PURPOSE: To perform automatic threshold voltage control having no waveform distortion with a simple circuit by connecting an operational amplifier, diodes, capacitors, and resistances and obtaining a high threshold voltage at the midpoint of the maximum value and ground level and a low threshold voltage at the midpoint of the minimum value and ground level.

CONSTITUTION: A received signal inputted through a terminal 1 is transmitted to a maximum value detecting circuit 21 containing a diode 3 and capacitor 5 and minimum value detecting circuit 22 containing a diode 4 and condenser 6 through an operational amplifier 2. The maximum value of the received signal is passed through the diode 3 and stored in the capacitor 5 and appears at an putput terminal 7. When the output of the operational amplifier 2 becomes the minimum value, an electric current flows from a positive voltage source +V to the diode 4 through the capacitor 6 and the minimum voltage value is stored in the capacitor 6 and appears at another output terminal 8. When resistance values of resistances 11 and 15 and 12 and 14 are properly selected, a high threshold voltage is obtained at the midpoint of the maximum value and ground level. Moreover, a low threshold voltage obtained at a terminal 17 is obtained at the midpoint of the minimum value and ground level.