BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to magnetic recording disk storage systems employing a preamplifier circuit for magnetoresistive (MR) elements. More particularly, the present invention relates to an improved preamplifier circuit for MR elements which has a high impedance so that the resultant system would not be sensitive to the series input inductance of the leads necessary to connect the MR element with the preamplifier. 
     2. Description of Related Art 
     A typical magnetic disk storage system includes at least one rotatable magnetic disk which is supported on a spindle and rotated by a disk drive motor. The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks on the disk. 
     At least one slider is positioned on the disk, each slider supporting one or more magnetoresistive elements for reading data from and writing data to the disks. As the disks rotate, the slider is moved radially in and out over the disk surface so that heads may access different portions of the disk where desired data is recorded. Each slider is attached to a positioner arm by a suspension. The suspension provides a slight spring force which biases the slider against the disk surface. Each positioner arm is attached to an actuator assembly. The actuator assembly often includes voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the current signals supplied by the actuator controller. 
     During operation of the disk drive system, the rotation of the disk generates an air bearing between the slider and the disk surface which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of the suspension and supports the slider off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk drive system are controlled in operation by control signals generated by a control unit. Control signals include, for example, control signals and internal clock signals. Typically, the control unit comprises logic control circuits, storage means and a microprocessor. The control unit generates control signals to control various system operations such as drive motor control signals and head position and seek control signals. The control signals provide the desired current profiles to optimally move and position the slider to the desired data track on the disk. Read and write signals are communicated to and from the read/write heads by means of a recording channel. 
     A need presented itself for a single ended (SE) preamplifier circuit for magnetoresistive (MR) elements which has a high input impedance with respect to the MR resistance so the resultant system would not be sensitive to the series input inductance of the leads necessary to connect the MR element with the preamplifier having low power dissipation. With former preamplifiers, all were low input impedance types, for example using BiCMOS or BiPolar technology, or differential, using BiPolar technology. The need became apparent to use a BiCMOS technology with a high input impedance, SE amplifier. 
     U.S. Pat. No. 5,323,278 (issued to Contreras et al.) was presented to solve the need for such a preamplifier which could work with very low supply voltages, for example, 3 volts. However, the circuit disclosed used a current bias for the MR element, and contained other limitations on noise performance, and offset performance. Other circuits found in the art include U.S. Pat. No. 5,548,453 which teaches a base-grounded amplifier with a single ended MR device; U.S. Pat. No. 5,444,579 which teaches a preamplifier that uses a current-mode amplifier; U.S. Pat. No. 5,619,386 which teaches a single ended MR element with a fast settling read amplifier; U.S. Pat. No. 5,122,915 teaches a low noise preamplifier for a single ended MR element; and U.S. Pat. No. 5,5443,979 teaches a preamplifier for common current flow in a transducer. 
     It is readily apparent that none of these references address the need for a high input impedance, single ended, low supply voltage MR pre-amplifier with dR/R response. Therefore, it can be seen that there is a need for a preamplifier circuit which requires low noise, low power supply operation, and dR/R response. 
     SUMMARY OF THE INVENTION 
     It is the object of the invention to provide a high input impedance, single ended (SE) preamplifier for an MR element capable of very low supply voltages and having low noise. 
     According to these objects, the present invention is a preampifier circuit for magnetoresistive (MR) elements which has a high imput impedance with respect to the MR resistance so that the resultant system would not be sensitive to the series input inductance of the leads necessary to connect the MR element with the preamplifier. With former preamplifiers, nearly all are low impedance types, (using either BiCMOS or BiPolar technology), or differential (using BiPolar technology). The present invention fills the need in the prior art by providing an SE preamplifier circuit using BiCMOS technology with a high input impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and advantages of the present invention, as well as the preferred mode of use, reference should be made to the following detailed description read in conjunction with the accompanying drawings. 
     FIG. 1 is a simplified perspective drawing of a magnetic recording disk drive system that the preamplifier circuit of the present invention could be used in; 
     FIG. 2 is a block diagram of a preamplifier in accordance with the present invention; and 
     FIG. 3 is a detailed circuit diagram of the gain stage of the preamplifier shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention is described in a preferred embodiment in the following description with reference to the figures, in which like numbers represent the same or similar elements. While this invention is described in terms of the best mode for achieving this invention&#39;s objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the invention. 
     FIG. 1, shows a simplified perspective view of a disk drive  20  embodying the present invention. As shown in FIG. 1, at least one rotatable magnetic disk  22  is supported on a spindle  26  and rotated by a disk drive motor  30 . The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks (not shown) on disk  22 . 
