Abstract:
In a conventional light amplifier, when an addition or subtraction operation is performed on currents output from a plurality of light-receiving elements to output a voltage amplified according to the current resulting from the addition or subtraction operation, increasing the gain results in reducing the loop gain, making it impossible to achieve a wide band width. A light amplifier of the invention is provided with a plurality of light amplifier circuits, each having a light-receiving element that outputs a current according to the intensity of light received and a current amplifier that outputs a current by amplifying the output current of the light-receiving element, and a first transimpedance amplifier whose input terminal is connected to the node at which the output terminals of the individual current amplifiers are connected together. This helps prevent the lowering of the loop gain when the gain is increased.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light amplifier device that performs an addition or subtraction operation on currents output from a plurality of light-receiving elements and that outputs a voltage amplified according to the current resulting from the addition or subtraction operation. More particularly, the present invention relates to a light amplifier device for use in an optical pickup device. 
     2. Description of the Prior Art 
     In an optical pickup device for optical discs, a current signal output from a light-receiving section is used not only for data reading but also for servo control to achieve focusing (focusing of a reading light beam) and tracking (positioning of the reading light beam), both essential for correct reading of data. To achieve this, the light-receiving section is usually provided with not a single light-receiving element but a plurality of light-receiving elements arranged next to one another so that the servo control is achieved on the basis of the differences in the amount of light received by the individual light-receiving elements when a spot of light is incident on the light-receiving section. 
     On the other hand, for the purpose of data reading, to minimize read errors, a signal obtained by adding together all the current signals output from the individual light-receiving elements is used. Formerly, this addition operation has been performed by a signal processing integrated circuit provided outside an optical pickup device. Recently, however, such an addition operation has come to be performed increasingly by a light amplifier provided within an optical pickup device. One reason is that read and write rates have recently been increasing dramatically. Another reason is that, for reproduction from a plurality of types of optical discs, a laser beam having a plurality of frequencies has come to be used, which lowers the S/N ratio of the output signals from the light-receiving elements and accordingly makes less negligible the noise induced in the leads connecting the light-receiving elements to the signal processing integrated circuit that processes the output signals of the light-receiving elements. Still another reason is that further reduction of costs and electric power consumption has been expected in optical pickup devices. 
     FIG. 6 shows the configuration of a conventional light amplifier that adds together the current signals output from a plurality of light-receiving elements. The cathode of a photodiode D 1  is connected to the input terminal of a transimpedance amplifier  26 , and the cathode of a photodiode D 2  is connected to the input terminal of a transimpedance amplifier  27 . The anodes of the photodiodes D 1  and D 2  are kept at the ground potential. It is to be noted that a transimpedance amplifier denotes an amplifier that converts a current signal it receives into a voltage signal it outputs. 
     The output terminal of the transimpedance amplifier  26  is connected to one end of a resistor R 6 , and the output terminal of the transimpedance amplifier  27  is connected to one end of a resistor R 7 . The other ends of the resistors R 6  and R 7  are connected together, and the node n 2  between them is connected to the input side of a non-inverting amplifier  28 . The output side of the non-inverting amplifier  28  is connected to a terminal  4 . 
     The non-inverting amplifier  28  is composed of an operational amplifier OP 2  and resistors R 8  and R 9 . The non-inverting input terminal of the operational amplifier OP 2  serves as the input side of the non-inverting amplifier  28 . One end of the resistor R 8  and one end of the resistor R 9  are connected to the inverting input terminal of the operational amplifier OP 2 , and the other end of the resistor R 9  is kept at the ground potential. The other end of the resistor R 8  is connected to the output terminal of the operational amplifier OP 2 , and the node between them serves as the output side of the non-inverting amplifier  28 . 
     The output voltage V O ′ of the light amplifier configured as described above is given as follows. Let the output voltage of the transimpedance amplifier  26  be V 26 , the output voltage of the transimpedance amplifier  27  be V 27 , and the potential at the node n 2  be V n2 . Then, the current I fed to the non-inverting input terminal of the operational amplifier OP 2  is given by equation (1) below, where r 6  represents the resistance of the resistor R 6  and r 7  represents the resistance of the resistor R 7 . 
     
