Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
1999-11-09
2001-11-20
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C360S046000
Reexamination Certificate
active
06320466
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an amplifier circuit that amplifies the voltage of an MR element in an MR (Magneto resistive) head.
BACKGROUND OF THE INVENTION
In an MR head, the resistive value of an MR element changes relative to the changes of a magnetic field. In other words, an MR element is handled in the circuit construction a variable resistor in which the resistance value changes in response to the strength of the magnetic field that is applied. Because of this, when a constant current flows in the MR element, since the voltage drop that is generated in the said MR element changes relative to changes in the magnetic field, it is possible to detect the magnetic field strength by detecting the voltage drop of the MR element. Also, it is possible to detect the magnetic field strength by applying a constant voltage to an MR element, and detecting the current that flows in said MR element.
The pre-amp that detects the voltage drop of an MR element depends on the application system for the voltage for the MR element. There are two types: a single-end type and a differential type.
FIGS. 6 and 7
respectively show the construction of a single-end type and a differential type pre-amp.
As is shown in
FIG. 6
, the system that uses an amplifier of the single-end type is constructed by means of current supply circuit (
10
) that supplies current (I
B
) to MR element (
20
), and amplifier (
30
a
).
Current supply circuit (
10
) supplies current (I
B
) that is set in response to a control signal (S
D
) to MR element (
20
), and since the resistance value of MR element (
20
) changes in response to the magnetic field strength, the voltage of node (ND
1
) is determined by said resistance value and current (I
B
). Since the voltage of node (ND
1
) is amplified by amplifier (
30
a
), and amplified signal (S
out
) is output, the magnetic field strength can be detected corresponding to signal (S
out
).
In a system that uses the differential type amplifier shown in
FIG. 7
, MR element (
20
) is connected between nodes (ND
1
) and (ND
2
). Also, resistive elements (R
1
) and (R
2
) are series-connected between nodes (ND
1
) and (ND
2
). Current (I
B
) is supplied to node (ND
1
) by current supply circuit (
10
). On the other hand, current (I
B1
) that flows in the direction of node (ND
2
) to current supply circuit (
12
) is supplied by current supply circuit (
12
).
Output current (I
B1
) of current supply circuit (
12
) is controlled corresponding to the voltage of contact point (ND
0
) for resistive elements (R
1
) and (R
2
). Normally, the supply current (I
B1
) of current supply circuit (
12
) is controlled such that the potential of node (ND
0
) becomes ground potential (GND).
Since the potential difference between nodes (ND
1
) and (ND
2
) is amplified by differential amplifier (
30
), and the amplified signal (S
out
) is output, the resistance value for MR element (
20
) corresponding to said output signal (S
out
) can be detected, and, furthermore, the magnetic field strength can be detected.
Because MR element (
20
) is susceptible to static electricity, it is necessary that the potential of MR element (
20
) in regard to the circuit construction be considered to make it as close as possible to ground potential (GND). In the system using the differential type amplifier shown in
FIG. 7
, since the middle point of MR element (
20
) is maintained at ground potential (GND), electrostatic breakdown is effectively prevented. Also, in the system using the single-end type amplifier shown in
FIG. 6
, due to the fact that the supply current (I
B
) of current supply circuit (
10
) is set small and the potential of node (ND
1
) is maintained at a level close to 0.1 to 0.2 V, its electrostatic breakdown can be prevented. Since the single-end type amplifier can be operated with a single power supply voltage, its circuit construction is simple.
Incidentally, with the system that uses the above-mentioned single-end type amplifier, in the event in-phase noise is applied to MR element (
20
) due to noise, in other words, in the event a noise component of the same phase appears at both ends of MR element (
20
), since said noise component is amplified by means of amplifier (
30
a
) and appears on output signal (S
out
), it is susceptible to in-phase noise. In other words, the common mode removal ratio (CMRR) for this type of amplifier is small.
On the other hand, in principle with the system using the differential type amplifier, the CMRR can be increased up to a high frequency region, but because the middle point of MR element (
20
) is maintained at almost ground potential (GND), a negative power supply voltage becomes necessary; thus there is the disadvantage that the circuit construction becomes complicated.
This invention was made based on this type of information, and its purpose is to offer an amplifier circuit used for an MR head that is operable with a single power supply voltage, and in which a large CMRR can be maintained.
SUMMARY OF THE INVENTION
In order to achieve the above-mentioned purposes, the amplifier circuit of this invention has an MR element in which the resistance value changes in response to the strength of a magnetic field, a current supply means that supplies a prescribed current to the first terminal of the above-mentioned MR element, a resistive element that is connected between the second terminal of the above-mentioned MR element and ground potential, a current control means that, when the potential of the above-mentioned second terminal changes, supplies a current to the above-mentioned second terminal that is equivalent to the current change amount of the above-mentioned resistive element that is generated by means of the said potential change of the terminal, and an amplifying means that amplifies the voltage difference of the above-mentioned first terminal and second terminal, and outputs an amplified signal.
In this invention, preferably, the above-mentioned current control means has a first current source and a first transistor that are series-connected between the supply line for the power supply voltage and the above-mentioned second terminal, a second current source, a second transistor, and a second resistive element that are series-connected between the supply line for the above-mentioned power supply voltage and the above-mentioned ground potential, a third resistive element that is connected between the connecting point for the above-mentioned first current source and first transistor and the connecting point for the above-mentioned second current source and second transistor, and a second current supply means that supplies almost the same current as the above-mentioned current supply means to a connecting point between the above-mentioned second transistor and the above-mentioned second resistive element; the control terminal for the above-mentioned first transistor is connected to the connecting point between the above-mentioned second transistor and the above-mentioned second resistive element, and the control terminal for the above-mentioned second transistor is connected to the above-mentioned second terminal.
Also, in this invention, preferably, the above-mentioned current control means has a first current source, a first transistor, and a second transistor that are series-connected between the supply line for the power supply voltage and the above-mentioned second terminal, a second current source, a third transistor, a fourth transistor, and a second resistive element that are series-connected between the supply line for the above-mentioned power supply voltage and the above-mentioned ground potential, a third resistive element that is connected between the connecting point for the above-mentioned first current source and first transistor, and the connecting point for the above-mentioned second current source and third transistor, a fourth resistive element that is connected between the control terminal of the above-mentioned second transistor and the above-mentioned second terminal, a fifth resistive element that is connecte
Choe Henry
Kempler William B.
Pascal Robert
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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