Magnetoresistive element input circuit

Details Magnetoresistive element input circuit Magnetoresistive element input circuit

2000-07-25

2003-02-18

Faber, Alan T. (Department: 2753)

Dynamic magnetic information storage or retrieval

General recording or reproducing

Specifics of biasing or erasing

C360S067000, C360S046000, C360S061000

Reexamination Certificate

active

06522491

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a magnetoresistive element input circuit, and more particularly, to an input circuit or preamplifier of an integrated circuit for driving a magnetoresistive head.
A magnetic head formed by a magnetoresistive (MR) element is used in magnetic recording devices, such as magnetic tape devices and hard disk devices, to increase the recording density. An input circuit supplies the magnetoresistive element with a bias current to read data signals from a recording medium and amplify the signals.
FIG. 1
is a schematic circuit diagram showing a first example of a prior art MR element input circuit
10
.
The input circuit
10
includes a first resistor
12
connected between an MR element
11
and a high potential power supply (first power supply) V
1
, a second resistor
13
and a current source
14
connected in series between the MR element
11
and a low potential power supply (second power supply) V
2
, and a differential amplifier
15
having two input terminals connected to the two terminals of the MR element
11
. The first and second resistors
12
,
13
have the same resistance. A capacitor
16
is connected to a node N
1
between the second resistor
13
and the current source
14
. The capacitor
16
substantially equalizes the alternating current impedance at the two terminals of the MR element
11
.
In the first example, since the input impedances at the two terminals of the MR element
11
are the same, the two signals respectively provided to the differential amplifier
15
include external noise of the same phase. Accordingly, the S/N ratio of the signal output by the differential amplifier
15
is improved and the affect of the external noises is reduced.
However, in the first prior art example, the current source
14
constantly supplies the MR element
11
with a bias current. Thus, the power is consumed even when the input circuit
10
is inactive.
Accordingly, an MR element input circuit
20
(second prior art example) shown in
FIG. 2
has been proposed to reduce power consumption. Like or same reference numerals are given to those components that are the same as the corresponding components of the first prior art example.
The second example further includes a third resistor
21
, a switch element
22
, and an NPN transistor
23
. The transistor
23
is connected between a first resistor
12
a
and a high potential power supply V
1
. The base of the transistor
23
is connected to the high potential power supply V
1
via the third resistor
21
and to the low potential power supply V
2
via the switch element
22
. The sum of the resistance of the first resistor
12
a
and the ON resistance of the transistor
23
is the same as the resistance of the second resistor
13
.
In the input circuit
20
, the current Im flowing through the MR element
11
is inhibited when a control signal (not shown) closes the switch element
22
and deactivates the current source
14
. This reduces power consumption when the input circuit
20
is inactive.
However, in the second prior art example, when the input circuit
20
switches from an active state to an inactive state (i.e., when the switch element
22
goes on and the current source
14
is deactivated), the charge stored in the capacitor
16
may cause an excessively large amount of current to flow through the MR element
11
.
Further, when the input circuit
20
is active, the charge stored in the capacitor
16
is spontaneously discharged, for example, through the second resistor
13
, the MR element
11
, and the differential amplifier
15
. Therefore, when the input circuit
20
shifts from an inactive state to an active state (i.e., when the switch element
22
goes off thereby activating the current source
14
and recharging the capacitor
16
), a current exceeding a tolerable level may flow through the MR element
11
, as shown in FIG.
3
.
Additionally, in the second prior art example, a stable signal cannot be obtained from the input circuit
20
unless the capacitor
16
is charged. The recharging time of the capacitor
16
(in
FIG. 3
, the time required to stabilize voltage VN
1
(the voltage at node N
1
)) thus determines the time for the input circuit
20
to switch from an active state to an inactive state, or the transition period. Since the required capacitance of the capacitor
16
is relatively large (e.g., 1 &mgr;F) to stabilize operation, the transition period is long.
To solve these problems, an MR element input circuit
30
(third prior art example) shown in
FIG. 4
has been proposed. Switch elements
31
,
32
are respectively connected to the two terminals of the MR element
11
to disconnect the flow of current Im through the MR element
11
. However, the number of elements connected between the MR element
11
and the high potential power supply V
1
and between the MR element
11
and the capacitor
16
is increased in the third embodiment. This makes it difficult to match the alternating current impedance at the two terminals of the MR element
11
. The differential amplifier
15
may thus be affected by external noises. Further, since the impedance at both terminals of the MR element is high when the input circuit
20
is inactive, an undesirable current may flow through the MR element
11
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an input circuit that is not overly affected by external noises, consumes less power when in an inactive state, and has a short transition period for shifting from an inactive state to an active state.
To achieve the above object, the present invention provides a magnetoresistive element input circuit having an active mode and an inactive mode. The input circuit includes a first resistor connected between a magnetoresistive element and a first power source. A first current source is connected between the magnetoresistive element and a second power source to supply a DC bias current to the magnetoresistive element in the active mode. A second resistor is connected between the magnetoresistive element and the first current source. A capacitor is connected to a node between the second resistor and the first current source and to the first power supply. A differential amplifier is connected to the magnetoresistive element. A voltage supply circuit is connected to the node to supply the node, when the input circuit is in the inactive mode, with a voltage substantially equal to that supplied to the node when the input circuit is in the active mode.


REFERENCES:
patent: 5444579 (1995-08-01), Klein et al.
patent: 5623378 (1997-04-01), Shibasaki et al.
patent: 8-255302 (1996-10-01), None
patent: 8-315304 (1996-11-01), None

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