     At least one slider  24  is positioned on the disk  22 , each slider  24  supporting one or more magnetic read/write heads  34 . As the disks rotate, slider  24  is move radially in and out over disk surface  36  so that heads  34  may access different portions of the disk where desired data is recorded. Each slider  24  is attached to an actuator arm  32  by means of a suspension  28 . The suspension  28  provides a slight spring force which biases slider  24  against the disk surface  36 . Each actuator arm  32  is attached to an actuator means  42 . The actuator means as shown in FIG. 2 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller  46 . 
     During operation of the disk drive storage system, the rotation of disk  22  generates an air bearing between slider  24  and disk surface  36  which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension  28  and supports slider  24  off and slightly above the disk surface by a small, substantially constant spacing during normal operation. 
     The various components of the disk storage system are controlled in operation by control signals generated by control unit  46 , such as access control and internal clock signals. Typically, control unit  46  comprises logic control circuits, storage means and a microprocessor. The control unit  46  generates control signals to control various system operations such as drive motor control signals on line  38  and head position and seek control signals on line  44 . The control signals on line  44  provide the desired current profiles to optimally move and position slider  24  to the desired data track on disk  22 . Read and write signals are communicated to and from read/write heads  34  by means of recording channel  40 . 
     The above description of a typical magnetic disk storage system, and the accompanying illustration of FIG. 1 is for representation purposes only. It should be apparent that disk drive storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders. 
     Reference is now made to FIG. 2, which shows a block diagram of a preamplifier in accordance with the present invention. 
     MR element RMR  50  is biased into operation by a first feedback loop comprising differential amplifier  52  and transconductance amplifier  54 . Differential amplifier  52  amplifies the difference between the (DC) voltage imposed across said MR element  50  and a bias reference  56 . The bias reference is set to a desired MR element  50  bias voltage level. This difference is then converted into a current by transconductance amplifier  54  and fed into the positive terminal of the MR element  50 . The negative terminal of said MR element  50  connects to ground for protection against electostatic and conductive asperity events. Bias capacitor  58  stabilizes the first feedback loop creating a first low frequency pole in the system. 
     The signal generated from the MR element  50  while reading data from some media (such as a disk  22 ) is first amplified by gain stage  66 , the details of which are shown and described in FIG.  3 . Terminating resistor R 1  serves to electrically terminate the cable from said MR element  50  at the preamplifier. Input device Q 1  (PFET) amplifies the signal in a common-source configuration, where current source  60  shown on FIG. 2 serves to bias said input device into the proper operating point at the source for optimal noise figure. The source of the input device is de-coupled to ground by means of capacitor  62  shown in FIG.  2 . Capacitor  62 , along with the parallel impedance generated from said operating point, creates a second low frequency pole in the system. A first amplified signal then exists at gain resistor R 2  (shown in FIG. 3) which is then re-directed by transistor Q 2  (PFET) toward cascade transistor Q 3  (NPN). A second amplified signal is then seen at final gain resistor R 3  as an interim single-ended signal. The operating point of the branch of said first gain stage comprising R 3 , Q 3  and Q 2  is adjusted by the base of Q 3 , which is further controlled by a second feedback loop to maintain a predetermined voltage  64  at the collector of Q 3 . This operating point is a strong contributor for overall noise performance due to the relatively low gain which can be achieved by the branch of the first gain stage comprising Q 1  and R 2 . In other words, the noise figure for the system is strongly dependent upon the first two gain stages rather that the first gain stage as in more conventional preamplifier circuits. 
     The second feedback loop comprises gain stage  66  in the forward direction (described earlier) and a second transconductance stage  68  in the reverse direction. The second feedback loop is stabilized by capacitor  70 . The second transconductance stage amplifies the voltage difference between predertermined voltage  64  and the interim single-ended signal, thereby minimizing the difference and producing a quasi-differential signal between R 3  and the predetermined voltage  10 . Capacitor  70  serves to stabilize the second feedback loop and creates a third low frequency pole into the system. 
     Final gain stage  72  produces a differential signal output from said quasi-differential signal, and comprises one or more standard differential amplifier stages. These stages serve to bring the signal from said MR element to a useable level for subsequent signal processing, such as in a magnetic recording disk drive system. 
     While the invention has been described in terms of a single preferred embodiment with optimal features, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.
The present invention is a preampifier circuit for magnetoresistive (MR) elements which has a high imput impedance with respect to the MR resistance so that the resultant system would not be sensitive to the series input inductance of the leads necessary to connect the MR element with the preamplifier. With former preamplifiers, nearly all are low impedance types, (using either BiCMOS or BiPolar technology), or differential (using BiPolar technology). The present invention fills a need in the prior art by providing a preamplifier curcuit using BiCMOS technology with a high input impedance, and single ended (SE) topology for minimum power dissipation.