       
           I =( V   26   −V   n2 )/ r   6 +( V   27   −V   n2 )/ r   7   (1) 
       
     
     The relationship between the voltage V n2  and the output voltage V O ′ is expressed by equation (2) below, where r 8  represents the resistance of the resistor R 8  and r 9  represents resistance of the resistor R 9 . 
     
       
           V   O ′=(1 +r   8   /r   9 )× V   n2   (2) 
       
     
     When equations (1) and (2) are integrated together, the output voltage V O ′ is given by equation (3) below. Here, the term including the current I, which is a very small current, is approximated as zero. 
       V   O ′=(1 +r   8   /r   9 )×( r   7   ×V   26   +r   6   ×V   27 )/( r   6   +r   7 )  (3) 
     When the resistance r 9  of the resistor R 9  is set as defined by equation (4) below, and equations (3) and (4) are integrated together, then the output voltage V O ′ is given by equation (5) below. 
     
       
           r   9 =( r   6   ×r   7 )/( r   6   +r   7 )  (4) 
       
     
     
       
           V   O ′=(1 +r   8   /r   9 )× r   9 ×( V   26   /r   6   +V   27   /r   7 )  (5) 
       
     
     In equation (5), V 26 /r 6  can be regarded as the output current of the transimpedance amplifier  26 , and V 27 /r 7  can be regarded as the output current of the transimpedance amplifier  27 . Moreover, the voltage V 26  is the result of the conversion of the output current of the photodiode D 1  by the transimpedance amplifier  26 , and the voltage V 27  is the result of the conversion of the output current of the photodiode D 2  by the transimpedance amplifier  27 . Hence, equation (5) shows that the output voltage V O ′ is a voltage amplified according to the value obtained by adding together the currents output from the photodiodes D 1  and D 2 . 
     In the conventional light amplifier shown in FIG. 6, if the gain of the operational amplifier OP 2  is assumed to be A 0 , the loop gain T′ of the non-inverting amplifier  28 , which is a negative feedback amplifier, is given by equation (6) below. 
     
       
           T′=A   0   ×r   9 /( r   9   +r   8 )  (6) 
       
