| 摘要 |
A switch element (SW1) is connected in series between an air-fuel ratio detecting cell (1) and a power source (Vo) and controlled by a control time generator (4). A potential ground (VPG) is connected to a cathode side of the detecting cell (1). An output voltage from the detecting cell (1) is input into a sample hold circuit (SW2, C1, A2) which is controlled by a control time generator (4) adapted to control the switching element (SW1) and sample hold circuit alternately. An output from the sample hold circuit is compared with an electromotive force control value (e) in a comparator circuit (A1), and output as an oxygen concentration-detected signal. A temperature setting resistor (r2) and a load resistor (R2) are connected between an output terminal of the sample hold circuit and the switching element (SW1). The voltage (e+, e-) from a node of the detecting cell (1) and an electric current detecting resistor (R1) and a node of the temperature setting resistor (R2) and load resistor (R2) are input into a differential amplifier (A3). The electric current flowing to a heater contained in the detecting cell (1) is controlled by a transistor (Tr) on the basis of an output from the differential amplifier (A3). As a result, the internal resistance of the detecting cell (1) is controlled to constant level. |
