JP4706683B2 - Amplifier circuit and optical pickup provided with the same - Google Patents

Description

  The present invention relates to an amplifier circuit, and more particularly to an amplifier circuit suitable for use in amplification of an output signal of a photodiode. The present invention also relates to an optical pickup provided with such an amplifier circuit.

  An optical recording / reproducing apparatus that records and reproduces data on an optical disc such as a CD, DVD, or Blu-ray disc includes an optical pickup. The optical pickup includes a laser diode that is a light source, a photodiode that receives a laser beam reflected by an optical disk, and the like. The photodiode is an element that converts a received laser beam into an electric signal, and information recorded on the optical disk can be read by referring to the output signal.

However, since the output signal of the photodiode is very weak, an amplifier circuit is required to output it to the outside. Therefore, such an amplifier circuit (light receiving amplifier) is also mounted on the optical pickup. Patent Document 1 shows an example of such an amplifier circuit.
JP 2000-332546 A

  By the way, the cutoff frequency of the amplifier circuit varies depending on the capacitance value of the capacitive load connected to the output terminal. A graph G1 in FIG. 8 is a graph showing this variation. Such fluctuations impose restrictions on circuit design and are undesirable.

  Accordingly, an object of the present invention is to provide an amplifier circuit that can suppress a variation in cutoff frequency due to a capacitance value of a capacitive load connected to an output terminal.

  Another object of the present invention is to provide an improved amplifier circuit for a photodiode using MOS transistors.

  Another object of the present invention is to provide an optical pickup provided with such an amplifier circuit.

  In order to achieve the above object, an amplifier circuit according to the present invention includes an amplifying unit, an output terminal that outputs an output signal from the amplifying unit to the outside, and a load capacitance that controls a capacitance of a load connected to the output terminal And control means.

  According to the present invention, since the load capacity control means is provided in the amplifier circuit, fluctuations in the cutoff frequency due to the capacitance value of the capacitive load connected to the output terminal can be suppressed.

  Further, in the amplifier circuit, the load capacity control means includes one or a plurality of load capacity controllers having one end connected between the amplifier and the output terminal and the other end grounded. The load capacity controller may have a configuration in which a capacitor and a switch are connected in series. According to this, the load capacity can be controlled by the on / off control of the switch.

  In each of the amplifier circuits, the input signal of the amplifier circuit may be an output signal of a photodiode. According to this, in the amplifier circuit that amplifies the output signal of the photodiode, it is possible to suppress variation in the cutoff frequency due to the capacitance value of the capacitive load connected to the output terminal.

  The optical pickup according to the present invention is an optical pickup including a photodiode that receives a laser beam and an amplification circuit that amplifies an output signal of the photodiode, and the amplification circuit includes an amplification unit and the amplification unit. An output terminal for outputting an output signal from the unit to the outside, and load capacity control means for controlling the capacity of a load connected to the output terminal. According to this, in the amplifier circuit in the optical pickup, it is possible to suppress variation in the cutoff frequency due to the capacitance value of the capacitive load connected to the output end.

  As described above, according to the present invention, it is possible to suppress the variation in the cutoff frequency of the amplifier circuit due to the capacitance value of the capacitive load connected to the output terminal of the amplifier circuit.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Embodiment 1]
FIG. 1 is a circuit diagram showing a basic configuration of an amplifier circuit 1 and a photodiode 2 according to Embodiment 1 of the present invention.

  As shown in FIG. 1, the amplifier circuit 1 has a configuration in which a photodiode 2 is connected to an inverting input terminal (−) of an operational amplifier 10 and a reference voltage source 14 is connected to a non-inverting input terminal (+) of the operational amplifier 10. Have. The amplifier circuit 1 also includes a feedback circuit that connects the output terminal of the operational amplifier 10 and the inverting input terminal (−). This feedback circuit includes a resistor 16. With these configurations, the operational amplifier 10 and the feedback circuit constitute an amplification unit, and this amplification unit constitutes a so-called inverting amplification circuit.

  The amplifying circuit 1 further includes an output terminal OT that outputs an output signal from the amplifying unit to the outside, and load capacity control means 12a that controls the capacity of a load connected to the output terminal OT. Details of these will be described later.

  On the other hand, the photodiode 2 is an element that receives a laser beam reflected by the optical disc and performs photoelectric conversion. In one example, the photodiode 2 includes a PN diode, a reverse bias power source, and a resistor, and the voltage across the resistor varies when the PN diode hits the laser beam. This voltage fluctuates up and down with a predetermined voltage value (bias voltage) as a reference and becomes the output Vin of the photodiode 2. Hereinafter, this bias voltage is assumed to be 1.65V.

