Semi-insulating GaAs/AlGaAs multiple quantum wells are photorefractive materials with high sensitivity and short response time. Semi-insulation of these structures is commonly obtained by proton implantation. We present the measurements of photoconductivity in the samples with different proton doses. The results were compared to the theoretically predicted relationship between photoconductivity and the donor to acceptor concentration ratio. This allows to estimate the impact of proton dose on deep donor concentration.