In the two-layer model of a stably stratified medium we investigate the stability of flows without inflection points on the velocity profile, which monotonically increases from zero at the bottom ($y=0$) to the maximum value of $U_0$ (when $y\to\infty$). It is shown that in general, instability sets in at an arbitrarily small density difference, and perturbations of all scales grow simultaneously. With an enhancement of the stratification, the real part $c_r$ of the phase velocity of unstable perturbation increases. The upper boundary of the instability region is determined by the fact that $c_r$ reaches $U_0$, the perturbation becomes out of the phase resonance with the flow and transforms to a neutral oscillation of the medium. An analysis is made of the role of the neutral modes associated with null-curvature points on the velocity profile in the formation of the instability region configuration.