A Ka-Band CMOS Phase-Invariant and Ultralow Gain Error Variable Gain Amplifier With Active Cross-Coupling Neutralization and Asymmetric Capacitor Techniques

This article presents a 23–28-GHz digital-controlled phase-invariant and ultralow gain error variable gain amplifier (VGA) for 5G applications. The mechanisms of gain control and phase variation in CMOS VGA based on single-level transistor structure are analyzed in detail. According to the analysis, an active cross-coupling neutralization (ACCN) technique is proposed. The ACCN technique not only compensates for phase variation, but also makes phase compensation insensitive to the process, supply voltage, and temperature (PVT) variations. For achieving ultralow gain error, an asymmetric capacitive gain resolution improvement (ACGRI) technique is adopted, which realizes a fine gain step with low control circuit complexity. Further to reduce gain error, greatly improving the accuracy and robustness against PVT of gain control is necessary. With regard to this, a current-type digital-to-analog converter (DAC) is also integrated onto the chip. The proposed VGA is demonstrated in 65-nm CMOS technology with a core chip area of 0.14 mm
²
. Under 5-b digital gain control conditions, the presented VGA achieves a measured linear gain tunability range of 6.2 dB with a 0.2-dB fine tuning step. The measured root mean square (rms) phase error is less than 0.92° across 24–28 GHz with a minimum value of 0.63° at 27.8 GHz. The rms gain error is less than 0.13 dB, with a minimum value of only 0.03 dB at 25 GHz. At the maximum gain state, the proposed CS VGA exhibits a peak gain of 29.4 dB with 3-dB bandwidth of 23.5–27.5 GHz and the minimum noise figure is 4.8 dB at 26 GHz. Besides, the measured OP
_1dB
and OP
_3dB
are 6.3 and 9.6 dBm, respectively. The influence of VT variations on the phase compensation and gain tuning is also measured, and the proposed VGA exhibits strong robustness.