  The amplifier circuit 1 amplifies and outputs the output Vin of the photodiode 2. As a result, an output corresponding to the intensity of the received laser beam is obtained as the output Vout of the amplifier circuit 1.

  FIG. 2 is a circuit diagram showing the amplifier circuit 1 according to the present embodiment in more detail.

  As shown in FIG. 2, the operational amplifier 10 includes a differential amplifier circuit 20 and a source follower circuit 30, and these are connected in cascade. That is, the output Vin of the photodiode 2 is first supplied to the differential amplifier circuit 20, and the output of the differential amplifier circuit 20 is further supplied to the source follower circuit 30.

  The differential amplifier circuit 20 is connected to P-channel MOS transistors 21 and 22 connected in a current mirror, N-channel MOS transistors 23 and 24 connected in series to the transistors 21 and 22, respectively, and sources of the transistors 23 and 24. The constant current source 25 is used. The gate of the transistor 23 is connected to the photodiode 2, and the gate of the transistor 24 is connected to the reference voltage source 14. The reference voltage source 14 generates a voltage (here, 1.65 V) having the same value as the bias voltage of the photodiode 2. The output of the differential amplifier circuit 20 is taken out from the connection point between the transistor 21 and the transistor 23.

  The source follower circuit 30 includes an N-channel MOS transistor 31 to which the output of the differential amplifier circuit 20 is supplied to the gate, and a constant current source 32 connected to the source of the transistor 31. The output of the source follower circuit 30 is extracted from the source of the transistor 31 and becomes the output Vout of the amplifier circuit 1. The transistor 31 is a so-called depletion type MOS transistor. That is, the threshold voltage Vth of the transistor 31 is zero or a negative value.

  Note that a power supply voltage Vdd is supplied to the sources of the transistors 21 and 22 and the drain of the transistor 31.

  Here, the voltage amplification degree of the amplifier circuit 1 is appropriately set by adjusting the resistance value of the resistor 16 and the internal resistance value of the photodiode 2. However, when the output voltage of the photodiode 2 is 1.65 V as the bias voltage, the output voltage of the source follower circuit 30 (source voltage of the transistor 31) is about 1.65 V regardless of the voltage amplification degree. .

  When the threshold voltage of the transistor 31 is Vth, the gate voltage of the transistor 31 is given by Vout + Vth. In this embodiment, since the transistor 31 is a depletion type, Vth ≦ 0. Therefore, the gate voltage of the transistor 31 is substantially the same as or lower than the source voltage. For example, if the threshold voltage Vth is approximately 0V, the gate voltage of the transistor 31 is substantially equal to the voltage of the output Vout. As a result, for example, if the voltage of the output Vout is about 1.65V, the gate voltage of the transistor 31 is also about 1.65V.

  As a result, the source-drain voltage of the transistors 21 and 22 constituting the current mirror circuit is increased compared to the conventional case. That is, when an enhancement type transistor is used as the transistor 31, the gate voltage of the transistor 31 becomes higher than the voltage of the output Vout by the threshold voltage, and as a result, the source-drain voltage of the transistors 21 and 22 is lowered, resulting in a dynamic range. Becomes narrower. For this reason, in order to ensure a sufficient dynamic range, it is necessary to increase the power supply voltage Vdd as described above.

  On the other hand, in the present embodiment, since a depletion type transistor is used as the transistor 31, a sufficient dynamic range can be secured even if the power supply voltage Vdd is relatively low. For example, when the power supply voltage Vdd is 3.3 V, the maximum amplitude of the source-drain voltage of the transistors 21 and 22 (the maximum amplitude of the signal that can be output by the differential amplifier circuit 20) is about 1.65 V. (= 3.3V-1.65V (voltage value of the bias voltage of the photodiode 2)), and sufficient amplitude is ensured. In this case, the upper limit value of the dynamic range of the amplifier circuit 1 is 3.3V.

  Now, the load capacity control means 12a will be described here. The load capacity control means 12a is composed of one or a plurality of load capacity controllers, one end of which is connected between the amplifier (the operational amplifier 10 + feedback circuit) and the output terminal OT, and the other end is grounded. Each load capacity controller has a configuration in which a capacitor 71 and a switch 72 are connected in series in this order from the ground end side. FIG. 2 shows an example of load capacity control means 12a provided with two load capacity controllers. That is, the load capacity control means 12a shown in FIG. 2 includes a load capacity controller composed of a capacitor 71-1 and a switch 72-1, and a load capacity controller composed of a capacitor 71-2 and a switch 72-2. And.

