Narrow Linewidth RLC Resonator Design Using Positive Feedback Loop

This paper presents a simple idea for improving the linewidth or quality factor (or Q-factor) of an RLC resonant circuit using positive feedback loop. This is achieved by considering a second-order resonant circuit which implements a band-pass filter structure (i.e. having a single zero at the origin). This allowed the loop to be closed without affecting the resonance frequency while at the same time the quality factor is modified. The larger the open-loop gain the more the improvement on the quality factor. It is shown that although in principle this approach has no upper limit to how much improvement can be made on the quality factor, it has the practical limitation which is dictated by the op-amp’s gain-bandwidth product used in the feedback loop.