Theoretical Study of Optimal Feedback LNA Design

It is known that designing an LNA for its minimum noise figure (NF) is not the best if the design goal is the lowest NF of the entire receiver (RX). A better LNA performance metric is the noise measure (NM). However, if feedback is allowed, the minimum NM can be realized with different combinations of NF and available gain, depending on which the RX NF changes. This paper theoretically studies the low-noise performance limit of an RX when lossless feedback is applied to its LNA. Somewhat counterintuitively, the lowest possible RX NF is shown to be realized when the LNA NM is slightly off its minimum NM. A new noise performance metric, $\Xi$, is introduced. The RX NF is shown to be minimized when $\Xi$ of LNA is minimized. Unlike NM, $\Xi$ is applicable even when the first stage of an RX is passive, as in a passive-mixer-first THz RX.