Strictly positive real problem with observers

We study the extension of what class of linear time invariant plants may be transformed into SPR systems introducing an observer. It is shown that for the open loop stable systems a cascaded observer achieves the result. For the open loop unstable systems an observer-based feedback is required to success. In general any system stabilizable and observable may be transformed into an SPR system. This overcomes the old conditions of minimum phase and relative degree one. The result is illustrated with some examples.