Patent Publication Number: US-2007108374-A1

Title: Photocurrent amplifier circuit and optical pick-up device

Description:
BACKGROUND OF THE INVENTION  
      (1) Field of the Invention  
      The present invention relates to a photocurrent amplifier circuit and an optical pick-up device in which the photocurrent amplifier circuit is used, and particularly to a technology which realizes a small-scale photocurrent amplifier circuit.  
      (2) Description of the Related Art  
      These days, optical disc media such as Compact Discs (CDs) and Digital Versatile Discs (DVDs) have been widely used for recording large amount of digital information such as video and audio. As is commonly known, in order to read and/or write information from and/or to these various types of optical disc media (hereinafter simply referred to as media), laser lights of different wavelengths are used depending on the type of media.  
      A conventional small-scale optical pick-up device compliant with both CDs and DVDs typically includes a two-wavelength semiconductor laser device which is used as a light source and a single optical system which is used in common for laser beams of the both wavelengths. Light receiving devices placed in the positions which differ in accordance with the distance between the emitting points of the lasers having the respective wavelengths, and perform photo-electric conversion of reflected laser light from the medium projected through the optical system. An electric signal is thus obtained, and the obtained electric signal is amplified and outputted.  
      As a light receiving amplifier device suitable for such an optical pick-up device, a light receiving amplifier device, which selectively obtains one output by switching preamplifiers placed for each of the light receiving devices of different wavelengths, is well-known (see  FIG. 3  and  FIG. 4  of Patent Reference  1 : Japanese Laid-Open Patent Application No. 2004-22051).  
     SUMMARY OF THE INVENTION  
      Although the conventional light receiving amplifier device is suitable for selectively outputting one of the signals obtained from a plurality of light receiving devices, it is not necessarily suitable for an application to output signals of adopted combinations. The application, for instance, can be seen when the push-pull method and the phase difference detection method, both well-known technologies for tracking detection, are dynamically used.  
      In the push-pull method, two sub-beams which are distantly positioned in the tangential direction are projected onto a media. Tracking detection is performed by utilizing the fact that imbalance of the reflected sub-beams arises in accordance with the direction of tracking errors (toward an inner track or an outer track).  
      In the phase difference detection method, tracking detection is performed by utilizing the fact that phase difference arises between the inner track side of the reflected sub-beams and the outer track side of the reflected sub-beams.  
       FIG. 3  is a diagram conceptually showing an example configuration of light receiving device which can be used in both methods. In this example, light receiving devices A and B are set to receive reflected light of the preceding sub-beam, and light receiving devices C and D are set to receive reflected succeeding sub-beam. In the push-pull method, intensity difference of light between (A+B) and (C+D) is used as a signal representing tracking errors, while in the phase difference detection method, the phase difference between (A+C) and (B+D) is used as a signal representing tracking errors.  
      The signals are calculated by adding signals from each of the photo electric devices using various combinations according to the detection methods, and it is easy to obtain the signal by processing each signal in operational amplifying circuits set for each combination. However, in such a case it is inevitable for the scale of the circuit to increase.  
      The present invention has been conceived in view of such circumstances, and the object of this invention is to provide a small-scale photocurrent amplifier circuit which can freely combine the photocurrents obtained from the light receiving devices and amplify the combined photocurrents.  
      In order to solve the above problem, the photocurrent amplifier circuit including: light receiving devices which generate photocurrent in accordance with the amount of light received; amplifier circuits; and device selector switches which are connected between the amplifier circuits and the light receiving devices, in which the number of device selector switches connected to each of the light receiving devices is the same as the number of the amplifier circuits.  
      In addition, the photocurrent amplifier circuit, in which each of the amplifier circuits is an operational amplifier, an input and output of which has a gain resistor connected therebetween, amplifies a current signal flowing from each of the light receiving devices to the gain resistor through each of the device selector switches which corresponds to each of the amplifier circuits.  
      In addition, The photocurrent amplifier circuit, further including a preamplifier circuit set for each of the light receiving devices which amplifies the photocurrent from a corresponding one of the light receiving devices.  
      The present invention can be realized not only as such a photocurrent amplifier circuit but also for an optical pick-up device equipped with the photocurrent amplifier circuit.  
