Design and Analysis of a Coupled-Line-Based Load-Modulated Balanced Amplifier MMIC With Enhanced Bandwidth Performance

This article presents the design theory and implementation of a fully integrated coupled-line-based load-modulated balanced amplifier (CLLMBA) monolithic microwave integrated circuit (MMIC). To facilitate the design, a novel design method is proposed for the CLLMBA to precisely control the load modulation and output power back-off (OBO) level by arranging the current ratio among the control amplifier (CA) and balanced amplifiers (BAs). Moreover, to further expand the working bandwidth, the coupled-line couplers are adopted in the CLLMBA. Subsequently, the physical dimensions and operating conditions of the three sub-amplifiers are selected accurately based on load modulation analysis at the fundamental frequency. It leads to properly modulated impedances and cancels the output matching networks for sub-amplifiers. Besides, meandering lange couplers are adopted by double metal layers and air-bridges for a compact layout. To validate the proposed techniques, a CLLMBA prototype is implemented and fabricated in a commercial 0.25- $\mu $ m GaN HEMT process with the die size of $3.1\times 2.3$ mm2. The measurement result exhibits a 38.1-39.3 dBm saturated output power with a 45.8%-57.6% saturated drain efficiency (DE), and a 31.7%-42.3% DE at 10-dB OBO from 4 to 6 GHz. Furthermore, under a 100 MHz orthogonal frequency division multiplexing (OFDM) signal with 8.5 dB peak-to-average power ratio (PAPR), the average DE is 32.8%-40.6% and the adjacent channel leakage ratio (ACLR) after digital predistortion is better than −47.5 dBc.