Design of a 39-GHz compact bi-directional variable gain amplifier in 65-nm CMOS

This paper presents a differential two-stage 39-GHz bi-directional variable gain amplifier (Bi-VGA) for 5G mmWave applications. Based on a phase compensated bi-directional structure with tail current tuning technique, a variable gain cell with small phase variation and compact area is realized. To boost the gain and ease the impedance matching, the cross-coupled transistor pairs are adopted. Analysis and simulations are undertaken to illustrate the working mechanism of accurate gain control and low phase variation. The measured Bi-VGA achieves peak gain of 15.6 dB with 3-dB bandwidth over 35-45 GHz, covering the 5G 39-GHz band (37-43.5 GHz), and a maximum OP1dB of 3.5 dBm. Fabricated in 65-nm CMOS process, the Bi-VGA consumes 67 mW from a 1.2 V power supply with a small core area of 0.46 x 0.14 mm2, and its minimum noise figure is about 6.4 dB at the maximum gain state. Across a gain tuning range of 12 dB, the phase variation is less than 6.1 degrees over the operating frequency range.