Control and evaluation circuit for a laser diode and a photodiode for determining the concentration of a gas, the laser diode being suitable to generate light in the range of an absorption line of the gas, comprising: a driver (10, 11, 12, 13) for generating a current for the laser diode, the driver comprising a first digital-analog converter for analog-converting a digital control signal for generating said current; an assembly (8, 9) for generating a reference signal; and a subtractor for subtracting the reference signal from the signal supplied by the photodiode; characterized in that the assembly for generating a reference signal further comprises: a digital memory for storing a characteristic, the characteristic being chosen, so that at zero concentration of the gas, a difference signal at the output of the subtractor is zero for all values of the control signal occurring during operation of the circuit, the digital memory being electrically connected to the input of the first digital-analog converter, in such a way that during operation the digital control signal is also supplied to the digital memory; a second digital-analog converter electrically connected to the digital memory in such a way that the second digital-analog converter receives digital values read out from the digital memory and the second digital-analog converter converts the received values into the reference signal. a driver (10, 11, 12, 13) for generating a current for the laser diode, the driver comprising a first digital-analog converter for analog-converting a digital control signal for generating said current; an assembly (8, 9) for generating a reference signal; and a subtractor for subtracting the reference signal from the signal supplied by the photodiode; characterized in that the assembly for generating a reference signal further comprises: a digital memory for storing a characteristic, the characteristic being chosen, so that at zero concentration of the gas, a difference signal at the output of the subtractor is zero for all values of the control signal occurring during operation of the circuit, the digital memory being electrically connected to the input of the first digital-analog converter, in such a way that during operation the digital control signal is also supplied to the digital memory; a second digital-analog converter electrically connected to the digital memory in such a way that the second digital-analog converter receives digital values read out from the digital memory and the second digital-analog converter converts the received values into the reference signal. a digital memory for storing a characteristic, the characteristic being chosen, so that at zero concentration of the gas, a difference signal at the output of the subtractor is zero for all values of the control signal occurring during operation of the circuit, the digital memory being electrically connected to the input of the first digital-analog converter, in such a way that during operation the digital control signal is also supplied to the digital memory; a second digital-analog converter electrically connected to the digital memory in such a way that the second digital-analog converter receives digital values read out from the digital memory and the second digital-analog converter converts the received values into the reference signal.