      With the photocurrent amplifier circuit of the present invention, the photocurrent amplifier circuit can amplify the photocurrent flowing together to gain resistors through the device selector switches to be switched on. In addition, on/off of the device selector switches can be freely set, and thus, a small-scale photocurrent amplifier circuit which can freely combine the photocurrents generated in a plurality of light receiving devices and amplify the combined photocurrents can be realized.  
     FURTHER INFORMATION ABOUT TECHNICAL BACKGROUND TO THIS APPLICATION  
      The disclosure of Japanese Patent Application No. 2005-328089 filed on Nov. 11th, 2005 including specification, drawings and claims is incorporated herein by reference in its entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects, advantages and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings that illustrate a specific embodiment of the invention. In the Drawings:  
       FIG. 1  is a circuit diagram showing an example of a photocurrent amplifier circuit of the first embodiment;  
       FIG. 2  is a circuit diagram showing an example of a photocurrent amplifier circuit of the second embodiment; and  
       FIG. 3  is a diagram conceptually showing a conventional configuration of a light receiving device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S)  
      The embodiments of the present invention are described hereinafter with reference to the diagrams.  
     First Embodiment  
       FIG. 1  is a circuit diagram showing an example of a photocurrent amplifier circuit according to the first embodiment of the present invention.  
      The photocurrent amplifier circuit is a circuit which freely combines photocurrents generated in the light receiving devices, amplifies the combined photocurrents. The photocurrent amplifier circuit is configured of light receiving devices  525  to  528 , amplifier circuits  538  and  540 , gain resistors  537  and  539 , NPN transistors  529  to  536 . Here, the NPN transistors  529  to  536  are examples of device selector switches. In accordance with the number of the amplifier circuits, two NPN transistors are placed for the respective light receiving devices  525  to  528  in the present embodiment. Each of the device selector switches are placed between the input of the amplifier circuit  538  and  540 , respectively. Note that the amplifier circuits  538  and  540 , for instance, may also be operational amplifiers.  
      The NPN transistors  529  to  536 , the device selector switches, provide photocurrent, generated in the light receiving devices  525  to  528  which are connected to the NPN transistors  529  to  536 , to the corresponding inputs of amplifier circuits, respectively.  
      The gain resistors  537  and  539  are feedback resistors respectively inserted to the negative feedback circuits of the amplifier circuits  538  and  540 , and provide the photocurrent from the output of the corresponding amplifier circuit. The amplifier circuits  538  and  540  respectively amplify the photocurrents flowing in the gain resistors  537  and  539  to voltage signals and output the voltage signals.  
      The photocurrent amplifier circuit, for instance, is suitable for the optical pick-up devices having light receiving devices utilized for both the push-pull method and the phase difference method.  
      In the case where this photocurrent amplifier circuit is used for such an optical pick-up device, the light receiving devices A to D shown in  FIG. 3  are the light receiving devices  525  to  528  shown in  FIG. 1 , respectively.  
      In the push-pull method, for example, by switching on the NPN transistors  529 ,  531 ,  534 , and  536 , while switching off the NPN transistors  530 ,  532 ,  533 , and  535 , a signal (A+B) is obtained from the amplifier circuit  538 , together with a signal (C+D) from the amplifier circuit  540 . The intensity difference between the two signals can be calculated in an external circuit which is not shown in the diagrams.  
      On the other hand, in the phase difference detection method, for example, by switching on the NPN transistors  529 ,  532 ,  533 , and  536 , while switching off the NPN transistors  530 ,  531 ,  534 , and  535 , a signal (A+C) is obtained from the amplifier circuit  538 , together with a signal (B+D) from the amplifier circuit  540 . The phase difference between the two signals can be calculated in the external circuit which is not shown in the diagram.  
      With this configuration, the photocurrents which flow together to the gain resistors  537  and  539  through the NPN transistors  529  to  536  when switched on can be amplified in the amplifier circuit  538  and  540 . In addition, since on/off of the NPN transistors  529  to  536  can be freely set, a small-scale photocurrent amplifier circuit, which can freely combine the photocurrents obtained from the light receiving devices  525  to  528  and amplify the combined photocurrents can be realized.  
     Second Embodiment  
       FIG. 2  is a circuit diagram showing an example of a photocurrent amplifier circuit according to the second embodiment of the present invention.  
