A Broadband Variable-Gain Low-Noise Amplifier With Low NF and Dual Phase Compensation

This brief proposes a broadband variable gain low-noise amplifier (VG-LNA) for multi-band 5G communications using 65-nm CMOS technology. Through theoretical derivation of input impedance and noise matching, a high-order matching network based on a weakly coupled asymmetrical transformer is introduced. The introduced transformer provides magnetic negative feedback, which further reduces the noise figure (NF) and achieves broadband input matching. Precise gain control of the VG-LNA is provided by five cascaded switchable attenuators in topologies of simplified T-type, T-type and
$\boldsymbol {\pi }$
-type. To reduce the phase error between the reference state and the attenuation state, compensation capacitors and transistors are added in the parallel and series branches of the attenuator, respectively, to realize dual phase compensation. The proposed VG-LNA achieves a measured linear gain tuning range of 15.5 dB with a 0.5-dB fine tuning step. At the maximum gain state, the presented VG-LNA exhibits a peak gain of 15.8 dB with a 3-dB bandwidth of 19–45 GHz, and the NF is 3.0-5.0 dB. The measured root mean square (RMS) phase error is less than 2.9° over the entire 3-dB bandwidth with a minimum value of 1.4° at 34 GHz. The measured RMS gain error is less than 0.27 dB, with a minimum value of 0.05 dB at 39 GHz. To the best of our knowledge, the proposed VG-LNA achieves the highest 3-dB bandwidth with low phase variation and low NF.