JPH0610842B2 - Resistance change detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Use of the Invention] The present invention relates to an element whose resistance changes due to an external factor,
For example, it relates to a circuit for detecting a resistance change of a magnetoresistive effect element.

[Background of the Invention]

There is a magnetoresistive effect element among resistance change elements whose resistance changes due to external factors such as pressure, heat, and magnetic field. This magnetoresistive effect element is an element whose specific resistance is changed by an external magnetic field, and is used as a reading head for a magnetic card reader, a magnetic tape, a magnetic storage device of a magnetic disk, or the like.

For example, there is a circuit shown in FIG. 1 as a resistance change detection circuit of a magnetoresistive effect type magnetic head (Japanese Patent Laid-Open No. 52-13570).
6). In FIG. 1, 1 is a two-terminal type magnetoresistive effect magnetic head, and 2 is a sense current source. A current from the sense current source 2 is passed through the magnetoresistive head 1 and the leakage magnetic field from the magnetization on the storage medium changes, so that the resistance value of the magnetoresistive head 1 changes. The high frequency signal voltage corresponding to the change of the magnetic field is amplified by the preamplifier 7 through the derived voltage source 13. At this time, the sense current I flowing through the magnetoresistive head 1
_MR is the magnetic head 1 itself in the resistance voltage I _MR · R _MR is derived voltage source to be equal to the derived voltage V _OS of 13 preamplifier 7 and fed back circuit 1 for generating the product of the R _MR
Controlled by 9.

In this circuit, one terminal 9 of the magnetoresistive head 1
Is an AC ground, while the other terminal 8 is a high-impedance signal line, so the magnetic head 1
With respect to the external noise applied to both terminals, noise is easily added to the terminal 8 with respect to the terminal 9, the noise remains as a differential component, and the S / N of the signal decreases. The derived voltage V _OS is obtained as the difference between the base-emitter voltage V _BE by biasing the transistors in the input part of the preamplifier 7 with different emitter currents in the embodiment, and for this reason, a large V _OS. It was difficult to obtain, etc.

[Object of the Invention]

An object of the present invention is to realize a large-scale integrated circuit with a wide band, and to keep the current density of the current flowing through the resistance change element constant to prolong the life of the resistance change element and to have a resistance excellent in external noise resistance. It is to provide a change detection circuit.

[Outline of Invention]

According to the present invention, the emitter terminals of a pair of base-grounded transistors having different base potentials are connected to both ends of the magnetoresistive effect magnetic head to keep the potential at both ends of the magnetoresistive effect magnetic head constant. Therefore, the current density does not change from the initial value even if the resistance value of the magnetoresistive head is changed.
Further, the change in the resistance value of the magnetoresistive head due to the change in the leakage magnetic field on the storage medium appears as the change in the flowing current value, which is the reverse of the collector currents of the pair of base-grounded transistors. The phase changes. At this time, since the current is supplied by the current source having a high impedance, the impedances at both ends of the magnetoresistive effect magnetic head are equal and differential amplification is possible. Therefore, in-phase external noise can be removed, and the signal S / N can be improved.

[Embodiment of the Invention] An embodiment of the present invention will be described below with reference to FIG. Second
In the figure, 1 is a magnetoresistive head, 13, 1
4 is a pair of grounded base transistors, 15 and 16 are load resistors, 7 is a preamplifier, 2 is a current source consisting of a transistor 17 and an emitter resistor 18, and 19 is terminals 9 and 1.
This is a feed back circuit that controls the current source 2 at the terminal 21 so that the DC potential of 0 becomes equal, and is composed of, for example, a differential amplifier circuit and a low pass filter. In FIG.
The initial value of the magnetoresistive head 1 is R _MR , the current value is I _MR , the collector load resistances 15 and 16 of the base-grounded transistors 13 and 14 are _RC , and the collector currents of each are I ₁ and I _MR , respectively.
I ₂

The base potentials of the grounded base transistors 13 and 14 are connected to the power supplies V _{ref +} and V _ref- , respectively. Further, the output 21 (potential V _B ) of the feedback circuit 19 controls the collector current I ₃ of the current source transistor 17, and the current I ₃
Is input to the sense circuit system from the connection point 8.

The operation of this embodiment will be described below. The potential difference across the magnetoresistive head 1 is determined by the voltage across the emitter terminals of the pair of base-grounded transistors 13 and 14, and is always constant. Therefore, the current I _MR flowing through the magnetoresistive head 1 is equal to the collector current I ₁ of the transistor 13, and I _MR = (V _{ref +} −V _ref− ) / R _MR = I ₁ (1).

At this time, the collector current I ₂ of the transistor 14 is controlled by the current source 2 through the feedback circuit 19 so that the DC potential difference between the input terminals 9 and 10 of the preamplifier 7 becomes zero. Since the load resistance 15 and 16 are both equal to _{R C,} as the collector current _{I 2} becomes equal to _{I 1,} the supply current _{I 3} of the current source 2 _{I 3 = I 1 + I 2} = 2I MR (2) and the control To be done.

