PURPOSE: To switch the constant current bias circuit of an operational amplifier at a low-impedance point to eliminate the noise, the leak current, the contact defect, etc., by switching this constant current bias circuit by an external signal to select a photodetector for photometry.
CONSTITUTION: When a bias switching control signal SC is L-level, a transistor TR Q 13 is turned on through an inverter IN 1 , and TRs Q 14 and Q 10 are turned off, and a differential amplifying circuit consisting of TRs Q 8 and Q 9 is not biased with a constant current and is made unoperated. Therefore, an FETQ 2 and a photodetector SPD 2 are disconnected from an operational amplifier OP 1 . Meanwhile, a TR Q 12 is turned off, and TRs Q 19 and Q 11 are turned on, and a constant current Ij is flowed to the TR Q 11 . Consequently, a differential amplifying circuit consisting of TRs Q 6 and Q 7 is biased with the constant current and is made operated, and a photodetector SPD 1 is set to the photometric state through FETs Q 3 and Q 1 . When the signal SC is H-level, the photodetector SPD 1 is disconnected, and the photodetector SPD 2 is set to the photometric state.