A 28-GHz Variable Gain Amplifier with Low Phase Variation in 90-nm CMOS

A 28 GHz variable gain amplifier is designed in a 90 nm CMOS technology. The circuit provides a maximum gain of 17.7 dB with a tunable range of 38 dB by the current-steering method. In this design, the phase variation of the voltage gain is minimized by introducing transformer coupling to cancel capacitive leakage current. A varactor provides the capability of frequency tuning. Consequently, the measured phase variation is less than $7^{\circ}$ over the gain tuning range at 28 GHz. The circuit consumes a dc current of 6.9 mA under the supply voltage of 1.2 V, including the output buffer of a two-stage cascode amplifier.