Pré-publication, Document de travail: This paper compares different implementations of monetary policy in a new- Keynesian setting. We can show that a shift from Ramsey optimal policy under short term commitment (based on a negative-feed back mechanism) to a Taylor rule (based on a positive-feed back mechanism) corresponds to a Hopfbifurcation with opposite policy advice and a change of the dynamic properties. This bifurcation occurs because of the ad hoc assumption that interest rate is a forward-looking variable when policy targets (inflation and out put gap) a reforward-looking variables in the new-Keynesian theory.