Abstract:
There is provided a light receiver including a photodiode converting an optical signal into an electrical signal, and a plurality of amplifiers respectively having different gains, each input end of the plurality of amplifiers being connected to one end of the photodiode, and one of the plurality of amplifiers being in operating state for outputting an output signal. At least one of the plurality of amplifiers and the photodiode are connected through a diode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a light receiver which is used for an optical pickup and has a photodiode as an input source and, particularly, to a light receiver with gain switching function.  
         [0003]     2. Description of Related Art  
         [0004]     An optical pickup is a device for reading and writing data on optical disks such as CD and DVD. The optical pickup typically includes a light receiver which receives reflected light from an optical disk as an optical signal by a photodiode, amplifies an electrical signal generated based on the optical signal and outputs the electrical signal.  
         [0005]     Recently, highly functional light receivers are required so as to ensure application to optical pickups with multi-compatibility feature (e.g. CD-ROM, DVD-ROM, DVD-R, DVD-RAM, BD(Blu-ray disk)-ROM). In an optical pickup with multi-compatibility, a light receiver receives optical signals of different intensity levels, such as optical signals for writing/reading, optical signals with different wavelengths, and so on. Still, the light receiver needs to provide an output signal of a constant intensity level.  
         [0006]     An optical pickup with multi-compatibility to receive optical signals with different wavelengths and different intensity levels can include a plurality of light receivers with different gains. However, this requires an optical system for inputting an optical signal to each light receiver and thus complicates the optical system of the optical pickup. Further, this hinders the miniaturization of the optical pickup. In addition, preparing a plurality of light receivers results in an increase in the number of components, which increases manufacturing costs.  
         [0007]     To address the above drawback, an optical pickup can include a single photodiode and a plurality of amplifiers which have different gains as usage and are connected to the photodiode, so that one amplifier can be selected and activated according to need.  
         [0008]      FIG. 3  is a circuit diagram of a light receiver  31  with multi-compatibility according to a related art. A cathode of a photodiode  1  which serves as an input signal source is connected to an input end of a first amplifier  4  and an input end of a second amplifier  3 . The first amplifier  4  and the second amplifier  3  are negative-feedback amplifiers which have resistors R 1  and R 2 , respectively, between the input and output. Outputs of the first amplifier  4  and the second amplifier  3  are input to an output circuit  7  through buffers  5  and  6 , respectively. The light receiver  31  also includes a switching circuit  8  which switches to cause one of the first amplifier  4  and the second amplifier  3  to enter operating state and the other to enter non-operating state, alternately. The first amplifier  4  is used for processing DVD-R, and the second amplifier  4  is used for processing DVD-RAM, for example. When processing DVD-RAM, an optical signal reading speed is higher, and optical signal intensity is smaller because reflected light from the optical disk is lower than when processing DVD-R. Accordingly, parameters of circuit elements are set so that the second amplifier  3  has a higher gain and a wider bandwidth than the first amplifier  4 .  
         [0009]     In the light receiver  31  of  FIG. 3 , the first amplifier  4  and the second amplifier  3  are connected to the photodiode  1 . The amplifier in the non-operating state affects the amplifier in the operating state as a load of resistance and capacitance, which leads to an increase in noise to cause an optical signal reading error. Particularly, the second amplifier  3  has a high gain and is largely affected by the first amplifier  4  as a load, and an optical signal reading error is likely to occur during operation of the second amplifier  3 . Related arts are described in Japanese Unexamined Patent Application Publication Nos. 2003-22537 and 2005-32282.  
       SUMMARY OF THE INVENTION  
       [0010]     According to an aspect of the present invention, there is provided a light receiver including a photodiode converting an optical signal into an electrical signal and a plurality of amplifiers having each input end connected to one end of the photodiode, wherein at least one of the plurality of amplifiers and the photodiode are connected through a diode.  
         [0011]     In this configuration, when the amplifier connected to the photodiode through the diode is in non-operating state, the diode is substantially zero-biased and in high-impedance state.  
         [0012]     The present invention provides a light receiver with a wide bandwidth where a reading error does not occur, which is suitable for use in optical pickups with multi-compatibility. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is a circuit diagram showing the configuration of a light receiver according to a first embodiment of the invention;  
         [0015]      FIG. 2  is a view showing simulation results of output characteristics of a second amplifier according to the first embodiment of the invention;  
         [0016]      FIG. 3  is a circuit diagram according to a related art;  
         [0017]      FIG. 4  is a circuit diagram showing the configuration of a light receiver according to a second embodiment of the invention;  
         [0018]      FIG. 5  is a circuit diagram showing a detail of a light receiver according to the first embodiment of the invention; and  
         [0019]      FIG. 6  is a view showing simulation results of output characteristics of a first amplifier according to the first embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposed.  