     
     Here, an attempt to increase the gain of the conventional light amplifier shown in FIG. 6 by increasing the amplification factor of the current signals fed to the transimpedance amplifiers  26  and  27  results, since the resistance r 9  is set as defined by equation (4), in reducing the resistance r 9 , with the result that, as equation (6) clearly shows, the loop gain T′ of the non-inverting amplifier  28  is reduced. 
     In the conventional light amplifier shown in FIG. 6, its characteristics are enhanced by a factor of [(loop gain)/(gain after negative feedback)] by configuring the non-inverting amplifier  28  as a negative feedback amplifier, as compared with a case where no negative feedback is present. However, as described above, when the amplification factor of the current signals fed to the transimpedance amplifiers  26  and  27  is increased with a view to increasing the gain of the conventional light amplifier shown in FIG. 6, the loop gain T′ of the non-inverting amplifier  28  is reduced, and thus the characteristics of the non-inverting amplifier  28  are degraded. This makes it impossible to achieve a high gain and a wide band width with the conventional light amplifier shown in FIG.  6 . 
     Incidentally, Japanese Patent Application Laid-Open No. H2-301879 discloses an adder that outputs a voltage amplified according to the current obtained by adding together the output currents of a plurality of amplifiers (conductance amplifiers or variable conductance amplifiers) provided within the adder itself. However, the amplifiers provided within this adder receive voltages as their inputs, and therefore this adder cannot be used as a light amplifier to which the output currents from light-receiving elements are fed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a high-gain, wide-band-width light amplifier that performs an addition or subtraction operation on currents output from a plurality of light-receiving elements and that outputs a voltage amplified according to the current resulting from the addition or subtraction, and to provide an optical pickup device employing such a light amplifier. 
     To achieve the above object, according to the present invention, a light amplifier is provided with: a plurality of light amplifier circuits, each having a light-receiving element that outputs a current according to the intensity of light received and a current amplifier that outputs a current by amplifying the output current of the light-receiving element; and a first transimpedance amplifier whose input terminal is connected to the node at which the output terminals of the individual current amplifiers are connected together. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which: 
     FIG. 1 is a diagram showing the configuration of a light amplifier according to the invention; 
     FIG. 2 is a diagram showing the configuration of the current amplifier provided in the light amplifier of FIG. 1; 
     FIG. 3 is a diagram showing the configuration of the transconductance differential amplifier provided in the current amplifier of FIG. 2; 
     FIG. 4 is a diagram showing the configuration in which the transimpedance amplifier provided in the current amplifier of FIG. 2 is replaced with a constant voltage source; 
     FIG. 5 is a diagram showing the configuration of the optical pickup device; and 
     FIG. 6 is a diagram showing the configuration of a conventional light amplifier. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a light amplifier according to the invention. It is to be noted that such circuit elements and signals as are found also in FIG. 6 are identified with the same reference symbols. 
     The cathode of a photodiode D 1  is connected to the input terminal of a current amplifier  1 , and the cathode of a photodiode D 2  is connected to the input terminal of a current amplifier  2 . The anodes of the photodiodes D 1  and D 2  are kept at the ground potential. 
     The output terminals of the current amplifiers  1  and  2  are connected together, and the node n 1  between them is connected to the input side of a transimpedance amplifier  3 . The output side of the transimpedance amplifier  3  is connected to a terminal  4 . 
     The transimpedance amplifier  3  is composed of an operational amplifier OP 1 , a resistor R 1 , and a constant voltage source  5 . The inverting input terminal of the operational amplifier OP 1  is connected to one end of the resistor R 1 , and the node between them serves as the input side of the transimpedance amplifier  3 . The output terminal of the operational amplifier OP 1  is connected to the other end of the resistor R 1 , and the node between them serves as the output side of the transimpedance amplifier  3 . The resistor R 1  acts as a negative feedback resistor for the operational amplifier OP 1 . The positive potential side of the constant voltage source  5  is connected to the non-inverting input terminal of the operational amplifier OP 1 , and the negative potential side of the constant voltage source  5  is kept at the ground potential. 
     The circuit configured as described above operates in the following manner. The current amplifier  1  receives the output current of I p1  of the photodiode D 1 , and amplifies it to output a current I O1 . The current amplifier  2  receives the output current of I p2  of the photodiode D 2 , and amplifies it to output a current I O2 . Thus, a current that is the sum of the currents I O1  and I O2  is extracted from the input side of the transimpedance amplifier  3 . The operational amplifier OP 1  evaluates the difference between the voltages fed to it, and outputs a voltage V O  according to the difference. Thus, equation (7) below holds, where r 1  represents the resistance of the resistor R 1  and V ref  represents the output voltage of the constant voltage source  5 . 
     
       
           V   O   =r   1 ×( I   O1   +I   O2 )+ V   ref   (7) 
       
     
     Here, if the impedance of the signal source connected to the input side of the transimpedance amplifier  3  is assumed to be Z i , then the loop gain T of the transimpedance amplifier  3  is given by equation (8) below, where the gain of the operational amplifier OP 1  is assumed to be A 0 . 
     
