Impact of the Base Resistance Noise and Design of a -190-dBc/Hz FoM Bipolar Class-C VCO

This letter presents the analysis of phase noise generated from the base resistance in the bipolar transistor Class-C oscillator. It is found that the efficient pulsed current of transistors enhances this noise contribution. Moreover, the impact of the base resistance noise rises quickly with the oscillation amplitude, limiting the benefits of rising the supply voltage to reduce phase noise and penalizing the Figure of Merit (FoM). Furthermore, the amplitude stability problem is addressed and a simple expression for the maximum tail capacitance to prevent instability is derived. The phase noise analysis is validated by the measurements on a 12-GHz Class-C HBT VCO. With 3.4-V supply, the VCO reaches -187-dBc/Hz FoM, limited by the base resistance noise. The impact of this noise source is reduced by decreasing the supply voltage, and with 1.8-V supply, the VCO maintains a state-of-the-art phase noise (-120 dBc/Hz at 1 MHz) with an excellent -190-dBc/Hz FoM.