I'm looking to build an amplifier circuit to amplify a small DC voltage signal (between 0-15mV) to the range of 0-1.3V. I tried messing around with a standard non-inverting op-amp amplifier circuit but have found that the gain is not consistent in this range, it increases as the input voltage increases until the input voltage reaches 20mV then stays steady until the output hits the rail (as expected).
My question is are there any good circuits that can be used to amplify DC voltages with a consistent gain at this level of input. I need to be able to math my way back to the original signal (from a photo-diode) so consistent gain is important. Also this is for a class so I need a circuit I can build with components, not one chip that solves the problem.