| 摘要 |
PURPOSE:To reduce a power supply voltage and to decrease widely a driving electric power by supplying a voltage necessary to operate normally a differential amplifier by the voltage which occurs with a resistance component of one focus coil, and limiting the resistance value of the detecting resistance to the necessary minimum. CONSTITUTION:An electric potential of a connecting point (A point) of a detecting resistance 5 and a focus coil 3 is inputted to a non-inverting input terminal of a differential amplifier 9, and to an inverting input terminal, the electric potential of an anode terminal (B point of a Zener diode 11 giving a reference voltage is inputted. The output of the differential amplifier 9 is connected to the base of a transistor 10 and set so that the electric potential of a A point and the electric potential of a B point can be always equal. Next, to supply a constant voltage to the C point, the electric potential of a dividing point (D point) is inputted to the non-inverting input terminal of the differential amplifier 8. To the inverting input terminal, the electric potential of an emitter (C point) of a transistor 7 to control the electric current is inputted. The output of the differential amplifier 8 is connected to the base of the transistor 7, and controls so that the voltage between (B-D) and the voltage between both ends of the detecting resistance can be equal. Thus, a constant electric current flows at focus coils 1-3 and a electro-magnetic focus is matched.
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