Abstract:
An apparatus for monitoring changes in a magnetic field using a magneto-resistive device situated in the field includes a first and second input locus coupled with the magneto-resistive device. An amplifier means for amplifying electrical signals has input terminals and output terminals. A first input terminal is coupled with the first input locus with a first capacitor coupled in series between the first input locus and the first input terminal. A second input terminal is coupled with the second input locus with a second capacitor coupled in series between the second input locus and the second input terminal. The apparatus receives a bias current at the first input locus that cooperates with the magneto-resistive element to affect electrical potential at the first input locus. The amplifier device presents at least one output signal at the output terminals indicating changes in the magnetic field.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is directed to apparatuses for monitoring changes in magnetic fields, and especially to such apparatuses employed in magnetic storage read devices, sometimes referred to as read heads.  
           [0002]    In preamplifiers associated with monitoring changes in magnetic fields, such as read heads for reading information from a rotating magnetic storage disk, a magneto-resistive device is situated within the magnetic field to sense magnetic properties in the field. An example of such a magneto-resistive device is a magnetic resistivity device that exhibits changes in resistance with changes in a magnetic field in which it is located. The magneto-resistive device is preferably located in close proximity with the rotating disk of a magnetic storage device in order to accurately monitor changes in the magnetic field of the disk that comprise data indicators. A danger with such a close proximal relation between the magneto-resistive device and the rotating disk is that if too much potential difference is present between the two components, arcing may occur between the components thereby damaging at least one of the components.  
           [0003]    In the past the need for keeping voltage levels at the magneto-resistive device low had a tendency to interfere with operation of the amplifier device of the preamplifier apparatus to which the magneto-resistive device is coupled. This tendency existed because the amplifier device requires a predetermined voltage potential to ensure reliable operation of its internal transistor components. The voltage potential required to assure reliable operation of transistor components within the amplifier device is significantly greater than the potential at which the magneto-resistive device should be maintained.  
           [0004]    This problem was overcome in the prior art by using a fully differential amplifier circuit arrangement in which the amplifier device is coupled with a power supply that provides both a positive supply voltage and a negative supply voltage. By such an arrangement, a circuit designer can substantially independently control voltage potential across transistor components within an amplifier and voltage potential at a circuit locus coupled with the amplifier device.  
           [0005]    With the current trend in industry toward cost savings in circuit construction and toward smaller, more compact products, such fully differential circuit construction has been supplanted in some products by a single supply circuit arrangement. A single supply circuit arrangement provides supply voltage only to one supply voltage input terminal (for example, and preferably the positive supply voltage input terminal) of an amplifier device. The other supply voltage terminal (for example, and preferably the negative supply voltage terminal) is connected to ground potential. Such single supply circuit arrangements provide advantages in their being less expensive to construct, less complex in their layout, requiring fewer connections and they may be produced using smaller semiconductor dies. However, such a single supply arrangement provides no independent control of voltage potential across transistor components within an amplifier and voltage potential at a circuit locus coupled with the amplifier device (e.g., a circuit locus at which potential at a magneto-resistive device is controlled).  
           [0006]    There is a need for an apparatus for monitoring changes in a magnetic field that can be configured in a single supply circuit arrangement and effect substantially independent control of voltage across transistor components within an amplifier device and voltage potential at a circuit locus coupled with the amplifier device.  
         SUMMARY OF THE INVENTION  
         [0007]    An apparatus for monitoring changes in a magnetic field using a magneto-resistive device situated in the field includes a first and second input locus coupled with the magneto-resistive device. An amplifier means for amplifying electrical signals has input terminals and output terminals. A first input terminal is coupled with the first input locus with a first capacitor coupled in series between the first input locus and the first input terminal. A second input terminal is coupled with the second input locus with a second capacitor coupled in series between the second input locus and the second input terminal. The apparatus receives a bias current at the first input locus that cooperates with the magneto-resistive element to affect electrical potential at the first input locus. The amplifier device presents at least one output signal at the output terminals indicating changes in the magnetic field.  
           [0008]    It is, therefore, an object of the present invention to provide an apparatus for monitoring changes in a magnetic field that can be configured in a single supply circuit arrangement and effect substantially independent control of voltage across transistor components within an amplifier device and voltage potential at a circuit locus coupled with the amplifier device.  
