Signal amplifying circuit for magnetoresistive element

Details Signal amplifying circuit for magnetoresistive element Signal amplifying circuit for magnetoresistive element

1999-12-15

2001-04-03

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C360S067000

Reexamination Certificate

active

06211736

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a signal amplifying circuit for a magnetoresistive element, as an element for a head of a magnetic recording medium such as a hard disk and a floppy disk, used in a hard disk drive (HDD), a floppy disk drive (FDD) or the like. More specifically this invention relates to a signal amplifying circuit for a magnetoresistive element capable of being integrated on one semiconductor chip.
BACKGROUND OF THE INVENTION
Recently, as an element for a head of a magnetic recording medium used in a hard disk drive, floppy disk drive or the like, a magnetoresistive element (hereinafter, referred to as “MR (magnetoresistive) element”) has been used widely. In the head using the MR element (hereinafter, referred to as “MR head”), since a reproduction output is stronger than that of a conventional head using a thin film element, the surface recording density of the magnetic recording medium can be improved greatly. Here, in the following description, the MR element means an element which shows a magnetoresistive effect where resistance changes due to application of an external magnetic field. The MR element includes a GMR (giant magnetoresistive) element or a TMR (tunneling magnetoresistive) element, for example.
FIG. 9
is a circuit diagram showing a conventional signal amplifying circuit of the MR element. The signal amplifying circuit shown in
FIG. 9
functions as an output detection circuit of the MR element, namely, a read amplifying circuit. In
FIG. 9
, both terminals T
1
and T
2
of the MR element MR are connected respectively with input terminals in
1
and in
2
of a differential amplifying circuit DA
1
. The terminal T
1
of the MR element MR is connected with a resistance R
11
in series, and the terminal T
2
is connected with an electric current source CS
1
again in series. The constant electric current source CS
1
discharges a bias electric current Ib from a power source line Vcc at high potential to a power source line Vee at low potential. Therefore, the bias electric current Ib flows in the MR element MR, and thus the MR element MR generates an electric potential difference which is in proportion to a difference in resistance at the terminals T
1
and T
2
.
The differential amplifying circuit DA
1
has transistors TR
1
and TR
2
which compose a differential pair. The collectors of these transistors TR
1
and TR
2
are connected with collector resistances RC
1
and RC
2
having a same resistance value Rc. These resistances RC
1
and RC
2
are connected with the power source line Vcc. Moreover, emitters of the transistors TR
1
and TR
2
are connected with each other through a capacitor C
1
, and the emitters of the transistors TR
1
and TR
2
are connected respectively with electric current sources CS
2
and CS
2
′ in series. The electric current sources CS
2
and CS
2
′ are connected with the power source line Vee. Bases of the transistors TR
1
and TR
2
function as input terminals in
1
and in
2
. Collector terminals of the transistors TR
1
and TR
2
are also connected respectively with output terminals out
1
and out
2
. The emitters of the transistors TR
1
and TR
2
composing the differential pair are connected with each other by the capacitor C
1
in order to cancel a DC potential difference between the terminals T
1
and T
2
of the MR element MR to be inputted into the differential amplifying circuit DA
1
.
In this signal amplifying circuit, the resistance value of the MR element MR in which the bias electric current Ib flows changes according to a magnetic signal from the outside, and thus a potential difference between the terminals T
1
and T
2
of the MR element MR changes Only an AC portion of the changed potential difference is amplified by the differential amplifying circuit DA
1
so as to be outputted as a potential difference between the output terminals out
1
and out
2
, namely, an output voltage.
Incidentally, since an input signal from the MR element MR is a weak input signal of less than 1 mvpp, the differential amplifying circuit DA
1
should physically be provided in a vicinity of the MR element MR, and hence it is usual to form an integrated circuit.
However, a capacitance of the capacitor C
1
which is realized on one semiconductor chip is maximum about several nF.
Since a cut-off frequency f of a low frequency becomes high, i.e., several tens MHz in such a capacitor C
1
having such a small capacitance, a capacitance of an external capacitor should be used. As a result, integration of the signal amplifying circuit of the MR element is hindered, and thus promotion of miniaturization and light weight is prevented.
The cut-off frequency f will be described below concretely with reference to FIG.
10
. When a base electric current is ignored, a gain Av of the differential amplifying circuit DA
1
shown in
FIG. 10
becomes:
Av=Rc/re=Rc·Ie/V
T
.
Here, “re” is an emitter resistance of the transistors TR
1
and TR
2
, and “V
T
” is thermal voltage defined as follows:
V
T
=kT/q~
26 mV at 300 K.
where,
q=electric charge,
k=Boltzman's constant,
T=temperature (in K).
In
FIG. 9
, since the capacitor C
1
is connected with the emitter resistances of the transistors TR
1
and TR
2
in series, when electric currents which flow in the constant electric current sources CS
2
and CS
2
′ are Ie as shown in
FIG. 9
,
Av=Rc/
(
re+
1/2
j&ohgr;C
)=
Rc/
(
V
T
/Ie+
1/2
j&ohgr;C
).
Here, “C” is a capacitance of the capacitor C
1
. Accordingly, the cut-off frequency f becomes:
f=Ie/
(4&pgr;
V
T
·C
).
When Ie=10 mA and C=5 nF, the cut-off frequency f becomes:
f=
10 mA/(4×3.14×26 mV×5 nF)=6.1 MHz,
and thus it can be understood that the cut-off frequency f is high.
SUMMARY OF THE INVENTION
The present invention has been achieved with such points in view. It therefore is an object of the present invention to provide a signal amplifying circuit for a magnetoresistive element which can be integrated on one semiconductor chip.
In the signal amplifying circuit according to the present invention, a first input terminal of a differential amplifying circuit is directly connected with a first terminal of a magnetoresistive element by a connection line, and a second input terminal of the differential amplifying circuit is connected with a second terminal of the magnetoresistive element by a first capacitor. An AC component corresponding to a change in the resistance, which is generated at both the terminals of the magnetoresistive element by the change of the external magnetic field, is outputted differentially by the differential amplifying circuit. Further, a DC offset, which is generated between the first input terminal and the second input terminal of the differential amplifying circuit, is canceled by a DC offset cancel circuit, and a cut-off frequency of a high-pass filter, which is formed by the first capacitor and the impedance of the DC offset cancel circuit, is determined.
Further, the DC offset cancel circuit is composed by a voltage follower. The first input terminal and the second input terminal of the differential amplifying circuit are connected with a first input terminal and a second input terminal of the voltage follower, and an output is obtained at the second input terminal of the differential amplifying circuit via a first resistance. The DC offset generated between the first input terminal and the second input terminal of the differential amplifying circuit is canceled by the voltage follower, and DC input impedance to the second input terminal of the differential amplifying circuit is set to a desired value by using the first resistance. A cut-off frequency of a high-pass filter, formed by the first capacitor and synthetic impedance of the output impedance of the voltage follower and the first resistance, is determined.
Further, when the input signals from the first terminal and second terminal of the magnetoresistive element are inputted into the first input te

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