A device that may include a DC power supply coupled to a fixed current source; an APD; a DC voltage regulator that comprises a regulating transistor, arranged to maintain a regulated voltage at a fixed value over different APD currents; a temperature control module that is arranged to maintain a portion of the temperature control module at a fixed temperature; and compensation circuit that comprises a compensation component that is thermally coupled to the APD. A voltage drop over the compensation component is smaller than a voltage drop over the APD. A sum of (a) a current that pass through the APD and (b) a current that passes through the compensation component is fixed. The portion of the temperature control module is thermally coupled to the compensation component and to the APD.