PURPOSE: To make it posible to memorize momentarily the quantity of light or a voltage by making use of characteristics of a field effect transistor.

CONSTITUTION: With switches S2 and S3 OFF, capacitor 5 is charged via FET4 by a photo current from photo diode 1. Here, a feedback value is so set that gate- source voltage VGS of FET4 will be zero or constant, and drain-source voltage VDS is extremely small, so that since the linear part of characteristics of drain current ID to voltage VDS is used, resistance RDS will be constant. While capacitor 5 is being charged through this constant resistance, transistor 11 conducts by time constants C10 and R9 to control FET4, and consequently the charge of capacitor 5 is stopped to memorize the value. In this way, the quantity of detected light can momentarily be meorized.