           [0009]    Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings, in which like elements are labeled using like reference numerals in the various figures, illustrating the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is an electrical schematic diagram illustrating a prior art fully differential preamplifier for a magnetic read head for a disk drive.  
         [0011]    [0011]FIG. 2 is an electrical schematic diagram illustrating a prior art single supply preamplifier for a magnetic read head for a disk drive.  
         [0012]    [0012]FIG. 3 is an electrical schematic diagram illustrating a novel single supply preamplifier for a magnetic read head for a disk drive configured according to the teachings of the present invention.  
         [0013]    [0013]FIG. 4 is an electrical schematic diagram illustrating an equivalent circuit for a portion of the novel single supply preamplifier illustrated in FIG. 3.  
         [0014]    [0014]FIG. 5 is a graphic representation of the sensitivity responses of a prior art single supply preamplifiers and a novel single supply preamplifier configured according to the teachings of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    [0015]FIG. 1 is an electrical schematic diagram illustrating a prior art fully differential preamplifier for a magnetic read head for a disk drive. In FIG. 1, a preamplifier apparatus  10  includes an amplifier  12 , a power supply circuit  14  and a magneto-resistive device  16 . An example of such a magneto-resistive device is a magnetic resistivity device that exhibits changes in resistance with changes in a magnetic field in which it is located. Amplifier  12  has a positive signal input terminal  20  and a negative signal input terminal  22 . Amplifier  12  also has a positive supply terminal  24  and a negative supply terminal  26 . Amplifier  12  has output terminals  30 ,  32  at which signals are presented that are representative of signals received at signal input terminals  20 ,  22 . Power supply circuit  14  is coupled with both supply terminals  24 ,  26  so that power supply circuit  14  provides a positive supply signal to positive supply terminal  24  and provides a negative supply signal to negative supply terminal  26 . Voltage potential at a circuit locus  11 , adjacent to magneto-resistive device  16 , is preferably kept at a voltage level low enough to preclude arcing between magneto-resistive device  16  and an adjacent magnetic component, such as a magnetic disk of a data storage device (not shown in FIG. 1). Maintaining voltage potential at circuit locus  11  at a first potential level that is significantly lower than the potential required at signal input terminal  20  may be effected in preamplifier apparatus  10  by adjusting the negative supply signal provided to negative supply terminal  26 .  
         [0016]    [0016]FIG. 2 is an electrical schematic diagram illustrating a prior art single supply preamplifier for a magnetic read head for a disk drive. In FIG. 2, a preamplifier apparatus  40  includes an amplifier  42 , a power supply circuit  44  and a magneto-resistive device  46 . An example of such a magneto-resistive device is a magnetic resistivity device that exhibits changes in resistance with changes in a magnetic field in which it is located. Amplifier  42  has a positive signal input terminal  50  and a negative signal input terminal  52 . Amplifier  42  also has a positive supply terminal  54  and a negative supply terminal  56 . Amplifier  42  has output terminals  60 ,  62  at which signals are presented that are representative of signals received at signal input terminals  50 ,  52 .  
         [0017]    Magneto-resistive device  46  is coupled between positive signal input terminal  50  and ground potential at ground locus  47 . Negative signal input terminal  52  is coupled with ground potential at ground locus  53 . Power supply circuit  44  is coupled with ground potential at ground locus  45 . Power supply circuit  44  is coupled with positive supply terminal  54  so that power supply circuit  44  provides a positive supply signal to positive supply terminal  54 . Negative supply terminal  56  is coupled to ground potential at ground locus  57 . No negative supply signal is supplied to amplifier  42  by power supply circuit  44 . Voltage potential at a circuit locus  41 , electrically adjacent to magneto-resistive device  46  and to positive signal input terminal  50 , is preferably kept at a voltage level low enough to preclude arcing between magneto-resistive device  46  and an adjacent magnetic component, such as a magnetic disk of a data storage device (not shown in FIG. 2).  
         [0018]    Maintaining voltage potential at circuit locus  41  at a sufficiently low potential to avoid arcing between magneto-resistive device  46  and a magnetic disk of a data storage device disadvantageously affects operation of amplifier  42  because circuit locus  41  is electrically common with positive signal input terminal  50 . The level at which circuit locus  41  must be maintained results in having to bias internal components of amplifier  42  in a manner that establishes a low pass pole that limits maximum operational frequency of apparatus  40  to a lower value than is achievable in a fully differential amplifier (e.g., amplifier  10 ; FIG. 1).  
