Optical: systems and elements – Optical amplifier
Reexamination Certificate
2002-07-10
2004-03-23
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
C250S2140AG
Reexamination Certificate
active
06710915
ABSTRACT:
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 (cond
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