The switch 72 is controlled by an external signal. When the switch 72 is turned on, the corresponding capacitor 71 forms a parallel circuit with the capacitive load LC connected to the output terminal of the amplifier circuit 1. Therefore, for example, when the capacitance of the capacitor 71-1 is C1, the capacitance of the capacitor 71-2 is C2, and the capacitance value of the capacitive load LC is CL, as shown in Table 1 along with the on / off of the switches 72-1 and 72-2. The load capacity as a whole fluctuates.

  This is because the capacity value of the capacitive load LC connected to the output terminal (output terminal OT) is controlled by the switch 72 if an appropriate number of load capacity controllers having an appropriate capacity capacitor 71 are prepared in advance. This means that the load capacity of the amplifier circuit 1 can be maintained at a constant value regardless of the case. That is, in the amplifier circuit 1, by using the load capacity control means 12a, it is possible to suppress variation in the cutoff frequency due to the capacity value of the capacitive load connected to the output terminal.

  The solid line graph G2 in FIG. 3 is a graph showing the relationship between the cutoff frequency of the amplifier circuit 1 including the load capacity control means 12a and the capacitance value of the capacitive load. The dotted line graph G1 in the figure is the same as that shown in FIG. FIG. 3 shows an example of the amplifier circuit 1 including a large number of load capacity controllers instead of two. As shown in the graph G2, the cutoff frequency of the amplifier circuit 1 including the load capacity control unit 12a is constant regardless of the capacitance value of the capacitive load connected to the output terminal.

  In the first embodiment, an example of an amplifier circuit using a two-stage operational amplifier in which the first stage is a differential amplifier circuit and the next stage is a source follower circuit has been described. However, the present invention provides a single-stage operational amplifier. The present invention can also be applied to other types of amplifier circuits such as an amplifier circuit used.

[Embodiment 2]
The amplifier circuit 1 described in the first embodiment can be integrated on the same chip as the photodiode 2, and such a chip is usually called a photodiode IC (PDIC). In the present embodiment, a specific example of the circuit configuration of an optical recording / reproducing apparatus using this PDIC will be given, and further, the configuration of an optical pickup used in the optical recording / reproducing apparatus will be described.

  First, FIG. 4 is a diagram showing an example of the appearance of the PDIC 100 used in an optical recording / reproducing apparatus that performs recording / reproducing of an optical disc. A PDIC 100 shown in the figure is used for a CD (Compact Disc) / DVD / BD (Blu-ray Disc (registered trademark)) compatible recorder, among other optical recording / reproducing apparatuses, and includes 20 photodiodes (A, B, C, D, E1, E2, E3, E4, F1, F2, F3, F4, a, b, c, d, e1, e2, f1, f2). Each photodiode is disposed in any one of the light receiving portions R-1 to R-6.

  The light receiving parts R-1 to R-3 are used for BD / DVD recording / reproduction. The light receiving unit R-1 includes four photodiodes A, B, C, and D, and receives the main beam MB reflected by the BD or DVD. The light receiving unit R-2 includes four photodiodes E1, E2, E3, and E4, and receives the sub beam SB1 reflected from the BD or DVD. The light receiving unit R-3 includes four photodiodes F1, F2, F3, and F4, and receives the sub beam SB2 reflected by the BD or the DVD.

  The light receiving parts R-4 to 6 are used for CD recording / reproduction. The light receiving unit R-4 is composed of four photodiodes a, b, c, and d, and receives the main beam MB reflected by the CD. The light receiving unit R-5 includes two photodiodes e1 and e2, and receives the sub beam SB1 reflected by the CD. The light receiving unit R-6 includes two photodiodes f1 and f2, and receives the sub beam SB2 reflected by the CD.

  5 and 6 are diagrams showing the internal circuit configuration of the PDIC 100. FIG. 5 and 6 show the internal circuit configuration of one PDOC 100 in the two diagrams, and the lower part of FIG. 5 and the upper part of FIG. 6 are actually connected. Since there are many parts that are similar to each other in the circuit configuration of each photodiode, the following description will be made by paying attention to the photodiode A and the photodiode a.

