| 摘要 |
PURPOSE:To maintain highly accurately the black level of a video signal at a reference electric potential by applying a video signal to one end of a capacitor, comparing the potential of the other end of the capacitor with a reference voltage and discharging the electrostatic charge of the capacitor based on the compared output. CONSTITUTION:When a video signal SGI is applied to one end of the capacitor C2, the gate of an FET 20 is raised at a fixed changing ratio by current I1 supplied from a constant current source 18. When the potential of the source S of the FET 20 exceeds the sum of the reference voltage Vr and the half- amplitude level of hysteresis of a comparator 21, the output Vc of the comparator 21 is inverted, and in a back poach period, i.e., in a period that a clamp pulse PCL is 'H', a leading switch SW is closed by a switch pulse PSW and the gate voltage VG is dropped by the current I2 of a constant current source 19. When the output of the comparator 21 is reduced less than the difference between the voltage Vr and the half-amplitude level, the output is turned to the 'L' level and the switch SW is turned off. Thereby, the voltage VG is led. By repeating said operation, the black level in the back poach period is almost equal to the voltage Vr.
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