| 摘要 |
PURPOSE:To detect accurately a beam current by providing a current supply circuit to the current side of a cathode of a cathode-ray tube and a current lead-in circuit to the ground side so as to subtracts a current applied from the current supply circuit from the current of the current lead-in circuit. CONSTITUTION:A drive circuit of the cathode of the cathode-ray tube 1 is divided into two systems; one is the current supply circuit 3a supplying current only and the current lead-in circuit 3b leading in the current only. When a high frequency signal is inputted as the signal, charge/discharge is applied to a parasitic capacitance 10. A charging current Ic to the parasitic capacitance 10 is supplied from the current supply circuit 3a. The addition of a current Ic from the parasitic capacitance 10 to a beam current Ib is led to the current lead-in circuit 3b. Thus, the beam current Ib is detected by subtracting the current Ic applied from the current supply circuit 3a from the current (Ib+Ic) lead-in to the current lead- in circuit 3b.
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