         [0019]    [0019]FIG. 3 is an electrical schematic diagram illustrating a novel single supply preamplifier for a magnetic read head for a disk drive configured according to the teachings of the present invention. In FIG. 3, a preamplifier apparatus  70  includes an amplifier  72 , a power supply circuit  74  and a magneto-resistive device  76 . An example of such a magneto-resistive device is a magnetic resistivity device that exhibits changes in resistance with changes in a magnetic field in which it is located. Amplifier  72  has a positive signal input terminal  80  and a negative signal input terminal  82 . Amplifier  72  also has a positive supply terminal  84  and a negative supply terminal  86 . Amplifier  72  has output terminals  90 ,  92  at which signals are presented that are representative of signals received at signal input terminals  80 ,  82 .  
         [0020]    Magneto-resistive device  76  is coupled between a first circuit locus  71  and a second circuit locus  73 . First circuit locus  71  is coupled with positive signal input terminal  50  via a direct current (DC) blocking capacitor  79 . Second circuit locus  73  is coupled with negative signal input terminal  82  via a DC blocking capacitor  81 . Second circuit locus  73  is also coupled with ground potential at ground locus  77  via a bias or isolation resistor  78 . Power supply circuit  74  is coupled with ground potential at ground locus  75 . Power supply circuit  74  is coupled with positive supply terminal  84  So that power supply circuit  74  provides a positive supply signal to positive supply terminal  84 . Negative supply terminal  86  is coupled to ground potential at ground locus  87 . No negative supply signal is supplied to amplifier  72  by power supply circuit  74 .  
         [0021]    A current supply  96  is coupled with first circuit locus  71  via an isolation resistor  75  so that a current I may be injected into preamp apparatus  70  at first circuit locus  71  to establish a controlled voltage drop across magneto-resistive device  76  and isolation resistor  78 . By controlling voltage drop across magneto-resistive device  76  and isolation resistors  75 ,  78 , potential at first circuit locus  71  may be controlled. Isolation resistors  75 ,  78  cooperate to isolate magneto-resistive device  76  from noise sources that are external or internal with respect to preamplifier apparatus  70 .  
         [0022]    First circuit locus  71  is electrically adjacent to magneto-resistive device  76  and to positive signal input terminal  80  (as was true in preamplifier apparatus  40 ; FIG. 2). However, DC blocking capacitor  79  blocks DC from reaching positive signal input terminal  80 . DC is therefore blocked from reaching internal components of amplifier  72  (not shown in FIG. 3). As a result, potential at the internal components of amplifier  72  may be maintained at a different DC level than the potential maintained at first circuit locus  71 .  
         [0023]    Preamplifier apparatus  72  is preferably configured as a product carried on a printed wiring board or as an integrated circuit on a chip, as indicated by a schematic chip boundary  98 .  
         [0024]    [0024]FIG. 4 is an electrical schematic diagram illustrating an equivalent circuit for a portion of the novel single supply preamplifier illustrated in FIG. 3. In FIG. 4, an equivalent circuit  100  represents a portion of a single supply preamplifier such as preamplifier apparatus  70  (FIG. 3). Equivalent circuit  100  includes a magneto-resistive device  102  coupled with a current source  104  via an isolation resistor  105 . Magneto-resistive device  102  is coupled with ground potential at ground locus  107  via an isolation resistor  106 . Magneto-resistive device  102  is akin to magneto-resistive device  76  (preamplifier apparatus  70 ; FIG. 3). Current source  104  is akin to current source  96  (preamplifier apparatus  70 ; FIG. 3). Isolation resistors  105 ,  106  are akin to isolation resistors  75 ,  78  (preamplifier apparatus  70 ; FIG. 3).  
