Tail Current-Shaping to Improve Phase Noise in LC Voltage-Controlled Oscillators

This paper introduces a tail current-shaping technique in LC-VCOs to increase the amplitude and to reduce the phase noise while keeping the power dissipation constant. The tail current is made large when the oscillator output voltage reaches its maximum or minimum value and when the sensitivity of the output phase to injected noise is the smallest; the tail current is made small during the zero crossings of the output voltage when the phase noise sensitivity is large. The phase noise contributions of the active devices are decreased and the VCO has a larger oscillation amplitude and thus better DC to RF conversion compared to a standard VCO with equal power dissipation. A circuit design to implement tail current-shaping is presented that does not dissipate any extra power, does not use additional (noisy) active devices and occupies a small area. The operation and performance of the presented circuit is extensively analyzed and compared to an ideal pulse biased technique. The presented analysis is confirmed by measurement results of two 2-GHz differential nMOS VCOs fabricated in 0.25-mum BiCMOS process