A Modified Darlington-Based Class-C VCO With Simultaneous Optimization of Phase Noise/FoM in GaAs Technology

This letter presents a modified Darlington-based (DB) class-C voltage-controlled oscillator (VCO) topology which achieves a cooptimization of phase noise and figure of merit (FoM) in GaAs technology. Both the Darlington pair and noise shifting techniques are introduced to improve the phase noise in class-C VCOs. Balanced and differential architectures are used to absorb the strength of both the Colpitts and class-C VCO topology with a simple trick of two cascaded stages in the VCO core, which elevates the VCO driving capability with no degradation of power efficiency. In addition, the startup conditions and capacitive feedback factors are derived and compared in both mathematical and simulation manners, which proves the superiority of the topology. The proposed VCO is fabricated in a GaAs bipolar high electron mobility transistor (BiHEMT) technology which demonstrates a frequency range of 2.76-2.91 GHz with a current consumption of 9.5 mA at 5 V supply, a phase noise of -138.6 to -135.9 dBc/Hz at 1 MHz offset from the carrier, an output power of -3.06 to -4.18 dBm, and a peak FoM of -191.2 dBc/Hz at 1 MHz offset. To the best of the authors' expertise, the proposed DB class-C VCO achieves both exceptional phase noise and FoM, which far outweigh that of the existing GaAs.