         [0021]     Exemplary embodiments of the present invention are described hereinafter with reference to the drawings.  
       First Embodiment  
       [0022]     Referring first to  FIG. 1 , in a light receiver  11  according to a first embodiment of the present invention, a cathode of a photodiode  1  which serves as an input signal source is connected to a cathode of a diode  2  and an input end of a second amplifier  3 . An anode of the diode  2  is connected to an input end of a first amplifier  4 . Output ends of the first amplifier  4  and the second amplifier  3  are connected to an input end of an output circuit  7  through buffers  5  and  6 , respectively. The light receiver  11  also includes a switching circuit  8  which switches to cause one of the first amplifier  4  and the second amplifier  3  to enter operating state and the other to enter non-operating state, alternately.  
         [0023]     The light receiver  11  may be integrated in one chip, thereby achieving the miniaturization of an optical pickup with multi-compatibility.  
         [0024]     The first amplifier  4  is used for processing DVD-R, and the second amplifier  3  is used for processing DVD-RAM, for example. When processing DVD-RAM, an optical signal reading speed is higher, and optical signal intensity is smaller because reflected light from the optical disk is lower than when processing DVD-R. Accordingly, parameters of circuit elements are set so that the second amplifier  3  has a higher gain and a wider bandwidth than the first amplifier  4 .  
         [0025]      FIG. 5  is a circuit diagram to describe the circuit diagram of  FIG. 1  in more detail.  
         [0026]     A transistor as an element of the light receiver  11  may be a bipolar transistor. The first amplifier  4  and the second amplifier  3  have the same circuit configuration. Each input portion is a differential amplifier composed of an NPN transistor pair. Outputs are connected from the emitter follower of a PNP transistor to the emitter follower of an NPN transistor, and are then negative fed back to the input through the resistors R 1  and R 2 , respectively. If output portions are directly connected with each other, the input/output of the amplifiers are common, the resistors R 1  and R 2  are parallel, which fails to perform switching operation. Thus, the buffers  5  and  6 , each composed of an NPN transistor, are placed at the outputs of the first amplifier  4  and the second amplifier  3 , respectively, so that each output of the first amplifier  4  and the second amplifier  3  is not connected to the output end of one another.  
         [0027]     The switching circuit  8  includes a differential circuit composed of a transistor pair Q 1  and Q 2  and a differential circuit composed of a transistor pair Q 3  and Q 4 . The output of Q 3  and the output of Q 4  are respectively connected to the differential amplifiers of the first amplifier  4  and the second amplifier  3 , the emitter follower of the PNP transistor and the emitter follower of the NPN transistor, and constant current supplies  9  and  10  for supplying constant currents to the buffers  5  and  6 . The light receiver  11  also has a switching terminal (SW) for receiving a switching signal from outside.  
         [0028]     Although the first amplifier  4  and the second amplifier  3  have the same circuit configuration in this example, the circuit configuration is not necessarily the same as long as it includes a constant current supply and a buffer.  
         [0029]     Referring now to  FIG. 5 , the operation of the light receiver  11  is described hereinafter.  
         [0030]     The switching circuit  8 , the first amplifier  4 , and the second amplifier  3  receive a voltage Vcc from a power supply (not shown). In the switching circuit  8 , the transistor Q 2  operates if an input voltage to the switching terminal (SW) is lower than a voltage determined by the voltage between Vcc and the ground and the resistors R 3  and R 4 , and the transistor Q 1  operates if the input voltage is higher than the same. An output of the differential portion, which is a voltage at a node between R 7  and the collector of the transistor Q 1  is switched depending on the operation state (operating or non-operating) of the transistor Q 1 , and if the voltage is lower than a voltage determined by the voltage between Vcc and the ground and the resistors R 5  and R 6 , the transistor Q 3  operates and the current of the constant current source  11  flows only to the transistor Q 3  side; if the voltage is higher than the same, the transistor Q 4  operates and the current flows only to the transistor Q 4  side. The selection of whether to flow a current to the bias line on the Q 3  side to turn ON the constant current supply  9  or to flow a current to the bias line on the Q 4  side to turn ON the constant current supply  10  is thereby made, thus switching between the operation and non-operation of the second amplifier  3 .  