  A circuit 40 shown in FIG. 5 includes an operational amplifier 10, a resistor 16, a switch 41, a limiter circuit 42, a switch 43, a resistor 44, and a load capacitance control unit 12a in addition to the photodiode A and the photodiode a. The operational amplifier 10, the resistor 16, and the load capacitance control unit 12 a are the same as those described in the first embodiment, and constitute the amplifier circuit 1 according to the first embodiment. However, in the present embodiment, a limiter circuit 42 is inserted between the operational amplifier 10 and the load capacitance control means 12a. Further, the synthesis circuit 60 is connected between the limiter circuit 42 and the load capacity control means 12a.

  The inverting input terminal (−) of the operational amplifier 10 is connected to the photodiode A when the recording / reproduction target medium is BD or DVD by the operation of the switch 41 according to the control from the outside. Is connected to the photodiode a. On the other hand, a power supply Vs having a predetermined voltage is connected to the non-inverting input terminal (+) of the operational amplifier 10. The amplifier circuit 1 including the operational amplifier 10 and the resistor 16 performs an amplification process on the output signal of the photodiode.

  The gain control unit 50 controls the switch 43 according to an instruction from the outside. As a result, it is possible to incorporate the resistor 44 into the feedback circuit and thereby control the gain of the amplifier circuit 1.

  When the amplitude of the signal output from the operational amplifier 10 exceeds a predetermined maximum value, the limiter circuit 42 clips the excess amplitude and outputs it to the subsequent stage.

  The output signal of the limiter circuit 42 is output from the terminals VA / Va and also input to the synthesis circuit 60. An output signal from the terminal VA / Va is used as a servo signal for detecting defocusing of the light spot or deviation from the track in a control circuit (not shown). In the present embodiment, the load capacity control means 12a is provided to suppress fluctuations in the cutoff frequency of the amplifier circuit 1 due to the capacitance value of the capacitive load connected to the terminals VA / Va. .

  In addition to the output signal of the limiter circuit 42, the output signal of the limiter circuit is similarly input to the combining circuit 60 from the photodiodes B, C, and D or the photodiodes b, c, and d. These output signals are combined and output from the terminal VRFP and the terminal VRFN. Note that the output signals of the terminal VRFP and the terminal VRFN are out of phase with each other. The signal output in this way is used as a data signal indicating recorded data in a control circuit (not shown).

  As shown in FIG. 5, the load capacity control means 12a is provided not only at the front stage of the servo signal output terminals (terminal VA / Va etc.) but also at the data signal output terminals (terminal VRFP and terminal VRFN). It is also provided in the previous stage. These load capacity control means 12a are provided in order to suppress fluctuations in the cutoff frequency of the amplifier circuit in the preceding stage due to the capacitance value of the capacitive load connected to the terminal VRFP or the terminal VRFN.

  Next, FIG. 7 is a schematic diagram illustrating a configuration of an optical pickup 101 including the PDIC 100.

  As shown in FIG. 7, the optical pickup 101 includes a laser light source 102, a diffraction grating 103 that divides the laser beam from the laser light source 102 into a plurality, and a collimator lens that collimates the laser beam emitted from the diffraction grating 103. 104, a mirror 105 that guides the parallel laser beam to the optical disc 200, a quarter-wave plate 110 that converts the laser beam reflected by the mirror 105 into circularly polarized light and enters the objective lens 106, and 1 An objective lens 106 for converging the laser beam incident from the quarter-wave plate 110 onto the disk surface, a beam splitter 107 for guiding the light reflected by the optical disk 200 and further reflected by the mirror 105 to the PDIC 100 side, and from the beam splitter 107 An anamorphic lens 108 for converging the reflected light; And a PDIC100 for receiving the reflected light is converged by Namo Fick lens 108. The PDIC 100 includes 20 photodiodes as described above, and each photodiode receives the reflected light, performs photoelectric conversion, and outputs a voltage signal corresponding to the intensity of the reflected light.

  Note that the position of the objective lens 106 with respect to the optical disc 200 is controlled with high accuracy by the objective lens driving device 109. Specifically, by driving the objective lens 106 in the focus direction, focus correction is performed to focus the beam spot on the recording surface of the optical disc 200, and by driving in the tracking direction, the beam spot is applied to the track of the optical disc 200. Tracking correction to follow is performed. Further, the tilt angle corresponding to the warp of the disk is corrected by rotating the objective lens 106 in the tracking direction with the tangential direction as the rotation axis.

  The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Needless to say, it is included in the range.

  For example, in the above embodiment, the case where the amplifier circuit according to the present invention is applied to a PDIC has been described as an example, but the scope of the present invention is not limited to this.

