Dynamic bias schemes for class-C VCOs

In class-C VCOs the trade-off between oscillation amplitude and start-up condition reduces the advantages of this topology with the respect to the traditional class-B operation. In this paper a dynamic bias scheme able to break this ultimate trade-off is presented. Two different topologies are reported. The first VCO uses an active current tail generator, while the second VCO has a passive resistive tail. Both solutions, simulated in a CMOS 65nm process, oscillate at 7.2GHz and show a phase noise of -126dBc/Hz @ 1MHz while drawing 11mA from a 1.2V supply voltage, resulting in a figure-of-merit of -191dBc/Hz.