       
           T=A   0   ×Z   i /( Z   i   +r   1 )  (8) 
       
     
     In the light amplifier of FIG. 1, the impedance Z i  is the reciprocal of the sum of the reciprocals of the output impedances of all the current amplifiers connected to the transimpedance amplifier  3 , specifically the current amplifiers  1  and  2 . The current amplifiers  1  and  2  both output currents, and therefore have high output impedances. Thus, Z i , which is the reciprocal of the sum of the reciprocals of the output impedances of all the current amplifiers, is far higher than the resistance r 1 . Accordingly, equation (8) shows that the loop gain T of the transimpedance amplifier  3  is approximately equal to the gain A 0 . 
     As described earlier, in a negative feedback amplifier, its characteristics are enhanced by a factor of [(loop gain)/(gain after negative feedback)], as compared with a case where no negative feedback is present. With the light amplifier of FIG. 1, even when its gain is increased by increasing the gain of the current amplifiers  1  and  2 , the loop gain T remains equal to the gain A 0 , and does not lower. This makes it possible to design the light amplifier of FIG. 1 to offer a high gain and a wide band width. 
     Next, a practical example of the current amplifier provided in the light amplifier of FIG. 1 will be described with reference to FIG. 2. A terminal  9  is connected to the input side of a transimpedance amplifier  6 , and the output side of the transimpedance amplifier  6  is connected to the non-inverting input terminal of a transconductance differential amplifier  8 . It is to be noted that a transconductance differential amplifier denotes an amplifier that converts the difference between two voltage signals it receives into a current signal it outputs. 
     The transimpedance amplifier  6  is composed of an amplifier A 1  and a resistor R 2 . The input terminal of the amplifier A 1  is connected to one end of the resistor R 2 , and the node between them serves as the input side of the transimpedance amplifier  6 . The output terminal of the amplifier A 1  is connected to the other end of the resistor R 2 , and the node between them serves as the output side of the transimpedance amplifier  6 . 
     The input side of a transimpedance amplifier  7  is connected to a terminal  11 , and the output side of the transimpedance amplifier  7  is connected to the inverting input terminal of the transconductance differential amplifier  8 . The transimpedance amplifier  7  is composed of an amplifier A 2  and a resistor R 3 . The input terminal of the amplifier A 2  is connected to one end of the resistor R 3 , and the node between them serves as the input side of the transimpedance amplifier  7 . The output terminal of the amplifier A 2  is connected to the other end of the resistor R 3 , and the node between them serves as the output side of the transimpedance amplifier  7 . The output terminal of the transconductance differential amplifier  8  is connected to a terminal  10 . 
     The terminals  9  and  10  serve as the input and output terminals, respectively, of the current amplifier  1  provided in the light amplifier of FIG.  1 . Thus, the transimpedance amplifier  6  receives, via the terminal  9 , the output current of I p1  of the photodiode D 1  and converts it into a voltage to output this voltage signal to the non-inverting input terminal of the transconductance differential amplifier  8 . When a bias voltage is applied to the cathode of the photodiode D 1 , the DC bias level of the output voltage of the transimpedance amplifier  6  varies according to the voltage applied there. 
     On the other hand, with the terminal  11  kept at the same potential as the bias voltage applied to the cathode of the photodiode D 1 , the transimpedance amplifier  7  performs current-to-voltage conversion, and outputs the resulting voltage signal to the inverting input terminal of the transconductance differential amplifier  8 . Since the transimpedance amplifier  7  outputs a DC bias voltage having the same level as the DC bias level of the output voltage of the transimpedance amplifier  6 , the differential operation of the transconductance differential amplifier  8  cancels variations in the DC bias level, which thus no longer exert any effect on the circuit in the succeeding stage. This eliminates the need to consider the matching of the DC bias level, and thus makes it possible to apply the optimum bias voltage to the photodiodes provided in the light amplifier of FIG.  1 . 
