A Ka-Band CMOS Variable Gain Amplifier with High Gain Resolution and Low Phase Variation

This paper presents a Ka-band variable gain amplifier (VGA) with high gain resolution and low phase variation for millimeter-wave (mm-wave) 5G communications in 65-nm CMOS technology. The VGA employs three-stage differential common source (CS) structure. The input and output stages provide fixed gain, while the middle stage provides variable gain control. Based on the capacitive cross coupled neutralization (CCCN) technique and asymmetric capacitor structure, the proposed three-stage differential VGA achieves a measured gain resolution of 0.2 dB with 7-bit digital gain control in 23 ~ 28 GHz. It realizes the phase variation lower than 2.3° with 6.2 dB gain control range at 28 GHz. The measured peak gain is 29.4 dB with a 3-dB bandwidth of 23.5 ~ 27.5 GHz. The core chip area is 0.36 mm2excluding pads.