Strictly positive real transfer functions revisited

The two most commonly used definitions of strictly positive real (SPR) transfer functions are reviewed. Contrary to what has been previously suggested, it is shown that the least restrictive (weak) definition of the two is clearly related to the Yacubovich-Kalman lemma. A relationship between time and frequency domain conditions pertaining to the weak definition of SPR systems is established. An alternative proof is given to show that a PR plant is stabilized by any (weak) SPR system connected in negative feedback.<>