| 摘要 |
PROBLEM TO BE SOLVED: To provide a focus enhancement circuit and a video display apparatus for suppressing excess correction of a shape of an electronic beam spot when the screen of the video display apparatus gets bright. SOLUTION: The focus enhancement circuit is configured with: a quadrupole coil 14 for applying a correction magnetic field to an electron beam to deform in advance the shape of the beam; a switching transistor 28 connected to the quadrupole coil 14 via a first resonance capacitor 22; and a DC bias source 50 applied to the quadrupole coil and connected between the quadrupole coil and first and second resonance capacitors, and decreases a terminal voltage (DC level) of the quadrupole coil when the screen gets bright to weaken the effect of the correction magnetic field on the electron beam thereby avoiding excess correction of the shape of the electronic beam spot. Even when the current beam shape is changed, deformation of the beam spot shape at the peripheral part of the screen is decreased so as to avoid in advance the focus on the entire screen from being deteriorated. Since the DC level of the quadrupole coil can be varied in response to the current beam shape, excessive correction to the shape of electronic beam spot is avoided thereby enhancing the focus of the entire screen when the screen gets bright. COPYRIGHT: (C)2004,JPO
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