Low-phase-noise SiGe HBT VCOs using trifilar-transformer feedback

The SiGe heterojunction bipolar transistor (HBT) voltage controlled oscillators (VCOs) using trifilar-transformer feedback at emitters, bases and collectors are demonstrated. The integrated trifilar transformer can allow dual voltage swings across the collectors and emitters of a cross-coupled differential pair and separate the bias between bases and collectors to optimize the output power. The tank inductance is also improved by the mutual coupling of the trifilar transformer. Thus, 191 dBc/Hz FOM (figure of merit) is achieved and is comparable to that of the state-of-the-art VCO.