This paper presents an approximate input-output linearization approach for nonminimum phase nonlinear systems with linear unforced dynamics. A special Byrnes Isidori normal form is introduced, where the internal dynamics are not affected by the inputs up to second order such that the corresponding transformation can be obtained by solving linear equations. After a factorization of the internal dynamics into a stable and into an antistable part a quadratic linear and internally stable input-output behaviour is achieved by state feedback in explicit form.