  We calculate power spectra of cosmological perturbations at high accuracy for
two classes of inflation models. We classify the models according to the
behaviour of the Hubble distance during inflation. Our approximation works if
the Hubble distance can be approximated either to be a constant or to grow
linearly with cosmic time. Many popular inflationary models can be described in
this way, e.g., chaotic inflation with a monomial potential, power-law
inflation and inflation at a maximum. Our scheme of approximation does not rely
on a slow-roll expansion. Thus we can make accurate predictions for some of the
models with large slow-roll parameters.
