**5. Conclusions**

It was shown with physical and theoretical analyses that the switched-biasing technique can improve the phase noise characteristics by modulating the current source in the VCO using the deep sub-micron CMOS process. The switched-biasing technique can be divided into external-biasing and self-biasing topologies, depending on the method of implementing the current modulation. Even though the external-biasing topology can apply an optimum waveform as the modulation signal, the self-biasing topology that can control the current source with a waveform correlated with the output signal shows higher improvement in performance. The self-biasing topology can be subdivided into the usage of a fixed DC voltage and an adaptive DC voltage. The self-biasing topology with an adaptive DC voltage can be expected to apply the optimized waveform to the modulation signal; however, there is no significant improvement in the VCO performance compared to the self-biasing topology with a fixed DC voltage because the implementation of additional circuits, which are required for the adaptive DC voltage, increases the noise injection to the VCO. In addition, the self-biasing topology with the adaptive DC voltage is not also suitable for the millimeter-wave VCO design because the additional circuits can increase the parasitic components that affect the oscillation frequency shift, tuning range limit, and design accuracy. Based on the improvement of the phase noise, ease of implementation, and overall *FoM* and *FoMT*, it could be concluded that the self-biasing topology with a fixed DC voltage is the most useful in the switched-biasing technique.

**Author Contributions:** Conceptualization, C.-W.K. and J.-R.Y.; methodology, C.-W.K.; software, C.-W.K.; validation, C.-W.K., H.M., and J.-R.Y.; formal analysis, C.-W.K. and J.-R.Y.; investigation, C.-W.K.; resources, C.-W.K. and H.M.; data curation, C.-W.K., H.M., and J.-R.Y.; writing—original draft preparation, C.-W.K., H.M., and J.-R.Y.; writing—review and editing, H.M. and J.-R.Y.; visualization, C.-W.K.; supervision, J.-R.Y.; project administration, J.-R.Y.; funding acquisition, J.-R.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partly supported by the Institute of Information & Communications Technology Planning & Evaluation (IITP) gran<sup>t</sup> funded by the Korean governmen<sup>t</sup> (MSIT) (No. 2018-0-00711) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1B03031357).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created in this study. Data sharing is not applicable to this article.

**Acknowledgments:** The authors would like to thank all authors of previous papers for approving the use of their published research results in this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
