GENERATOR FOR FOCUS CONTROL SIGNAL OF LUMINOUS FLUX

摘要

PURPOSE:To form a focus servo system which is free from variation of its characteristics due to the variance of incident light quantity to a detector, by detecting the shift of position between the focal point of an objective lens and the recording surface of information with a fixed level of intensity regardless of the total quantity of light which is incident to the detector. CONSTITUTION:A difference DELTAP is obtained by a subtractor 10 between the outputs P1+P3 and P2+P4 of a pin diode 6 and amplified by an amplifier 11. This amplified difference is delivered in the form of a focus control signal S. While the sum KP of both outputs is delivered from an adder 12 and amplified by an amplifier 13. A comparator 15 compares the DC reference voltage V of a DC reference source 14 with the output V' of the amplifier 13. The output of this comparison is applied to a variable impedance element 16 which forms a negative feedback loop to control the amplification factor of the amplifier 13. The output of this amplification is kept at a fixed value V' approximately equal to the voltage V. AT the same time, a variable impedance element 17 having the same characteristics as the element 16 controls the amplfication factor having the same characteristics as the amplifier 13. Thus the focus shift/focus control signal characteristics are kept constant regardless of the total light quantity KP.