Ultra-Wideband Power Amplifier Using Non-Foster Characteristics of Coupled Transmission Lines

This paper presents a simplified matching network using coupled transmission lines (CTLs) for broadband power amplifiers. The proposed structure consists of a CTL with an electrical length shorter than λ/4 and a single shunt component, exhibiting excellent frequency characteristics across a wide bandwidth at both the input and load networks of the transistor. The reactance variation of the non-Foster elements in the equivalent circuit of the CTL with respect to frequency was analyzed, and the external reactive components were accordingly optimized to extend the bandwidth of the matching network. The proposed network was applied to the input and load networks of a GaN HEMT-based power amplifier. It was designed to maintain required performances over a wide frequency range of 1.9–4.9 GHz, covering both LTE and sub-6 GHz 5G bands, thereby achieving a fractional bandwidth (FBW) of 88.2%. The CTLs were fabricated on a two-layer printed-circuit board (PCB), and the additional shunt components were designed using surface-mount devices (SMDs). The overall power-amplifier module occupied a small area of 40 × 35 mm2. Using the continuous-wave (CW) signal, the proposed power amplifier exhibited a power gain of 10–14.8 dB and a drain efficiency (DE) of 47.5–60% at a saturated output power of 7.1–9.3 W across the entire operating frequency band. Using a 5G New Radio (NR) signal with a 100 MHz bandwidth and a peak-to-average power ratio (PAPR) of 7.8 dB, the amplifier achieved an average output power of 30 dBm, a DE of 20–27.5%, and an adjacent-channel leakage power ratio (ACLR) better than −30 dBc.