Tracking error balance adjustment circuit and a current...

Details Tracking error balance adjustment circuit and a current... Tracking error balance adjustment circuit and a current...

2002-11-25

2004-01-13

Hindi, Nabil (Department: 2655)

Dynamic information storage or retrieval

Control of storage or retrieval operation by a control...

Control of information signal processing channel

C369S059150, C369S124100

Reexamination Certificate

active

06678224

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a tracking error signal generation circuit for an optical disk playback apparatus for playing back an optical disk and, more particularly, to a tracking error balance adjustment circuit. The present invention is applied to an audio compact disk (CD) player, the CD-ROM playback apparatus of a computer system, and the like.
The present invention also relates to a current control circuit, variable gain amplifier circuit using the same, and optical disk playback apparatus and, more particularly, to a current control circuit for supplying a control current to a gm amplifier whose transconductance gm is changed by controlling the collector current of a differential amplifier transistor. The present invention is applied to an audio compact disk (CD) player, the CD-ROM drive of a computer system, and the like.
Generally, in tracking control in a CD-ROM playback apparatus, the positional shift between a track on a rotating disk and the irradiation position of a laser beam emitted by an optical pickup is detected to control the position of the optical pickup so as to accurately irradiate the track with the laser beam.
FIGS. 1A
to
1
C show different examples of the irradiation positions of laser beams (main beam M and two sub-beams E and F) on a track T of a disk.
The irradiation positions of the two sub-beams E and F are set to slightly shift from each other in opposite directions along the track width on the two sides of the irradiation position of the main beam M along the track.
As shown in
FIG. 1D
, the main beam M is photoelectrically converted by four photodiodes A, B, C, and D in correspondence with four divided regions of the main beam M. Outputs from the four photodiodes are input to a head amplifier where the outputs are added.
Each of the two sub-beams E and F is photoelectrically converted by one photodiode. An output (E or F signal) from the photodiode is input to the tracking error signal generation circuit of the head amplifier.
This tracking error signal generation circuit removes the high-frequency components of the E and F signals, differentially adds the resultant E and F signals, and outputs the sum as a tracking error signal. Tracking servo control reduces the tracking error signal to 0.
FIG. 2
shows a conventional tracking error signal generation circuit.
A first current-to-voltage conversion circuit
71
receives an output (RF signal) from a first photodiode arranged in correspondence with one (first sub-beam E) of the two sub-beams, converts a current to a voltage, and removes a high-frequency component.
A second current-to-voltage conversion circuit
72
receives an output (RF signal) from a second photodiode arranged in correspondence with the other one (second sub-beam F) of the two sub-beams, converts a current to a voltage, and removes a high-frequency component.
Output voltages from the two current-to-voltage conversion circuits
71
and
72
are adjusted by a tracking error balance adjustment circuit
80
, and differentially input to an adder circuit
73
via resistive elements R
2
. An output from the adder circuit
73
is input to a gain adjustment circuit
74
via a resistive element R
3
.
In the adder circuit
73
, a feedback resistive element Rf and a capacitive element Cf for passing a high-frequency component are parallel-connected between the (−) input terminal and output terminal of an operational amplifier circuit OA
3
. A resistive element Ro and a capacitive element Co for removing a high-frequency component are parallel-connected between a reference potential and the (+) input terminal of the operational amplifier circuit OA
3
.
In the gain adjustment circuit
74
, a feedback resistor Ru adjustable by the user is connected (e.g., externally connected to an LSI) between the (−) input terminal and output terminal of an operational amplifier circuit OA
4
. A resistive element R
4
is connected between the reference potential and the (+) input terminal of the operational amplifier circuit OA
4
.
If a tracking error signal TE output from the gain adjustment circuit
74
is 0, the two sub-beams E and F are at ideal irradiation positions; and if the tracking error signal TE shifts in a (+) or (−) direction, the two sub-beams E and F shift from ideal irradiation positions to one side along the track width.
The role of the tracking error balance adjustment circuit
80
will be explained.
Even if the irradiation position of a laser beam on the track of a disk is accurate as in a case wherein tracking control is accurately done, characteristics may vary between the first system including the first sub-beam E and corresponding first photodiode and the second system including the second sub-beam F and corresponding second photodiode. In this case, an output (E signal) from the first photodiode and an output (F signal) from the second photodiode exhibit different amplitudes. To correct such variations, tracking error balance adjustment must be performed. Thus, output voltages OE and OF from the two current-to-voltage conversion circuits
71
and
72
are adjusted by the tracking error balance adjustment circuit
80
, and then differentially added by the adder circuit
73
. As a control signal input to the adjustment circuit
80
, a DC tracking error balance adjustment voltage TEB generated by another means is input.
The tracking error balance adjustment circuit
80
comprises two current-controlled variable gain control circuits in correspondence with the voltage OE prepared by converting the E signal by the first current-to-voltage conversion circuit
71
, and the voltage OF prepared by converting the F signal by the second current-to-voltage conversion circuit
72
.
The gains of the two variable gain control circuits are controlled by an output current (control current) prepared by converting an input tracking error balance adjustment voltage TEB into a current by a current control circuit
90
. In general, the two circuits are differentially controlled to increase the gain of one circuit and decrease the gain of the other circuit.
The conventional tracking error balance adjustment circuit
80
shown in
FIG. 2
will be explained.
The input voltages OE and OF of these circuits are respectively input to current-controlled variable gain control circuits
82
via attenuation circuits
81
. The output side of each variable gain control circuit
82
is connected to an operational amplifier circuit
83
having an RNF (feedback resistive element) connected between the input and output terminals. These circuits
81
to
83
are formed from bipolar transistors.
The attenuation circuit
81
is made up of an input resistor R
1
connected between a signal input node and an attenuation output node, and a first gm amplifier A
1
of constant gm type having an equivalent resistor between the attenuation output node and an AC ground node.
The current-controlled variable gain control circuit
82
uses a second gm amplifier A
2
of variable gm type in which gm changes in accordance with a control current from the current control circuit
90
. By connecting the first gm amplifier A
1
to the second gm amplifier A
2
, as described above, gm variations including the temperature coefficient of the second gm amplifier A
2
are cancelled.
To perform tracking error balance adjustment using the tracking error balance adjustment circuit
80
shown in
FIG. 2
, the tracking error balance adjustment voltage TEB is changed to change an output current (control current) from the current control circuit
90
. In accordance with this control current, the operating current of the second gm amplifier A
2
is changed (e.g., 50 to 150 &mgr;A). At this time, the two circuits are differentially controlled to increase gm of the second gm amplifier A
2
in one circuit and decrease gm of the second gm amplifier A
2
in the other circuit. Accordingly, the tracking error signal is controlled to 0.
In this case, the control current to one circuit is increased. Then, the operating current of the second gm

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