It has been shown in the literature that a cross-coupled CMOS LC VCO will outperform an equivalent Colpitts VCO. In the case of bipolar devices, the jury is still out. This paper reports a comparative analysis of phase noise (PN), tuning range (TR), dissipated DC power and Figure of Merit (FoM) in cross-coupled and differential Colpitts LC VCOs topologies designed in 180 nm Si-Ge HBT technology for operation around 5 GHz. SpectreRF simulations show that the cross-coupled topology exhibits a minimum PN equal to -108 dBc/Hz, a tuning range of 17.5% and a dissipated DC power of 12.6 mW, with a FoM equal to 204 dB, while the Colpitts topology exhibits a minimum PN over the tuning range equal to -113 dBc/Hz, a tuning range of 21.6% and a dissipated DC power of 14.1 mW, with a FoM equal to 212 dB. This suggests that, for the considered technology, the differential Colpitts can exhibit better overall performance than the cross-coupled VCO.
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