  The chiral lattice structure of twisted bilayer graphene with D6 symmetry
allows for intrinsic photogalvanic effects only at off-normal incidence, while
additional extrinsic effects are known to be induced by a substrate or a gate
potential. In this work, we first compute the intrinsic effects and show they
reverse sign at the magic angle, revealing a band inversion at the {\Gamma}
point. We next consider different extrinsic effects, showing how they can be
used to track the strengths of the substrate coupling or displacement field. We
also show that the approximate particle-hole symmetry implies stringent
constraints on the chemical potential dependence of all photocurrents. A
detailed comparison of intrinsic vs. extrinsic photocurrents therefore reveals
a wealth of information about the band structure and can also serve as a
benchmark to constrain the symmetry breaking patterns of correlated states.
