An evolution of Colpitts VCO for simultaneous optimization of phase noise and FoM in GaAs technologies

This paper presents a novel co-optimization configuration to simultaneously improve phase noise and FoM in order to solve the long-existing issue of excellent phase noise and mediocre FoM in the light of GaAs technologies. Considering traditional GaAs based Colpitts/class-C VCOs, a first step is taken with the introduction of noise shifting structure while a second step is carried out with the presentation of Darlington pair. Afterwards, a detailed outline of pros and cons of five VCO topologies, including Colpitts/class-C/noise shifting (NS) Colpitts/noise shifting class-C/noise shifting Darlington-based (NSDB) class-C VCO, is given in theory and in practice while respective start-up conditions and capacitive feedback factors are derived mathematically. It can be found that iterative phase noise and FoM improvement is achieved in GaAs based VCOs above in a gradual manner. Furthermore, the output power is stabilized with a simple trick of two-stage Darlington-based topology and thus, the power-hungry buffer is avoided. Ultimately, three VCOs out of the five are fabricated in GaAs technologies while their specifications are thoroughly compared with previous research. The fabricated Colpitts VCO demonstrates a frequency tuning range of 3.59-3.69 GHz with a current consumption of 11.2 mA@5 V supply, a phase noise of - 130.1 to - 129.1 dBc/Hz at 1 MHz offset from the carrier and the output power is around - 5 to - 5.1 dBm. The fabricated noise shifting Colpitts VCO demonstrates a frequency tuning range of 3.58-3.67 GHz with a current consumption of 10.8 mA@5 V supply, a phase noise of - 132.4 to - 130.2 dBc/Hz at 1 MHz offset from the carrier and the output power is around - 2.5 to - 3.5 dBm. The fabricated Darlington-based class-C VCO demonstrates a frequency tuning range of 2.76-2.91 GHz with a current consumption of 9.5 mA@5 V supply, a phase noise of - 138.6 to - 135.9 dBc/Hz at 1 MHz offset from the carrier and the output power is around - 3.06 to - 4.18 dBm. To the best of the authors' expertise, the proposed Darlington-based class-C VCO achieves both exceptional phase noise and FoM, which far outweigh that of existing GaAs ones.