     Of a plurality of current amplifiers provided in a light amplifier according to the invention, those connected to particular light-receiving elements may be configured so as to have not precisely the configuration shown in FIG. 2 but a modified version of it in which the output side of the transimpedance amplifier  6  is connected to the inverting input terminal of the transconductance differential amplifier  8  and the output side of the transimpedance amplifier  7  is connected to the non-inverting input terminal of the transconductance differential amplifier  8 . This causes the current amplifiers to output current signals having the opposite polarity in response to the current signals from those particular light-receiving elements, and thus permits those signals to be handled as factors to be subtracted. 
     Even when the current amplifies connected to particular light-receiving elements are configured in this way, the transimpedance amplifiers  6  and  7  have the same configuration as otherwise, and therefore does not affect the operation of the transconductance differential amplifier  8  in any way. Accordingly, even when other input currents are subjected to an addition or subtraction operation in various combinations, the characteristics of the light amplifier as a whole is little affected thereby. This greatly facilitates the design of the light amplifier in terms of the addition or subtraction operation it performs. 
     Next, a practical example of the transconductance differential amplifier  8  provided in the current amplifier  1  of FIG. 2 will be described with reference to FIG.  3 . The emitters of two PNP-type transistors Q 1  and Q 2  are connected together, and their node is connected to a terminal  16 , to which a constant voltage V CC  is supplied. The bases of the PNP-type transistors Q 1  and Q 2  are also connected together. Moreover, the base and collector of the transistor Q 2  are connected together. 
     The collector of the transistor Q 1  is connected to the collector of an NPN-type transistor Q 3 , and the node between them is connected to an output terminal  14 . The collector of the transistor Q 2  is connected to the collector of an NPN-type transistor Q 4 . 
     The base of the transistor Q 3  is connected to a non-inverting input terminal  12 , and the base of the transistor Q 4  is connected to an inverting input terminal  13 . The emitter of the transistor Q 3  is connected to one end of a resistor R 4 , and the emitter of the transistor Q 4  is connected to one end of a resistor R 5 . The other ends of the resistors R 4  and R 5  are connected together, and the node between them is connected to the positive potential side of a constant current source  15 . The negative potential side of the constant current source  15  is kept at the ground potential. 
     The transconductance differential amplifier  8  configured as described above operates in the following manner. The transistors Q 1  and Q 2  together form a current mirror circuit, and therefore the collector currents of the transistors Q 1  and Q 2  are equal. Let this collector current be I C . Then, the emitter current of the transistor Q 3  equals I C +I O1 , and the emitter current of the transistor Q 4  equals I C . 
     On the other hand, between the emitter current I E  and the base-to-emitter voltage V BE  of a transistor, the relationship V BE =V T ×ln(I E /I S ) holds, where V T  represents the thermal voltage of the transistor, ln represents the natural logarithm operator, and I S  represents the saturation current. Moreover, the current that is the sum of the emitter currents of the transistors Q 3  and Q 4  is equal to the output current I CC  of the constant current source  15 . Thus, if the resistances r 4  and r 5  of the resistors R 4  and R 5  are set equal, equation (9) below holds. 
       V   +   −V   −   =r   4   ×I   O1   +V   T×n[(   I   CC   +I   O1 )/( I   CC   −I   O1 )]  (9) 
     With equation (9), if consideration is given only to operation where |I O1 |&lt;&lt;I CC , that is, the output current of the transconductance differential amplifier  8  is far smaller than the drive current of the transconductance differential amplifier  8 , then equation (9) above can be rearranged as 
     
