A differential common drain Colpitts VCO circuit suitable for sub-1V low phase noise

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This letter proposes a differential common drain Colpitts VCO circuit suitable for low phase noise oscillation at the sub-1V supply voltage. Oscillation with low phase noise at the sub-1V supply voltage is facilitated by employing inductors as the current sources of the proposed circuit. One of the two feedback capacitors in the half circuit of the proposed VCO circuit is replaced by a MOS varactor in order to further increase negative resistance. Post-layout simulation results using a 0.18µm RF CMOS technology show that the phase noise performances of the proposed circuit at the sub-1V supply voltages are much better than those of the well-known cross-coupled VCO at the same supply voltages.