Now, considering the case where the resistance value of the magnetoresistive head 1 changes from R _MR to (R _MR + ΔR _MR ) due to the magnetization of the storage medium, the potential difference across the magnetoresistive head 1 is The current changes from I _MR to (I _MR + ΔI _MR ) because the grounded base transistors 13 and 14 are fixed by the respective emitters. Here, ΔI _MR is ΔI _MR = I _MR × (ΔR _MR / R _MR ) (3). At this time, since the current I ₃ supplied from the current source 2 does not change, this current change appears in the opposite phase as the collector current I ₂ of the transistor 14. That is, I ₁ = I _MR + ΔI _MR (4) I ₂ = I _MR −ΔI _MR (5) Therefore, the signal voltage between the input terminals of the preamplifier 7 is
When the potentials of the terminals 9 and 10 are V ₁ and V ₂ , respectively, | V ₁ −V ₂ | = 2ΔI _MR · R _C (6) Assuming that the amplification factor of the preamplifier 7 is G, from the equation (3), the signal output amplitude E ₀ in the whole circuit is Becomes

According to the present embodiment, the terminal voltage of the magnetoresistive effect magnetic head is determined by the base potential difference of the pair of base-grounded transistors and is constant, so that the resistance value of the magnetoresistive effect magnetic head changes due to, for example, abrasion. However, it is possible to keep the current density constant. Further, the signal due to the change of the magnetic field from the magnetoresistive head type storage medium becomes a differential input, and in-phase external noise can be removed. Further, since the DC voltage difference between the input terminals of the preamplifier becomes zero by the feedback circuit, the capacitor for removing the DC component of the signal line is not required, and the circuit can be integrated.

At this time, as is well known, transistors 13 and 1
By connecting a plurality of transistors 4 and 17 in parallel, it is possible to reduce random noise generated by the circuit. Further, when the present embodiment is applied in a high frequency band, by connecting a current source in which a minute current flows to the connection point 20 or by grounding with a capacitor having the same capacity as the output capacity of the current source 2, It is also clear that the effect of removing the in-phase external noise can be enhanced by correcting the impedance difference between both ends of the magnetoresistive effect magnetic head so as to be equal. Further, in this embodiment, the feedback is made from the input terminals 9 and 10 of the preamplifier 7, but it is clear that the same effect can be obtained even if the feedback is made from the output terminals 11 and 12 of the preamplifier 7. Is.

Further, as shown in the embodiment of FIG. 3, the feedback destination of the output terminal 21 of the feed back circuit 19 in FIG. 2 is set as the base of the transistor 14 and the base potential of the transistor 17 is set to V _Bref. It is also apparent that a detection circuit for driving a constant current sense source in which 2 is a constant current source of 2 · I _MR can be configured. According to the present embodiment, it is possible to realize a detection circuit which is driven by a constant current source and has excellent resistance to external noise.

〔The invention's effect〕

As described above in detail, according to the present invention, the current density of the variable resistance element can be kept constant, so that the variable resistance element can have a long self-life and a capacitor for removing a direct current component is not required. It will be easy. Moreover, impedances at both ends of the resistance change element are equal, and differential amplification is possible, so that an in-phase external noise is removed.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a conventional resistance change detection circuit, and FIG. 2 is
FIG. 3 is a circuit diagram of an embodiment of a resistance change detection circuit to which the present invention is applied, and FIG. 3 is a circuit diagram of another embodiment of the present invention. 1 ... Magnetoresistive magnetic head, 2 ... Constant current source, 3, 4 ... Capacitor for removing DC voltage, 7 ... Preamplifier, 13, 14 ... Grounded base transistor, 19 ... Feed back circuit.

Front page continuation (72) Minor Kosuge 2880, Kozu, Odawara-shi, Kanagawa Hitachi Ltd., Odawara Plant (72) Inventor Shinichi Arai, 2880, Kozu, Odawara, Kanagawa Hitachi Ltd., Odawara Plant (56) ) Reference JP-A-58-189803 (JP, A) JP-B-58-27561 (JP, B2)

Claims (4)

[Claims] 1. A base potential having an emitter terminal connected to both ends of the resistance change element in a circuit for detecting a resistance change of a resistance change element of two terminals in which a resistance value of the resistance change element is changed by an external factor when a current is applied. Of a pair of transistors different from each other, a current source connected to one end of the resistance change element for flowing the current, and a collector terminal of the pair of transistors for detecting a change in the current flowing through the resistance change element. Change detection circuit having differential amplifier. 2. A feedback means for controlling the current source so that the DC potential difference between the collector terminals of the pair of transistors becomes zero based on the input or output of the differential amplifier. The resistance change detection circuit according to item 1. 3. A feedback that controls the base potential of at least one of the pair of transistors based on the input or output of the differential amplifier so that the DC potential difference between the collector terminals of the pair of transistors becomes zero. The resistance change detection circuit according to claim 1, further comprising means. 4. The second feedback means comprises a differential amplifier circuit and a low-pass filter.
The resistance change detection circuit according to item 3 or item 3.