Signal amplifying circuit for an MR element

Details Signal amplifying circuit for an MR element Signal amplifying circuit for an MR element

1999-06-10

2001-03-27

Hindi, Nabil (Department: 2651)

Dynamic magnetic information storage or retrieval

General recording or reproducing

Specifics of the amplifier

C360S046000, C360S061000

Reexamination Certificate

active

06208482

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a signal amplifying circuit for an MR element, and more particularly relates to a configuration of a signal amplifying circuit for an MR element for reducing the influence of a DC current offset voltage to the minimum at the time of amplification of signals.
BACKGROUND OF THE INVENTION
In recent years, an MR (magnetoresistive) element has been used as an element for the head of a magnetic recording medium such as a hard disk drive (HDD) and a floppy disk drive FDD). The head that uses an MR element has a larger output for reproduction as compared to a head that uses a conventional thin-film element, therefore, disk recording density of the magnetic recording medium can largely improve.
Herein the MR element according to the present invention means an element that shows a magnetoresistive (MR) effect in which resistance of the element varies when an external magnetic field is applied to the element. Examples of such elements are a GMR (giant magnetoresistive) element or a TMR (tunneling magnetoresistive) element.
FIG. 9
shows configuration of a signal amplifying circuit for an MR element based on the conventional technology.
In
FIG. 9
, designated at the reference numeral
1
a
is a first MR element, at
1
b
a second MR element, at
2
a
and
2
a
′ a pair of first switching circuits, at
2
b
and
2
b
′ another pair of first switching circuits, at
2
c
and
2
c
′ a pair of second switching circuits, at
3
a constant current source, at
4
an amplifier, at V
1
a first potential (a power-supply potential in FIG.
9
), at V
2
a second potential (a ground potential in FIG.
9
), at MR
1
a first terminal of an MR element, at MR
2
a second terminal of the MR element, at OUT
1
a first output terminal of the amplifier
4
, and at OUT
2
a second output terminal of the amplifier
4
, respectively.
Next, operation of the signal amplifying circuit for an MR element shown in
FIG. 9
is described.
The first switching circuits
2
a,
2
a
′ and the other first switching circuits
2
b,
2
b
′ are paired, respectively. Those first switching circuits operate in such a way that, when one of the paired switching circuits (e.g.,
2
a
) is ON the other (e.g.,
2
a
′) is also ON, and when one of the paired switching circuits is OFF the other is also OFF. Further, those switching circuits are controlled by a control circuit not shown in the figure such that when the first switching circuits
2
a,
2
a
′ are ON the other first switching circuits
2
b,
2
b
′ are OFF, and when the first switching circuits
2
a,
2
a
′ are OFF the other first switching circuits
2
b,
2
b
′ are ON. Through the controls provided as described above, a current is supplied to either the first MR element
1
a
or the second MR element
1
b.
Furthermore, the second switching circuits
2
c,
2
c
′ are paired, and controls are provided by the control circuit not shown in the figure such that when the first switching circuits
2
a,
2
a
′ are ON the second switching circuits
2
c,
2
c
′ are connected to respective terminals a, and when the other first switching circuits
2
b,
2
b
′ are ON the second switching circuits
2
c,
2
c
′ are connected to respective terminals b.
Next, description is made the operation when the first MR element
1
a
is selected (when the first switching circuits
2
a,
2
a
′ are ON). In this case, as described above, the other first switching circuits
2
b,
2
b
′ are OFF, and further the second switching circuits
2
c,
2
c
′ are connected to respective terminals a. Therefore, a current is supplied to the first MR element
1
a
from a power supply unit as the first potential V
1
through the first terminal MR
1
of the MR element by the constant current source
3
. The first terminal MR
1
and the second terminal MR
2
of the first MR element
1
a
are connected to respective inputs of the amplifier
4
and, a signal from the first MR element
1
a
is amplified by the amplifier
4
and outputted from the first output terminal OUT
1
as well as the second output terminal OUT
2
of the amplifier
4
.
FIG. 10
shows a configuration using emitter-follower transistor circuits for the components corresponding to the pair of second switching circuits
2
c,
2
c
′ in the conventional signal amplifying circuit for an MR element in FIG.
9
. In
FIG. 10
, the reference numerals are assigned to the same sections corresponding to those in FIG.
9
.
In
FIG. 10
, the first terminal MR
1
of the first MR element
1
a
is connected to the base of a NPN transistor Tr
1
, and the second terminal MR
2
of the MR element is connected to the base of a NPN transistor Tr
2
, and the emitter of the NPN transistor Tr
1
and the emitter of the NPN transistor Tr
2
are connected to respective inputs of the amplifier
4
.
The conventional of signal amplifying circuit for an MR element in
FIG. 10
performs the same operation as that of the circuit in
FIG. 9
, but comprises the emitter-follower transistor circuit for the components corresponding to the second switching circuits
2
c,
2
c
′, thus the circuit operates such that signals of an MR element selected by the first switching circuits
2
a,
2
a
′ or by the other first switching circuits
2
b,
2
b
′ are inputted into the amplifier
4
as they are.
However, the conventional signal amplifying circuit for an MR element described above has drawbacks. Namely, in the conventional signal amplifying circuit for an MR element, signals comprising DC and AC current components obtained from both the terminals of the MR element are directly inputted into the amplifier, so that, when an offset voltage (voltage difference) arises in the signals from the MR element, an imbalance arises at the two inputs of the amplifier. Therefore, the dynamic range becomes narrow or amplification of a weak signal cannot accurately be performed.
SUMMARY OF THE INVENTION
The present invention is made for solving the problems described above, and it is an object of the present invention to provide a signal amplifying circuit for an MR element which can amplify signals with high precision even when there occurs an offset in the voltage of the signals from an MR element.
In the signal amplifying circuit for an MR element according to the present invention, a first terminal of a selected MR element is connected to one input of an amplifier through a first resistor and the second terminal of the selected MR element is connected to the other input of the amplifier through a second resistor, further, the second terminal of the selected MR element is connected to the other input of the amplifier through a capacitor. Thus, when the first MR element
1
a
is biased, the inputs of the amplifier are also biased, further, the DC current component of the MR element is cut and only the AC current component is inputted into the other input of the amplifier.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.


REFERENCES:
patent: 5757566 (1998-05-01), Ngo et al.

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