28-GHz In-Band Full-Duplex Power Amplifier Integrated With an Impedance Matched Hybrid Transformer

A 28-GHz in-band full-duplex (IBFD) power amplifier (PA) is presented which integrates a PA with an impedance matched hybrid transformer in a 28-nm bulk CMOS process. The hybrid transformer has a 1:2 turn ratio to match from the optimum load impedance of 25 Ω for a PA to the antenna and balance ports of 100 Ω. Since this allows the additional matching circuits of a PA and an antenna to be obviated, the full-duplex loss is reduced. It shows 28.4-dB isolation between the PA and LNA ports at 28 GHz. For comparisons, a 50-Ω matched PA and an IBFD PA are designed. The IBFD PA has core sizes of 0.24 × 0.43 mm
²
. Since the impedance matched hybrid transformer has the similar size to the output matching network of the 50-Ω matched PA, a full duplexer is integrated into the PA without additional chip area. The IBFD PA has 10.6-dB gain and a drain efficiency of 8.9% at 12.3-dBm P1dB.