JPH0441446Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Industrial application field This invention is a magnetoresistive magnetic head (hereinafter referred to as MR).
Regarding inverting amplifier circuits (referred to as heads),
The purpose is to protect the MR head.

(b) Prior Art As a signal amplifier for an MR head, an inverting amplifier circuit using an operational amplifier as shown in FIG. 2 has conventionally been used. (For example, see AES Tokyo Conference '85 Proceedings P170.) In other words, a detection current is passed through the magnetoresistive element 2 from the constant current source 1, and after converting the resistance change into voltage, it is input to the inverting input terminal of the operational amplifier 3, and then the output terminal It is designed to obtain a signal output with a higher voltage amplification.
Incidentally, the coupling capacitor 4 is used to take out the alternating current variation of the magnetoresistive element 2, and the reference numeral 5 is a feedback resistor of the operational amplifier 3.

By the way, some types of operational amplifiers have a pair of protection diodes connected in antiparallel between an inverting input terminal and a non-inverting input terminal.

Therefore, when this type of operational amplifier is used as a signal amplifier for an MR head, it will be constructed as shown in FIG.

(c) Problems to be solved by the invention Namely, in FIG. 3, the magnetoresistive element 2 of the MR head is connected to the operational amplifier via the connectors 6 and 7; MR
When the head is removed, as shown in Fig. 4A, a series circuit is formed of the diode 8 and the capacitor 4 in the forward direction with respect to the constant current source 1 among the protective diodes 8 and 9, and the constant current source 1 drives the diode 8 and the capacitor 4. Voltage +V
will be charged up to. When the magnetoresistive element 2 of the MR head is connected in this state, a discharge current i flows through the magnetoresistive element 2 due to the series circuit of the protection diode 9 and the capacitor 4, as shown in FIG. 4B.

The heak value of this discharge current is i=V/R, where R is the value of the magnetoresistive element of the MR head.
Normally, V is approximately equal to the power supply voltage, about +12V, and the resistance value R of the magnetoresistive element is around 100Ω, so
The peak current value is 100mA or more. On the other hand, since the permissible value of the detection current of the magnetoresistive element is about 10 mA at most, the excessive current causes destruction or deterioration of the magnetic properties (extreme decrease in sensitivity). The above-mentioned excessive current will not occur if the connector is removed after the amplifier side power is turned off, but it may equivalently occur due to poor contact of the connector, and therefore
This is a problem in amplifier circuits that require a detection current, such as MR heads.

Therefore, a method of inserting a resistor 10 in series as shown in FIG. 5 can be considered, but in this case, the amplification factor decreases, so the value of the feedback resistor must be increased accordingly. Problems include a direct current offset voltage at the output end due to the bias current of the operational amplifier, and the generation of thermal noise due to an increase in signal source impedance.

(d) Means for Solving the Problems In view of the above problems, the present invention has been developed to solve the problems described above. A current limiting resistor is connected in series with a coupling capacitor that takes out the alternating current, the resistor is connected to the inverting input terminal of an operational amplifier, and a feedback resistor that feeds back the signal from the output of the operational amplifier is connected to the capacitor and the current limiting resistor. It provides a circuit configuration connected to the connection point of the resistor.

(e) Effect The circuit configuration of the present invention limits the discharge current of the coupling capacitor via the protection diode to the permissible value of the magnetoresistive element, and does not increase the signal source impedance, thereby increasing the DC offset voltage and noise. can solve the problem.

(F) Embodiment An embodiment of the circuit of the present invention shown in FIG. 1 will be described below.

In FIG. 1, parts that are the same as those in FIG. 3 are given the same numbers. In the present invention, a current limiting resistor 11 is connected between the coupling capacitor 4 and the inverting input terminal of the operational amplifier 3, and a feedback resistor 5 is connected to the connection point between the capacitor 4 and the resistor 11.

Therefore, according to the circuit configuration of the present invention, the sixth
As shown in the figure, a current limiting resistor 11 is connected in series to the discharge path of the capacitor 4 via the diode 9 of the operational amplifier 3, so the resistor 11 is set to an appropriate value (for example, 1KΩ to 2KΩ). By doing so, the discharge current can be limited to the permissible value of the magnetoresistive element 2.

Next, regarding the problem of increase in signal source impedance, in an inverting amplifier circuit using an operational amplifier as shown in FIG. For,
As shown in FIG. 5, there is no change in the impedance of the series element due to the resistor 10. Incidentally, in FIG. 7, the capacitor is omitted because the impedance is sufficiently low.

(g) Effects of the invention As described above, according to the invention, the discharge current of the coupling capacitor via the operational amplifier's protection diode can be limited to the permissible current value without increasing the signal source impedance. be.

Therefore, it is useful for use in an inverting amplifier circuit such as an MR head that requires a detection current, a coupling capacitor is connected between it and the amplifier, and low noise characteristics are required.

[Brief explanation of the drawing]

Figure 1 shows an example of the circuit of the present invention, and Figure 2 shows a conventional circuit.
MR head amplifier circuit. Figure 3 is an example of a circuit using an operational amplifier with an input protection diode connected to the MR head amplifier circuit. Figures 4A and B show an input protection diode that prevents excessive damage to the magnetoresistive element. An equivalent circuit explaining the flow of current, Figure 5 is an example of a circuit normally used to prevent excessive current, and Figure 6 is an equivalent circuit that explains the flow of current.
7 and 7 are diagrams for explaining the operation of the present invention. 1... Constant current source, 2... Magnetoresistive element, 3...
Operational amplifier, 4 coupling capacitors, 5... feedback resistors, 8, 9... diodes, 11... current limiting resistors.

Claims (1)

[Scope of utility model registration request] A constant current source, a magnetoresistive element to which the constant current source is supplied, a coupling capacitor for taking out an alternating current component of a signal output from the magnetoresistive element, a current limiting resistor connected in series with the capacitor, an operational amplifier having an inverting input terminal connected to a resistor; a pair of diodes connected in antiparallel between the inverting input terminal and the non-inverting input terminal of the operational amplifier; and a feedback resistor for feeding back a signal from the output of the operational amplifier. 1. An inverting amplifier circuit for a magnetoresistive head, characterized in that the output signal of the operational amplifier is fed back to a connection point between the coupling capacitor and the current limiting resistor.