Patent ID: 7481913

Claim:
A gas concentration measuring apparatus comprising: a gas concentration sensor equipped with a sensor element formed by a lamination of a solid electrolyte plate, a diffusion layer, and an insulating layer and working to produce an electric signal as a function of a concentration of a preselected component of gasses over a given wide gas concentration measuring range; a sensor circuit including a current-measuring resistor, a plurality of amplifiers, and A/D converters, said sensor circuit working to apply a voltage to the sensor element, the current-measuring resistor functioning to measure a current signal flowing through the sensor element produced upon application of the voltage to the sensor element, the amplifiers having predetermined amplification factors different from each other and each working to amplify the current signal as measured by the current-measuring resistor to output the amplified current signal to the A/D converters to determine the concentration of the preselected component of the gasses within a corresponding one of a plurality of measurement ranges defined within the given wide gas concentration measuring range, respectively; clamping circuits disposed in output lines leading to output terminals of the amplifiers, respectively, each of the clamping circuits working to hold the amplified current signal outputted from a corresponding one of the amplifiers within an operating voltage range of a corresponding one of the A/D converters; and a resistance measuring circuit working to change one of a voltage and a current applied to the sensor element in an ac form and measure one of a resultant change in voltage and a resultant change in current, as sampled through the current-measuring resistor, to determine a resistance of the sensor element in order to control a degree of activation of the sensor element; wherein each of the clamping circuits includes a pnp transistor and a constant voltage source, the pnp transistor being connected at an emitter thereof to the output lines of a corresponding one of the amplifiers and at a base thereof to a reference voltage source, the constant voltage source generating a constant voltage substantially identical with a maximum operating voltage of a corresponding one of the A/D converters, and wherein each of the clamping circuits is so designed that an input voltage appearing at the base of the pnp transistor is identical with a value derived by subtracting a base-emitter voltage drop of the pnp transistor from the constant voltage generated by the constant voltage source.