A comparison of bandwidth setting concepts for Q-enhanced LC-tanks in deep-sub micron CMOS processes

In this paper LC-resonator bandwidth setting techniques in deep-sub micron CMOS processes are examined and compared with the main focus on Q-enhancement, linearity, noise, power consumption and controllability at low supply voltages. Simulations have been performed using an ideal amplifier model to achieve results independent of the purpose of the circuit. The Q-enhancement is achieved by positive feedback into the gates of a cross coupled MOSFET transistor pair. The different Q-tuning concepts include current control techniques as well as capacitive subdivision in the feedback path. As result of this work, a combination of tail current controlled and capacitive subdivision has been found to be the best mean to achieve highest linearity at good controllability. Simulation results show that a quality factor around 100 is possible at a current consumption of maximum 3 mA. The equivalent input noise does not exceed 1.5 nV/radic(Hz) with total harmonic distortion around 0.005% which makes the circuit reasonable for application in LNAs as well as for transmitter structures.