Inflationary Perturbations and Exponential Potentials
  We consider the spectrum of primordial fluctuations produced by inflationary models where the inflaton potential is the sum of two exponential terms. A wide range of spectra result, with the only constraint being that the scalar spectrum must have more tilt than the tensor spectrum. This model can mimic the spectra of most inflationary models, as well as producing combinations of scalar and tensor perturbations not found in common varieties of slow-rolling inflation. 
Richard Easther