PURPOSE: To enlarge the dynamic range of a detecting circuit which detects the change of a threshold value.

CONSTITUTION: A J1 is a GaAS field-effect transistor for a current source, constituting a low current circuit along with a self-bias resistance RS. A J2 is a GaAS field effect transistor for a voltage source, having the drain connected to the resistance RS and the source connected to a grounding terminal. The output voltage is obtained from the gate of the J2. The gate width of the GaAS field-effect transistors J1, J2 is different, and therefore the change of the saturation current due to the change of a threshold value appears in the output voltage. The dynamic range of the output voltage can be enlarged by the difference of the gate width.