PURPOSE: To obtain a stable discrimination circuit with less current consumption, by utilizing the source voltage of a field effect transistor FET as a comparison voltage.

CONSTITUTION: An N channel depletion type FETQ2 having a circuit threshold voltage VTD is used; a voltage developed at its source is inputted as Vcomp to a voltage comparing circuit 1" and the inverted voltage value of the circuit 1" is compared with the input Vcoomp to output the result. When the potential difference VGS between the source and gate rises up to -VTD, the FETQ2 is turned off and the source voltage becomes equal to -VTD. The relation between the potential difference VBS between a substrate and the source, and a threshold voltage is shown by the curve 2 in a figure. Since VBS=VTD in the circuit in the figure, its characteristics are shown by a straight line 3 and the value of the -VTD corresponding to their intersection E is equal to the potential at a point C. Even when the potential at the point C is to rise or fall, it returns to a point E because of the relation between the -VBS and -VTD. When VDD>- VTD, the potential at the point C is determined by the VBS-VTD characteristics of the Q2 regardless of the level of the VDD and stable.