         [0025]    An inductor  108  is coupled to a circuit locus  109  between isolation resistor  105  and magneto-resistive device  102 . Inductor  108  represents the inductance present in leads extending between amplifier  72  and magneto-resistive device  76  (preamplifier apparatus  70 ; FIG. 3). Leads between an amplifier and a magneto-resistive device are typically relatively long in preamplifier devices employed in storage disk read apparatuses. This is so because the mass of the read head is desirably kept to a minimum in order that the head can accelerate rapidly in moving to read data on a storage disc. The masses of the amplifier (e.g., amplifier  72 , FIG. 3) and associated components, such as a signal source, power supply circuit, DC Blocking capacitors and isolation resistor (e.g., signal source  96 , power supply circuit  74 , capacitors  79 ,  81  and isolation resistor  78 ; FIG. 3) are preferably situated at a location remote from the reader head itself to avoid contributing to inertia of the reader head.  
         [0026]    In equivalent circuit  100  (FIG. 4), a DC blocking capacitor  110  is connected between inductor  108  and an impedance  112 . Impedance  112  is coupled with ground potential at ground locus  113 . DC blocking capacitor  110  is akin to DC blocking capacitor  79  (preamplifier apparatus  70 ; FIG. 3). Impedance  112  represents the internal impedance presented by amplifier  72  to positive signal input terminal  80  (preamplifier apparatus  70 ; FIG. 3). An indication that impedance  112  is an internal impedance within an amplifier device is illustrated by a dotted-line representation of an amplifier device  120  in FIG. 4.  
         [0027]    Inductance contributed by inductance in leads between a magneto-resistive device and an amplifier in a preamplifier apparatus results in a decrease, or “roll-off” of signal response of the preamplifier apparatus as frequency of signals traversing the leads increases (FIG. 5). The capability to isolate voltage across magneto-resistive device  76  at first circuit locus  71  from voltage at positive signal input terminal  80  in preamplifier apparatus  70  (FIG. 3) permits operation of preamplifier apparatus  70  so that response “roll off” occurs at a higher frequency than occurs in prior art single supply preamplifier apparatuses (e.g., preamplifier apparatus  40 ; FIG. 2). Operation of a reader apparatus at a higher frequency permits the head reader device to operate with a greater bandwidth. Higher signal frequency also permits data to be more tightly arranged or packed on a disk so that more data may be stored on the disk.  
         [0028]    [0028]FIG. 5 is a graphic representation of the sensitivity responses of a prior art single supply preamplifiers and a novel single supply preamplifier configured according to the teachings of the present invention. In FIG. 5, a graphic representation  150  represents output signal strength from an amplifier (in decibels) plotted with respect to an axis  152 , as a function of signal frequency plotted with respect to an axis  154 . A first curve  160  represents a response curve of output signal strength (e.g. strength of signals appearing at output terminals  60 ,  62  of preamplifier apparatus  40 ; FIG. 2) as a function of signal frequency for a prior art single supply preamplifier apparatus (e.g., preamplifier apparatus  40 ; FIG. 2). A second curve  162  represents a response curve of output signal strength (e.g. strength of signals appearing at output terminals  90 ,  92  of preamplifier apparatus  70 ; FIG. 3) as a function of signal frequency for a single supply preamplifier apparatus configured according to the teaching of the present invention (e.g., preamplifier apparatus  70 ; FIG. 3). Acceptable performance for a preamplifier apparatus is typically expressed as a signal degradation less than a predetermined amount ΔdB as indicated in FIG. 5. An exemplary typical ΔdB in the telecommunications industry is 3 dB. Inspection of FIG. 5 reveals that output signals represented by response curve  160  (prior art preamplifier devices) degrade a maximum permitted ΔdB amount at a frequency f 1 . Inspection of FIG. 5 also reveals that output signals represented by response curve  162  (preamplifier devices configured according to the teaching of the present invention) degrade a maximum permitted amount ΔdB at a frequency f 2 . It is this decreasing, or “rolling off” of response curves from a maximum level toward zero that is the “roll off” of response referred to earlier in connection with describing the effect of inductor  108  (FIG. 4). Frequency f 2  is a higher frequency than frequency f 1 , indicating that single supply preamplifier apparatuses configured according to the teaching of the present invention can operate at higher speeds than prior art single supply preamplifier apparatuses.  
         [0029]    It is to be understood that, while the detailed drawings and specific examples given describe preferred embodiments of the invention, they are for the purpose of illustration only, that the apparatus and method of the invention are not limited to the precise details and conditions disclosed and that various changes may be made therein without departing from the spirit of the invention which is defined by the following claims: