The invention relates to an actuation and evaluation circuit for a laser diode (1) and a photodiode (3) for determining the concentration of a gas. The laser diode can generate light in the range of an absorption line of the gas. The circuit comprises a driver (10, 11, 12, 13) for generating a driving signal (17) for the laser diode (1), an assembly (8, 9) for generating a reference signal (20), and a subtractor (5) for subtracting the reference signal (20) from the signal (21) supplied by the photodiode. The invention further relates to a measuring device for determining the concentration of a gas by means of such an actuation and evaluation circuit. Finally, the invention relates to a corresponding method.