A power-efficient 4-element beamformer in 120-nm SiGe BiCMOS for 28-GHz cellular communications

A 4-element beamformer designed in 120-nm SiGe BiCMOS technology for 28-GHz mobile millimeter-wave broadband system is presented in this paper. Each element of the beamformer consists of a 4-bit active phase shifter and a two-stage Power Amplifier (PA). A two-stage PA design with a Class-C pre-driver and a 2nd-harmonic-tuned Class-AB driver stage is adopted for high gain and high efficiency at both peak and backed-off power levels. The active phase shifter employs in-phase/ quadrature phase current steering and digital control of transconductance (Gm). Measurement results show a 33-dB gain, 16.5-dBm saturated output power, 15.7-dBm oP1dB, 27.5% peak PAE and 8.2% 7-dB back-off PAE at 27 GHz for a single element. The minimum (maximum) RMS gain and phase errors across the 27-29 GHz band were 0.5 dB (3 dB) and 1.5°(12°). The beamformer also includes a 1:4 power splitter and a serial interface for digital control and occupies a die area of 5.32mm2.