The condition for the positivity of a cubic non-linearity cascaded with a simple time-invariant operator (s + δ)/(s2+ αs + β) is obtained, which is most useful in studies of the Instability of feedback systems using positive operators (Zames 1954,1966). The result obtained is an improvement on the existing result of Narendra and Cho (1967) who considered a general odd-monotone non-linearity in the feedback path.