       
           V   +   −V   −   =r   4   ×I   O1   (10) 
       
     
     As will be clear from Equation (10), the gain of the transconductance differential amplifier  8  depends on the resistance r 4  (=r 5 ). On the other hand, the gain of the transimpedance amplifier  6  provided in the current amplifier  1  of FIG. 2 depends on the resistance r 2 . Thus, the relationship between the input current I p1  and the output current I O1  of the current amplifier  1  is expressed as I O1 =r 2 /r 4 ×I p1 . By forming the resistors R 2 , R 4 , and R 5  in the same process, even if there are variations in the individual resistances, it is possible to keep the value of r 2 /r 4  constant, and thereby keep the gain of the current amplifier  1  constant. By forming also the resistor R 3  in the same process as the resistors R 2 , R 4 , and R 5 , even if there are variations in the individual resistances, it is possible to cancel the DC bias level. 
     It is to be noted that, when the aforementioned presupposition |I O1 |&lt;&lt;I CC  does not hold, that is, in operation where the output current of the transconductance differential amplifier  8  is not negligible relative to the drive current of the transconductance differential amplifier  8 , the transconductance differential amplifier  8  loses linearity, and therefore the gain of the current amplifier does not remain constant. 
     The current amplifiers  1  and  2  provided in the light amplifier of FIG. 1 are configured as shown in FIGS. 2 and 3, and therefore their gain can be varied easily by appropriately setting the resistances of the resistors included in them. Thus, the gain of each of the current amplifiers  1  and  2  provided in the light amplifier of FIG. 1 can be set independently, and therefore the addition or subtraction operation can be performed with different factors set for the individual currents output from a plurality of light-receiving elements. 
     Incidentally, also in the conventional light amplifier shown in FIG. 6, by setting the gain of each of the amplifiers  26  and  27  independently, it is possible to perform an addition or subtraction operation with different factors set for the individual currents output from a plurality of light-receiving elements. Here, however, setting the factors in that way affects the loop gain of the non-inverting amplifier  28 . As a result, if there are large differences among the factors for the individual currents output from a plurality of light-receiving elements, the highest factor among them affects the non-inverting amplifier  28  in such a way as to degrade its characteristics, and this makes it inevitable to set a limit to the factors that can be set. 
     By contrast, in the light amplifier of FIG. 1, each current amplifier has a sufficiently high output impedance to prevent the factor set for one current amplifier from affecting another current amplifier or the transimpedance amplifier  3 . Thus, in the light amplifier of FIG. 1, there is no need to set a limit to the factors that can be set, and this increases the flexibility with which factors can be set for the individual currents output from a plurality of light-receiving elements. 
     Next, the input impedance of the transimpedance amplifier  3  will be described. As described above, the current amplifiers  1  and  2  provided in the light amplifier of FIG. 1 have high input impedances. This lowers the frequency of the pole that appears in the time constant calculated as the input impedance at the node n 1  multiplied by the parasitic capacitance ascribable to the wiring connected to the node n 1  and the like. 
     However, since signal transmission at the node n 1  is achieved by a current, the current signal obtained by adding together the output currents of the current amplifiers  1  and  2  has only to be fed, as a current signal, to the input side of the transimpedance amplifier  3 , and therefore, as long as no variations appear in the voltage at the node n 1  meanwhile, the appearance of the pole does not cause degradation of the band width. That is, by making the input impedance of the transimpedance amplifier  3  as low as possible, it is possible to lower the input impedance at the node n 1 , and thereby make the frequency of variations in the voltage at the node n 1  lower than the frequency of the pole to obtain a wide band width. 
     In a case where the transimpedance amplifier  3  is designed to have a low input impedance in this way, the voltages at the inverting and non-inverting input terminals of the transimpedance amplifier  3  are equal. Thus, the voltage at the output terminals of the current amplifiers  1  and  2  is also equal to the voltage at the non-inverting input terminal of the transimpedance amplifier  3 . 
     Here, the output circuit portion of the current amplifier is, to offer a high output impedance, so configured as to control the outflow and inflow of a current by connecting together the collector outputs of common-emitter or common-base circuits. In the configuration shown in FIG. 3, the collector outputs of common-emitter circuits are connected together. Therefore, in the transconductance differential amplifier  8  shown in FIG. 3, a high output impedance is obtained only when the transistors Q 1  and Q 3  are both in an active state. 
