G base-station applications owing to its high breakdown voltage, excellent power density, and thermal performance [4], [5], [6], [7]. Besides, wideband GaN MMIC PA is highly desirable for achieving compact size, cost-effectiveness and compatibility. Commonly used wideband PA techniques include the distributed PA [8], [9] and the reactive matching (RM) PA [10], [11], [12], [13], [14], [15], [16], [17]. Among them, the RM PA can provide relatively uniform thermal distribution and high Power-added Efficiency (PAE) [14], thus has been extensively studied in recent years. Recently, an RM network using minimum inductance bandpass filter topology is proposed in [18] which presents state-of-the-art bandwidth-efficiency performances over 2–4 GHz. However, different design challenges oriented from Bode-Fano criteria [14], [15], [19] are encountered by designers at mm-wave band. According to [19], the available bandwidth of the PA output matching network (OMN) is limited by the optimal drain load and the parasitic capacitance [, see Fig. 1(a)] of the transistor, and can be estimated by (1) under the assumption of a constant reflection coefficient over the band of interest: \begin{equation*} BW \leq \frac {1}{2R_{opt}C_{out}\ln \left |{ \Gamma _{L} }\right |^{- 1}} = \frac {\pi f_{0}}{Q_{opt}\ln \left |{ \Gamma _{L} }\right |^{- 1}} \tag{1}\end{equation*} where is the center frequency of operation, and (=) is the Q-factor of the target drain impedance. The above equation illustrates that the available bandwidth of the PA OMN is inversely proportional to the target impedance Q-factor. Meanwhile, GaN transistors are often biased under high drain voltage for enhanced power density and efficiency. Especially for GaN-on-SiC technologies at mm-wave band, the transistor nominal supply voltage could be high enough to yield unexpectedly large values of and , thus reducing the maximum OMN bandwidth as well as increasing the impedance transformation ratio of the OMN. When applying conventional LC ladder or filter-based structures [13], [20], [21] for OMN realization, a high-order network is generally required to support the large transformation ratio and compensate for high , which often leads to increased circuit complexity, power loss and die size.
(a) Circuit model of transistor and PA OMN; (b) Maximum OMN bandwidth versus 27GHz and Return Loss=15dB).