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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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When implemented in Si-Ge HBT technology, the differential Colpitts LC Voltage-Controlled Oscillator (VCO) can deliver low close-in phase noise by virtue of its low flicker noise corner frequency. The Armstrong topology reuses bias current and therefore should be more power-efficient than Colpitts. By comparing differential Colpitts and Armstrong architectures side-by-side, this paper shows that the Armstrong topology can deliver similar noise performance with an order of magnitude lower power consumption. This gives the Armstrong an 11 dB advantage in terms of Figure of Merit (FoM)

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