JPH0740324B2 - Resistance change detection circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for detecting a resistance change of an element whose resistance changes due to an external factor, for example, a magnetoresistive effect element.

[0002]

2. Description of the Related Art A magnetoresistive effect element is known as a variable resistance element whose resistance changes due to external factors such as pressure, heat and magnetic field. This magnetoresistive element is an element whose specific resistance changes with an external magnetic field, and is used as a read head for a magnetic card reader, a magnetic tape, a magnetic storage device for 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. 52-135706). In FIG. 1, 1 is a two-terminal type magnetoresistive effect type magnetic head, and 2 is a sense current source. A current from the sense current source 2 is passed through the magnetoresistive effect magnetic head 1, and the leakage magnetic field from the magnetization on the storage medium changes, whereby the resistance value of the magnetoresistive effect magnetic head 1 changes, and the magnetic field The high-frequency signal voltage corresponding to the change is passed through the derived voltage source 3 and amplified by the preamplifier 7. At this time, the sense current IMR flowing through the magnetoresistive head 1 is pre-set so that the voltage IMR.RMR generated by the product of the resistance RMR of the magnetic head 1 itself becomes equal to the derived voltage VOS of the derived voltage source 3. It is controlled by the amplifier 7 and the feedback circuit 19.

In this circuit, one terminal 9 of the magnetoresistive effect type magnetic head 1 is an AC ground, while the other terminal 8 is a high impedance signal line.
For external noise applied to both terminals of the magnetic head 1,
Noise is likely to be 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 VOS is obtained as the difference between the base-emitter voltage VBE by biasing the transistors in the input part of the preamplifier 7 with different emitter currents in the embodiment, and it is difficult to obtain a large VOS. There were drawbacks such as.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to realize a large-scale integrated circuit having a wide band, and to maintain a constant current density of a current flowing through a resistance change element to extend the life of the resistance change element. Another object of the present invention is to provide a resistance change detection circuit having excellent resistance to external noise and resistance to offset.

[0006]

SUMMARY OF THE INVENTION According to the present invention, in a circuit for detecting a resistance change amount of a resistance change element whose resistance value changes due to an external factor, a difference in which an emitter is connected to both terminals of the resistance change element. A pair of grounded base transistors forming a dynamic pair, a current source connected to one terminal of the variable resistance element, a voltage source connected to each of the base terminals of a pair of base grounded transistors, and a pair of bases A differential amplifier that obtains a differential output signal from a load resistance connected to the collector terminal of the ground transistor and a collector output, and a feedback circuit for zeroing the DC potential difference on the input side of the differential amplifier are provided. To do. The feedback circuit is configured to provide the control signal to the current source or the voltage source.

[0007]

In the present invention, the emitter terminals of a pair of grounded base transistors forming a differential pair are connected to both ends of the magnetoresistive magnetic head, and the differential amplifier is connected to the collector terminal for differential amplification. To do. The change in the resistance value of the magnetoresistive effect magnetic head due to the change in the leakage magnetic field on the storage medium appears as a change in the flowing current value, which is the opposite phase of the collector currents of the pair of base-grounded transistors. It will change. 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 better. Further, a feedback circuit for controlling the collector currents of the base-grounded transistors to be equal to each other is provided, and the DC potential difference between the input terminals of the operational amplifier becomes zero, and the DC offset can be removed.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. In FIG. 2, 1 is a magnetoresistive effect magnetic head, and 1
3, 14 are a pair of grounded base transistors, 15 and 16
Is a load resistor, 7 is a preamplifier, 2 is a current source composed of a transistor 17 and an emitter resistor 18, and 19 is a feedback circuit for controlling the current source 2 at a terminal 21 so that the DC potentials at the terminals 9 and 10 are equal. And is composed of, for example, a differential amplifier circuit and a low-pass filter.

In FIG. 2, the initial value of the magnetoresistive head 1 is RMR, the current value is IMR, and the collector load resistors 15 and 16 of the base-grounded transistors 13 and 14 are R.
c, the collector currents of each are I ₁ and I ₂ .

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

The operation of this embodiment will be described below. The potential difference across the magnetoresistive head 1 is determined by the voltage between the emitter terminals of the pair of base-grounded transistors 13 and 14, and is always constant. Therefore, the current IMR flowing in the magnetoresistive head 1 is equal to the collector current I ₁ of the transistor 13, and IMR = (Vref + −Vref −) / RMR = I ₁ (1).

At this time, the collector current I ₂ of the transistor 14 passes through the feedback circuit 19 and the preamplifier 7
Is controlled by the current source 2 so that the DC potential difference between the input terminals 9 and 10 becomes zero. Both load resistors 15 and 16 are R
It is equal is c, as the collector current I ₂ becomes equal to I _1, the supply current I ₃ of the current source 2 is controlled and _{I 3 = I 1 + I 2} = 2IMR (2).