      This photocurrent amplifier circuit is a circuit which freely combines the photocurrents generated in a plurality of the light receiving devices and amplifies the combined photocurrents. The photocurrent amplifier circuit is configured of light receiving circuits  804 ,  808 ,  812 , and  816 , and output amplifier circuits  841  and  842 .  
      Each of the light receiving circuits  804 ,  808 ,  812 , and  816  is a circuit which amplifies the photocurrents generated in the light receiving devices to voltage signals according to the amount of light received, and output the voltage signals. The light receiving circuit  804  is configured of a light receiving device  801 , a resistor  802 , and an operational amplifier  803 . Here, the operational amplifier  803  is an example of the abovementioned preamplifier circuit which amplifies the photocurrent from the light receiving device to a voltage signal. Accordingly, the light receiving circuits  808 ,  812 , and  816  are configured in the same manner.  
      The output amplifier circuits  841  and  842  are circuits which combine the signals from the light receiving circuits selected by the device selector switches and amplify the combined signals. The output amplifier circuit  841  is configured of: the PNP transistors  821 ,  823 ,  825 , and  827 ; input resistors  822 ,  824 ,  826 , and  828 ; and an amplifier circuit  830  in which a gain resistor  829  is connected between the input and the output. Here, the PNP transistors  821 ,  823 ,  825 , and  827  are the examples of the device selector switches. Note that the amplifier circuit  830 , for instance, may also be an operational amplifier.  
      In each of the PNP transistors  821 ,  823 ,  825 , and  827  to be switched on, a current flow in accordance with the respective output voltages from the light receiving circuits  804 ,  808 ,  812 , and  816 , a reference voltage Vref, and respective input resistors  822 ,  824 ,  826 , and  828 . These currents flow together to the gain resistor  829 , and are amplified as a voltage signal by the amplifier circuit  830 .  
      With this configuration, an output corresponding to the total of current signals flowing in the PNP transistors  821 ,  823 ,  825 , and  827  to be switched on can be obtained, while on/off of the PNP transistors  821 ,  823 ,  825 , and  827  can be freely set. Thus, it is possible to obtain the result of the amplification from free combination of the photocurrents from the light receiving devices  801 ,  805 ,  809 , and  813 .  
      The configuration and operation of the output amplifier circuit  842  can be described in the same manner.  
      The photocurrent amplifier circuit, like the photocurrent amplifier circuits in the first embodiment, for instance, is suitable for an optical pick-up device having the light receiving devices, which can be used both for the push-pull method and the phase difference detection method as described in the Related Art.  
      In the case where the photocurrent amplifier circuit is used for such an optical pick-up device, the light receiving devices A to D are the light receiving devices  801 ,  805 ,  809 , and  813  shown in the  FIG. 2  respectively.  
      In the push-pull method, for instance, by switching on the PNP transistors  821 ,  823 ,  835 , and  837 , while switching off the PNP transistors  825 ,  827 ,  831 , and  833 , the signal (A+B) is obtained from the amplifier circuit  830 , and the signal (C+D) is obtained from the amplifier circuit  840 . The intensity difference between the two signals can be calculated in an external circuit which is not indicated in the diagram.  
      On the other hand, in the phase difference detection method, for instance, by switching on the PNP transistors  821 ,  825 ,  833 , and  837 , while switching off the PNP transistors  823 ,  827 ,  831 , and  835 , the signal (A+C) can be obtained from the amplifier circuit  830 , and the signal (B+D) can be obtained from the amplifier circuit  840 . The phase difference between the two signals can be calculated in an external circuit which is not indicated in the diagrams.  
      With this configuration, a small-scale photocurrent amplifier circuit which can freely combine the photocurrents obtained from the light receiving devices  801 ,  805 ,  809 , and  813  and amplify the combined photocurrents can be implemented.  
      Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.  
     INDUSTRIAL APPLICABILITY  
      The photocurrent amplifier circuit of the present invention can be widely used as an amplifier circuit which can freely combine photocurrents obtained from a plurality of light receiving devices and amplify the combined photocurrents. In particular, the photocurrent amplifier circuit is suitable for a small-scale optical pick-up device which can dynamically utilizes the push-pull method and the phase difference detection method for tracking detection.