Reconfigurable GaN Digital Tx Applying BST Bandpass Filter

The paper presents the first tunable GaN-based digital transmitter chain for 0.8–1.8 GHz range, suitable for MIMO systems in software-defined radio (SDR) installations. The demonstrator includes a 4-stage digital GaN power amplifier (PA) MMIC and a continuously tunable bandpass output filter based on a barium strontium titanate (BST) thin-film varactor. The filter shows a fractional bandwidth of 7% as well as a minimum insertion loss and maximum input return loss of 0.6 dB and 13 dB, respectively. The module delivers a maximum output power of 3 W and efficiency stays above 40% over the whole tuning range, peaking at 89% for 1.2 GHz. This is to the author`s best knowledge the first reconfigurable digital Tx chain fully demonstrating the inherent advantages of the digital PA approach including broadband operation, compactness, efficiency and reconfigurability.