A Ka-band 8-Element 4-Beam Transmitter Front End With Hybrid VGA and Symmetrical Transformer-based Doherty PA

This paper presents a 27.7–31.2 GHz highly integrated 8-element 4-beam transmitter front end in 65-nm CMOS for phased-array applications. The proposed hybrid variable-gain amplifier (VGA) combines digital common-source (CS) VGA and analog Gilbert-cell-based VGA to realize a large tuning range and high resolution simultaneously. Furthermore, the proposed VGA scheme also integrates with an active combining network to implement a compact multi-beam structure. In the vector-modulation-based phase shifter (PS), low/high pass lumped delay lines are inserted between CS transistors and common-gate (CG) transistors to form I/Q signals which drive four pairs of Gilbert-cell based G
m
-arrays for different phase states. In addition, the output element leverages a symmetrical transformer-based Doherty power amplifier (PA) and an analog adaptive bias to improve the power-back-off (PBO) efficiency with a high peak-to-average-ratio (PAPR) modulation signal. With a 1.1-V supply voltage, this front end achieves a saturated output power (P
sat
) of 19.2 dBm with a 20.5% channel efficiency and a 1-dB compression point (OP
1dB
) of 18.2 dBm. The measured maximum drain efficiency (DE) is 40.8% and 6-dB PBO DE is 28%. Compared with class-B/A PAs, the DE is improved by 40%/180%. The proposed hybrid VGA achieves a 27-dB tuning range and a 0.25-dB gain step with 0.26 dB/1.9° RMS gain/phase errors. The active PS implements 6-bit 360° phase shifting with 0.25 dB/2.5° RMS gain/phase errors.