The duality of parallel feedforward and negative feedback

The duality between the property of the negative feedback to stabilize a system and the property of positive Parallel Feedforward to render a system minimum phase is rigorously established. The importance of this duality is that all results on stability with negative feedback apply vis-a-vis to the property of positive parallel feedforwad to render the system minimum phase. With further restriction on the relative degree this result gives the parameterization of all the parallel feedforward transfer functions that render a system Almost Strictly Positive Real. Finally, an example is presented to illustrate the results.