Abstract:
An H-type bridge circuit having transistors Tr1 to Tr4, resistors R1 and R2 and a write head is disposed. A write current supplied from a current supply circuit having the transistor Tr1 and the resistor R1 flows through the write head and is received in a current receiving circuit having the resistor R2 and the transistor Tr4, and another write current supplied from a current supply circuit having the transistor Tr2 and the resistor R2 flows through the write head and is received in a current receiving circuit having the resistor R1 and the transistor Tr3. An impedance of the write head matches with an output impedance of the current supply circuit and matches with an input impedance of the current receiving circuit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a current driver circuit in which data of a magnetic signal is induced from a write current pre-amplified and is written to a magnetic storing device such as a magnetic disk driven by a hard disk drive (HDD) or a floppy disk drive (FDD).  
           [0003]    2. Description of Related Art  
           [0004]    [0004]FIG. 9 is a circuit view showing a conventional current driver circuit. In FIG. 9, Tr 1  and Tr 2  indicate current drivers formed of P-channel metal oxide semiconductor (PMOS) transistors respectively. Tr 3  and Tr 4  indicate current receivers formed of N-channel metal oxide semiconductor (NMOS) transistors respectively. Tr 5  to Tr 8  indicate transistors formed of NMOS transistors respectively. WH indicates a write head for inducing a magnetic signal from a write current. CS indicates a current source for supplying a reference current Iref. A voltage of a high voltage source is applied to gates of the current receivers Tr 3  and Tr 4  and the transistor Tr 7  through the current source CS so as to turn on the current receivers Tr 3  and Tr 4  and the transistor Tr 7 .  
           [0005]    [0005]FIG. 10 is a timing chart showing an operation of the conventional current driver circuit.  
           [0006]    Next, an operation of the conventional current driver circuit will be described below.  
           [0007]    The conventional current driver circuit is shown in FIG. 9 as an example to write data of a magnetic signal to a magnetic disk driven by an HDD or the like. Each of the current drivers Tr 1  and Tr 2  functions as a switch. When a gate terminal A of the current driver Tr 1  is set to a low level of Vl 1 , the current driver Tr 1  is turned on. When a gate terminal B of the current driver Tr 2  is set to a low level of Vl 1 , the current driver Tr 2  is turned on. A current mirror circuit is composed of the current receivers Tr 3  and Tr 4  and the transistor Tr 7 . In cases where a high voltage of Vh2 is applied to a gate terminal of the transistor Tr 8 , the transistor Tr 8  is turned on, and the current mirror circuit functions as a current source circuit. When the transistor Tr 5  or Tr 6  is turned on, the current mirror circuit functioning as a current source circuit is operated. That is, a switching operation of the current source circuit is performed in the current receivers Tr 3  and Tr 4  according to the on-off of the transistor Tr 5  or Tr 6 .  
           [0008]    The gate terminals A and B of the current drivers and gate terminals C and D of the transistor Tr 5  or Tr 6  are set to levels respectively as shown in the flow chart of FIG. 10. In a time period T 1 , the current driver Tr 1  and the transistor Tr 6  are turned on. Therefore, when the current receiver Tr 4  is turned on in the switching operation, a write current Iwc flows through the write head WH in a right direction in FIG. 9. Also, in a time period T 2 , the current driver Tr 2  and the transistor Tr 5  are turned on. Therefore, when the current receiver Tr 3  is turned on in the switching operation, a write current Iwc flows through the write head WH in a left direction in FIG. 9. The write head WH is formed of a coil. Therefore, a magnetic field is induced in the write head WH due to the write current Iwc flowing through the write head WH, the direction of the magnetic field is changed according to the change of the flow direction of the write current, and a magnetic signal is produced. The write head WH is disposed near to a magnetic disk (not shown), and data of the magnetic signal is written to the magnetic disk.  
           [0009]    The current drivers Tr 1  and Tr 2  and the current receivers Tr 3  and Tr 4  are disposed in the inside of an integrated circuit, and the current drivers Tr 1  and Tr 2  and the current receivers Tr 3  and Tr 4  are connected with external devices (for example, the write head WH) through pins of the integrated circuit. Also, the write head WH is connected with pins of the integrated circuit through inter-connectors.  
           [0010]    Recently, a data transfer rate of the magnetic signal has been increased more and more in case of the use of the HDD or the like. To write data of the magnetic signal in the magnetic disk at a high transfer rate, it is required to rapidly change the flow direction of the write current Iwc. Therefore, to heighten the data transfer rate, it is required to shorten both a rise time Tr and a fall time Tf of the write current Iwc. Because an operation frequency has been recently increased, current driving performance of the transistors is temporarily increased during a flow direction changing operation of the write current Iwc. Therefore, the rise time Tr and the fall time Tf of the write current Iwc can be shortened. However, in cases where the impedance matching between the write head WH and a group of pins of the integrated circuit connected with the write head WH is not obtained, energy of the current generated in the integrated circuit is not perfectly consumed as the write current Iwc, and energy not consumed is returned to a current supply end as a reflected component. The returning of the reflected component is called an undershoot phenomenon.  
