Re: Output buffer of VCO
Source follower is the most frequently used topology, i've seen some modified edtion of source follower such as bias-T structure. The following is the reference:
N. H. W. Fong, J. O. Plouchart, N. Zamdmer, D. Liu, L. F. Wagner, C. Plett, and N. G. Tarr, "A 1-V 3.8-5.7-GHz Wide-Band VCO With Differentially Tuned Accumulation MOS Varactors for Comon-Mode Noise Rejection in CMOS SOI Technology", IEEE Trans. on Microwave Theory and Techniques, vol. 51(8), pp. 1952-1959, Aug. 2003.
I've heard that open-drain could also be used as output buffer, but i've not find any reference mentioned about it.