  We report on the systematic study of infrared/terahertz spectra of
photocurrents in (Bi,Sb)Te based three dimensional topological insulators. We
demonstrate that in a wide range of frequencies, ranging from fractions up to
tens of terahertz, the photocurrent is caused by the linear photogalvanic
effect (LPGE) excited in the surface states. The photocurrent spectra reveal
that at low frequencies the LPGE emerges due to free carrier Drude-like
absorption. The spectra allow to determine the room temperature carrier
mobilities in the surface states despite the presents of thermally activate
residual impurities in the material bulk. In a number of samples we observed an
enhancement of the linear photogalvanic effect at frequencies between
30{\div}60 THz, which is attributed to the excitation of electrons from helical
surface to bulk conduction band states. Under this condition and applying
oblique incidence we also observed the circular photogalvanic effect driven by
the radiation helicity.
