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Approximate formulae describing modulated photocurrents in a sandwich configuration of sample electrodes are derived. The carrier transport is described in terms of the multiple-trapping model. The formulae refer to arbitrary spatial distribution of generated carriers. The case of exponential light absorption in the sample is considered in detail. It is shown that for limiting low and high modulation frequencies, both the amplitude and the phase shift of the photocurrent are almost independent of the light absorption coefficient. In the intermediate-frequency range, for the case of surface carrier generation, the phase shift of the photocurrent usually shows oscillations. This makes it possible to obtain the energetic density of states from a simple formula. Examples of calculated frequency dependencies of the photocurrent phase shift and amplitude are given for exponential and Gaussian trap distributions as well as for a single trap level.