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.