Research on Common-Mode Stability of Ka-Band Differential Common-Source Amplifiers

This article presents the analysis of the common-mode (CM) instability mechanism after introducing parallel capacitors at the center tap of the input balun of Ka-band differential amplifiers. Designed and implemented in 65nm CMOS technology with a 1-V supply, the Ka-band differential single-stage amplifier is fabricated for verification. The proposed circuit self-oscillates after introducing the parallel capacitor at the center tap of the input balun, but achieves tested stabilization after disconnecting this capacitor. The presented amplifier without the parallel capacitor reaches a maximum gain of 11.7 dB with a 3-dB bandwidth from 23-30.5 GHz, and the saturated output power (PSAT) is 8.8 dBm with 25% power-added efficiency (PAE) at 26 GHz.

I. Introduction

The evident trend of the recent development of millimeter wave (mm-Wave) amplifiers is to intensively utilize differential structure with transformer-based balun [1], which can realize conversion from single-ended to differential signal. However, capacitive couplings in the transformer-based balun may result in the imbalance of differential signals such that many designs have proposed many ways to alleviate this problem [1]-[4]. One of the methods is to introduce a parallel capacitor at the center tap of the balun to let the center tap be forced to be virtual ground and then the balance performance could be improved [4]. However, our research finds that when the parallel capacitor is introduced at the input balun, it might lead to common-mode (CM) instability.