Analysis of Optimum Aspect Ratio of Transistor for a Low-Power and Low-Phase Noise VCO for Biomedical Applications

Implantable electronics systems comprise small and low-power devices, and designing low-power, low-phase noise and area-efficient voltage-controlled oscillators (VCOs) are critical for biomedical applications. A new methodology to design them is described in this study. Ring oscillators were examined because they do not require external inductors and capacitors, and are area-efficient, when compared with LC oscillators. A model of CMOS ring oscillators was used to derive a phase noise model. Phase noise versus power consumption of five-stage ring oscillators was formulated and calculated for different aspect ratios of transistors in the VCO for the evaluation of its effect on phase noise and power consumption and determination of its optimum point. The calculations derived were compared with post layout simulated results for evaluation of this model. The calculated phase noise and power consumption were found to have an error of less than 2 percent, implying that this model is sufficiently accurate and allow optimizing this class of VCOs.