  Further, the configuration of the differential amplifier circuit is not limited to the configuration shown in FIG. 2, and may have a circuit configuration different from this.

1 is a circuit diagram showing a basic configuration of an amplifier circuit according to a first embodiment of the present invention. 1 is a circuit diagram showing in more detail an amplifier circuit according to a first embodiment of the present invention. FIG. It is the figure which graphed the relationship between the cutoff frequency of the amplifier circuit concerning Embodiment 1 of this invention, and the capacitance value of a capacitive load. It is a figure which shows the example of an external appearance of PDIC concerning Embodiment 2 of this invention. It is a figure which shows the internal circuit structure of PDIC concerning Embodiment 2 of this invention. It is a figure which shows the internal circuit structure of PDIC concerning Embodiment 2 of this invention. It is a schematic diagram which shows the structure of an optical pick-up provided with PDIC concerning Embodiment 2 of this invention. It is a figure which shows the relationship between the cutoff frequency of an amplifier circuit, and the capacitance value of the capacitive load connected to the output terminal.

Explanation of symbols

1 amplifying circuit 2, A, B, C, D, a, b, c, d, E1, E2, E3, E4, F1, F2, F3, F4, e1, e2, f1, f2 photodiode 10 operational amplifier 12a, 12b, 12c Load capacity control means 14 Reference voltage source 16, 44 Resistor 20 Differential amplifier circuit 21, 22 P channel MOS transistor 23, 24 N channel MOS transistor 31 Depletion type transistor 25, 32 Constant current source 30 Source follower circuit 40 circuit 41, 43 switch 42 limiter circuit 50 gain controller 60 combining circuit 71 the capacitor 72 switch 100 PDIC
DESCRIPTION OF SYMBOLS 101 Optical pick-up 102 Laser light source 103 Diffraction grating 104 Collimating lens 105 Mirror 106 Objective lens 107 Beam splitter 108 Anamorphic lens 109 Objective lens drive device 110 1/4 wavelength plate 200 Optical disk LC Capacitive load OT Output terminal

Claims (5)

An amplification unit;
An output terminal for outputting an output signal from the amplifying portion to the outside,
Via the output terminal, and a wiring connecting the output terminal of the amplifying section to a load,
And a load capacity control means,
The load capacity control means is composed of a plurality of load capacity controllers, one end of which is connected to the portion of the wiring between the amplifier and the output terminal, and the other end of which is grounded.
Each load capacitance controller has a configuration in which the capacitor and the switch are connected in series,
Wherein each capacitor and the switch are provided for each of the load capacitance controller,
By the on / off control of each of the plurality of switches constituting each load capacity controller, among the plurality of capacitors constituting each load capacity controller, the capacity and the load of the corresponding switch being on By controlling the load capacitance of the amplifier circuit, which is a combined capacitance with the capacitance of the capacitor, fluctuations in the cutoff frequency of the amplification circuit due to fluctuations in the capacitance value of the load are suppressed.
Amplifier circuit according to claim and this. The plurality of switches are on / off controlled so that the cut-off frequency is constant regardless of the capacitance value of the load.
The amplifier circuit according to claim 1. Each of the load capacity controllers has a configuration in which the capacitor and the switch are connected in series in this order from the ground end side.
The amplifier circuit according to claim 1 or 2. Amplifier circuit according to any one of claims 1 to 3 input signal of the amplifier circuit is characterized in that the output signal of the photodiode. An optical pickup comprising a photodiode for receiving a laser beam and an amplification circuit for amplifying an output signal of the photodiode,
The amplifier circuit is
An amplification unit;
An output terminal for outputting an output signal from the amplifying portion to the outside,
Via the output terminal, and a wiring connecting the output terminal of the amplifying section to a load,
And a load capacity control means,
The load capacity control means is composed of a plurality of load capacity controllers, one end of which is connected to the portion of the wiring between the amplifier and the output terminal, and the other end of which is grounded.
Each load capacitance controller has a configuration in which the capacitor and the switch are connected in series,
Wherein each capacitor and the switch are provided for each of the load capacitance controller,
By the on / off control of each of the plurality of switches constituting each load capacity controller, among the plurality of capacitors constituting each load capacity controller, the capacity and the load of the corresponding switch being on By controlling the load capacitance of the amplifier circuit, which is a combined capacitance with the capacitance of the capacitor, fluctuations in the cutoff frequency of the amplification circuit due to fluctuations in the capacitance value of the load are suppressed.
The optical pick-up which is characterized a call.

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