     To fulfill this condition, it is advisable, as shown in FIG. 4, to replace the transimpedance amplifier  7  provided in the current amplifier of FIG. 2 with a constant voltage source  17  and make the output voltage of the constant voltage source  17  equal to the output voltage of the constant voltage source  5  within the transimpedance amplifier  3 . 
     When the output voltages of the constant voltage sources  17  and  5  are made equal, the collector voltage of the transistor Q 3  is always equal to the base voltage of the transistor Q 4 . Then, simply by keeping the base voltage of the transistor Q 3  lower than the base voltage of the transistor Q 4 , it is possible to keep both the transistors Q 1  and Q 3  in an active state and thereby maintain a high output impedance. Thus, by appropriately setting the output voltages of the constant voltage sources  5  and  17  according to the range of the output current signal of the current amplifier, and then setting the base voltage of the transistor Q 3  lower than the base voltage of the transistor Q 4 , it is possible to always keep the output impedance of the current amplifier high. This makes it possible to increase the gain of the light amplifier without reducing the loop gain of the transimpedance amplifier  3 , and thus to realize a light amplifier that offers a high gain and a wide band width. 
     The constant voltage sources  5  and  17  do not necessarily have to be direct-current voltage sources, but may be alternating-current voltage sources that output mutually synchronous alternating-current voltages. Instead of replacing the transimpedance amplifier  7  with the constant voltage source  17 , the transimpedance amplifier  7  may be operated as a constant voltage source that outputs the same voltage as the constant voltage source  5 . 
     Next, an optical pickup device according to the invention will be described. The optical pickup device according to the invention is provided with the light amplifier according to the present invention shown in FIG.  1 . FIG. 5 shows the construction of the optical system of the optical pickup device according to the invention. 
     The laser beam emitted from a semiconductor laser  18  is formed into a parallel beam by a collimator lens  19 , is then transmitted through a beam splitter  20 , is then passed through a quarter-wave plate  21 , and is then condensed by an objective lens  22 . The condensed light beam is reflected from an optical disc  23 , is then formed into a parallel beam by the objective lens  22 , is then passed through the quarter-wave plate  21 , is then reflected from the beam splitter  20 , and is then condensed by a condenser lens  24  so as to reach a light amplifier  25 . The light amplifier  25  adds together the output currents of the photodiodes D 1  and D 2  and outputs a voltage V O  according to their sum. 
     The light amplifier  25  is a high-gain, wide-band-width amplifier. However, the light amplifier  25 , since it performs signal processing by using current signals, is affected by the Early effect of transistors, leak currents through various circuit elements, and the like and is thus inferior in level transmission accuracy to signal processing using voltage signals. Specifically, the light amplifier of FIG. 1 suffers from larger variations in the output voltage when no signal is present than the conventional light amplifier shown in FIG.  6 . 
     Incidentally, in the reproduction of a data signal in an optical pickup device, whereas no high accuracy is required in terms of direct-current levels, high-speed and high-gain amplification is required. On the other hand, in the processing of a servo signal for focusing and tracking in an optical pickup device, whereas the frequency band width required is far lower than that required for the reproduction of a data signal (about 20 kHz for the reproduction of audio from a CD), high accuracy is required in the amplification factor and in the offset level when no signal is present in order to produce the servo signal from very small differences between signals. 
     For these reasons, in the optical pickup device according to the invention, not only is a data signal produced on the basis of the output voltage V O  of the light amplifier  25 , but also a conventional light amplifier (not shown) is provided so that a servo signal is produced on the basis of the output voltage of the conventional light amplifier. It is advisable to share the photodiodes, which serve as light-receiving elements, between the light amplifier  25  and the conventional light amplifier for producing the servo signal. 
     In this way, separately providing a light amplifier suitable for producing a data signal and a light amplifier suitable for producing a servo signal eliminates the need to pursue a wide band width and high accuracy, which are conflicting objectives, within a single amplifier. This makes it easier to realize a high-performance optical pickup device. 
     The embodiments described above deal only with a light amplifier provided with two photodiodes. However, the present invention can be applied to any other configuration; for example, it can be applied to a configuration in which four photodiodes are provided as light-receiving elements, and accordingly four current amplifiers are provided to separately amplify the individual output currents from those photodiodes, with the output terminals of those current amplifiers connected together and to the input side of a transimpedance amplifier.