Now, the resistance value of the magnetoresistive head 1 is changed from RMR to (RMR + ΔRM) due to the magnetization of the storage medium.
Considering the case of changing to R), the potential difference between both ends of the magnetoresistive head 1 is fixed at the emitters of the pair of grounded base transistors 13 and 14, so that the current changes from IMR to (IMR + ΔIMR). Change. Here, ΔIMR is ΔIMR = IMR × (ΔRMR / RMR) (3). At this time, the supply current I ₃ from the current source 2 does not change, so this change in current appears as a change in the collector current I ₂ of the transistor 14 in antiphase. That is, I ₁ = IMR + ΔIMR (4) I ₂ = IMR−ΔIMR (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 | = 2ΔIMR · RC (6) Assuming that the amplification factor of the preamplifier 7 is G, from the equation (3), the signal output amplitude E ₀ in the entire circuit is Becomes

According to this embodiment, since the terminal voltage of the magnetoresistive effect magnetic head is constant and determined by the base potential difference of the pair of grounded base transistors, the magnetoresistive effect magnetic head has a resistance value due to, for example, abrasion. It is possible to keep the current density constant even when is changed. In addition, the signal due to the change in the magnetic field from the storage medium of the magnetoresistive head becomes a differential input, and in-phase external noise can be removed.
Furthermore, since the feedback circuit reduces the DC potential difference between the input terminals of the preamplifier to zero, a capacitor for removing the DC component of the signal line is not required, and the circuit can be integrated.

At this time, it is possible to reduce the random noise generated in the circuit by connecting a plurality of transistors 13, 14 and 17 in parallel, as is well known. Further, when the present embodiment is applied in a high frequency band, the magnetic resistance is changed by connecting a current source in which a minute current is flown to the connection point 20 or 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 at both ends of the effect type magnetic head to be equal. Further, in this embodiment, the input terminals 9, 10 of the preamplifier 7 are used.
Although it is fed back from the output terminals 11 and 1 of the preamplifier 7,
It is clear that the same effect can be obtained by returning from 2.

As shown in the embodiment of FIG. 3, the output terminal 21 of the feedback circuit 19 shown in FIG. 2 is fed back to the base of the transistor 14, and the base potential of the transistor 17 is set to VBref.
It is also clear that a detection circuit for driving a constant current sense source can be configured by using as a 2 · IMR constant current source. 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.

[0017]

As described in detail above, according to the present invention,
Since the current density of the resistance change element can be kept constant, the life of the resistance change element can be extended and a capacitor for removing a DC component is not required, which facilitates integration. 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.

FIG. 2 is a circuit diagram of an embodiment of a resistance change detection circuit to which the present invention is applied.

FIG. 3 is a circuit diagram of another embodiment of the present invention.

[Explanation of symbols]

1 ... Magnetoresistive effect magnetic head, 2 ... Constant current source, 3, 4
... DC voltage removing capacitor, 7 ... Preamplifier, 13,
14 ... Base grounded transistor, 19 ... Feedback circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Minor Kosuge 2880, Kozu, Odawara-shi, Kanagawa Stock company Hitachi Ltd. Odawara factory (72) Inventor Shinichi Arai 2880, Kozu, Kanagawa prefecture Hitachi Ltd. Odawara factory Within

Claims (2)

[Claims] 1. A circuit for detecting a resistance change amount of a resistance change element, the resistance value of which is changed by an external factor,
A pair of base-grounded transistors forming a differential pair in which emitters are connected to both terminals of the resistance change element; a current source connected to one terminal of the resistance change element; and a pair of base-grounded transistors of the pair. A voltage source connected to each of the base terminals, a differential amplifier for obtaining a differential output signal from a load resistance and a collector output connected to the collector terminals of the pair of grounded base transistors, and an input side of the differential amplifier A resistance change detection circuit is provided with a feedback circuit for giving a control signal for making the DC potential difference of 0 to the current source. 2. A circuit for detecting a resistance change amount of a resistance change element, the resistance value of which is changed by an external factor,
A pair of base-grounded transistors forming a differential pair in which emitters are connected to both terminals of the resistance change element; a current source connected to one terminal of the resistance change element; and a pair of base-grounded transistors of the pair. A voltage source connected to each of the base terminals, a differential amplifier for obtaining a differential output signal from a load resistance and a collector output connected to the collector terminals of the pair of grounded base transistors, and an input side of the differential amplifier A resistance change detection circuit is provided with a feedback circuit for giving a control signal for reducing the DC potential difference of the above to the voltage source.