Concept of quasi-capacitive tapping of bipolar voltage-controlled oscillators

Generally, bipolar transistors have better performance than MOS transistors if higher transconductances for the same operating point or higher transition frequencies are required. On the other hand, compared to MOS transistors, bipolar transistors often have limited use in the design of VCO´s, due to a limited voltage swing across the resonator and therefore limited phase-noise performance. However, applying quasi-capacitive tapping between the active part of the oscillator and the LC-tank, as proposed in this paper, the voltage swing over the tank can be increased accordingly, while at the same time the transistor remains far from heavy saturation. The presented analytical expressions give an insight into the trade-offs between quasi-tapped and non-tapped oscillators with respect to their phase-noise and power consumption.