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 (P
SAT
) is 8.8 dBm with 25% power-added efficiency (PAE) at 26 GHz.