         [0031]     The first amplifier  4  and the second amplifier  3  always receive a reference voltage Vref. A case where the first amplifier  4  is in the non-operating state and the second amplifier  3  is in the operating state is as follows. Because no current flows to the constant current supply  9  of the first amplifier  4 , no current flows to the first amplifier  4 . Consequently, no current flows to the diode  2 , so that the diode  2  is substantially zero-biased and high-impedance. Although the first amplifier  4  serves as a load of resistance and capacitance even when it is in the non-operating state, the diode  2  and the first amplifier  4  are connected in series with respect to the photodiode  1 , and the affect as a load is reduced by the diode  2  in the high-impedance state. The impedance value when the diode  2  is in the high-impedance state is set larger enough than the impedance when the first amplifier  4  serves as a load.  
         [0032]      FIG. 2  shows results of simulation of the output characteristics of the light receiver  11  to the output circuit  7  under the condition that the second amplifier  3  is in the operating state and the first amplifier  4  is in the non-operating state in comparison with the output characteristics of the light receiver  31  without the diode  2  under the same condition. The circuit configuration and the simulation conditions are the same except for the presence/absence of the diode  2 .  
         [0033]     The upper left column in  FIG. 2  shows the frequency characteristics of output signal gain when the light receiver has no diode, the lower left column shows the frequency characteristics of noise when the light receiver has no diode, the upper right column shows the frequency characteristics of output signal gain when the light receiver has a diode, and the lower right column shows the frequency characteristics of noise when the light receiver has a diode.  
         [0034]     When the light receiver has the diode  2 , regarding the frequency characteristics of noise, a peak in the high-frequency area decreases from 5.2 dB to 1.2 dB compared with when it has no diode. The noise characteristics are thus significantly improved. Further, regarding the frequency characteristics of output signal gain, a peak in the high-frequency area decreases, and a cutoff frequency is also improved from 147 MHz to 150 MHz. This is because the diode  2  becomes high-impedance and the affect of the second amplifier  3  in the non-operating state as a load is eliminated. Since the intensity level of the noise is smaller than the output signal, the effect of peak improvement is more significant.  
         [0035]      FIG. 6  shows results of simulation of the output characteristics of the light receiver  11  to the output circuit  7  under the condition that the first amplifier  4  is in the operating state and the second amplifier  3  is in the non-operating state in comparison with the output characteristics of the light receiver  31  without the diode  2  under the same condition. The circuit configuration and the simulation conditions are the same regardless of the presence/absence of the diode.  
         [0036]     The upper left column in  FIG. 6  shows the frequency characteristics of output signal gain when the light receiver has no diode, the lower left column shows the frequency characteristics of noise when the light receiver has no diode, the upper right column shows the frequency characteristics of output signal gain when the light receiver has a diode, and the lower right column shows the frequency characteristics of noise when the light receiver has a diode. When the first amplifier  4  is in the operating state and the second amplifier  3  is in the non-operating state, a current flows to the constant current supply  9  and a current flows to the diode  2 , so that the diode  2  is low-impedance. Accordingly, substantially the same characteristics as when the circuit does not have the diode  2  are maintained.  
       Second Embodiment  
       [0037]     A second embodiment describes a case where a diode is disposed for each amplifier.  FIG. 4  is a circuit diagram of a light receiver  41  in such a case. A detailed circuit configuration is the same as shown in  FIG. 5  except that the photodiode  1  and the second amplifier  3  are connected through a diode  13 .  
         [0038]     In this example, the first amplifier  4  is used for processing DVD-R, and the second amplifier  3  is used for processing BD-ROM. When processing BD-ROM, reflected light from the optical disk is still lower than when processing DVD-RAM, and the input optical signal intensity is smaller. Accordingly, it is necessary in this case to reduce the affect of the load of the amplifier in the non-operating state which causes an increase in noise during the operation of both amplifiers.  
         [0039]     In this embodiment, low noise output can be obtained when either of the first amplifier  4  and the second amplifier  3  is in the operating state, enabling obtainment of suitable characteristics for the above usages.  
         [0040]     Although the cathode of the photodiode is connected to the input of the amplifier in the first and second embodiments as described above, the present invention may be applied to the case of using an amplifier with inverted polarity and connecting to the anode of the photodiode. The connection of the diode is also inverted in such a case.  
         [0041]     Further, the element connected between the photodiode and the amplifier is not necessarily the diode but may be an element with emitter-base short-circuit or base-collector short-circuit.  
         [0042]     The present invention may be applied to a light receiver of an optical pickup which is used in a disk drive such as a personal computer, a DC/DVD/BD player, and so on.  
         [0043]     It is apparent that the present invention is not limited to the above embodiment and it may be modified and changed without departing from the scope and spirit of the invention.