           [0011]    [0011]FIG. 11 shows a waveform view showing the undershoot phenomenon. In cases where a degree of the “undershoot” in the phenomenon exceeds a certain level, the magnetic field is not sufficiently induced to write data of the magnetic signal in the magnetic disk. As a result, when the data is read out from the magnetic disk, there is high probability that a reading-out error occurs. Therefore, to heighten the data transfer rate, it is required to solve the problem of the returning of the reflected component, and the impedance matching between the write head WH and the group of pins of the integrated circuit connected with the write head WH is necessary in the current driver circuit.  
           [0012]    In the conventional current driver circuit shown in FIG. 9, current flows through the current driver Tr 1  or Tr 2  functioning as an MOS transistor switch, the write head WH and the line of the current source circuit (that is, a group of the current receiver Tr 3  and the transistor Tr 5  or a group of the current receiver Tr 4  and the transistor Tr 6 ) in that order. An output impedance of the current driver Tr 1  or Tr 2  is equal to 1/(β×(Vgs−Vth)), and an input impedance of the current source circuit is equal to 1/(λ×Id). Here, Id denotes a drain current. Vgs denotes a difference in voltage between a gate and a source of the current driver Tr 1  or Tr 2 . Vth denotes a threshold voltage of the current driver Tr 1  or Tr 2 . β and λ are described later. Though the output impedance of the current driver Tr 1  or Tr 2  and the input impedance of the current source circuit depend on process and bias conditions, the input impedance of the current source circuit is considerably higher than the output impedance of the switch.  
           [0013]    An example of an output impedance of the MOS transistor switch and an example of the input impedance of the current source circuit are described. An output resistance of the MOS transistor switch is expressed by Rout(SF), and an input resistance of the current source circuit is expressed by Rin(CS).  
               Rout        (   SF   )       =     1   /     (     β   ×     (     Vgs   -   Vth     )       )                   =     1   /     (     k   ×     (     W   /   L     )     ×     (     Vgs   -   Vth     )       )                           k   =     μ                 n   ×   C                 ox                 =     μ                 n   ×   ɛ                   ox   /   t                   ox                                 
 
           [0014]    Here, W denotes a gate width, L denotes a gate length, μn denotes a mobility of electrons, Cox denotes a fixed capacitance of an oxide film (SiO 2 ), ∈ox denotes a dielectric constant of the oxide film, and tox denotes a thickness of the oxide film. In the 0.25 μm process, μn=0.05 m 2 /(V*s), ∈ox=34.5 pF/m and tox=6 nm are satisfied. Therefore, k=290 μA/V 2  is obtained. In cases where W=500 μm and L=0.25 μm are satisfied, W/L=2000 is obtained. When the write current Iwc set to 50 mA, Vgs set to 2.5 V and Vth set to 0.7 V are satisfied, Rout(SF)=1.4Ω is obtained. 
             Rin ( CS )=1/(λ× Iwc )) 
           λ=(½ L ) {square root}{square root over ( )}(2 ∈s/ ( q×Nimp× ( Vds−Veff+φo ))) 
           [0015]    Here, ∈s denotes a dielectric constant of silicon, q denotes an electric charge, Nimp denotes a density of impurities, Vds denotes an electric potential difference between the drain and the source, Veff denotes an effective voltage indicated by an electric potential difference between Vgs (an electric potential difference between gate and source) and Vth (a threshold voltage), and φo denotes a built-in potential. In case of ∈s=Ks×∈o=104 pF/m, Nimp=1×10 23  m −3 , Vds=2 V, Veff=Vgs−Vth=0.5 V and φo=0.9 V, λ=0.15 V −1  is obtained. Therefore, Rin(CS)=133Ω is obtained.  
           [0016]    Therefore, the input resistance of the current source circuit is considerably higher than the output resistance of the source follower (Rout(SF)&lt;&lt;Rin(CS)).  
           [0017]    Because the conventional current driver circuit has the above-described configuration, the output impedance of the current driver Tr 1  or Tr 2  differs from the input impedance of the current source circuit. Therefore, it is difficult to obtain both the impedance matching at a connection point between the current driver Tr 1  or Tr 2  and the write head WH and the impedance matching at a connection point between the write head WH and the current source circuit. As a result, when data of the magnetic signal is written to the magnetic disk, “undershoot” undesirably occurs, and a problem has arisen that data of the magnetic signal cannot be accurately written to the magnetic disk.  
         SUMMARY OF THE INVENTION  
         [0018]    An object of the present invention is to provide, with due consideration to the drawbacks of the conventional current driver circuit, a current driver circuit in which data is accurately written at high data transfer rate.  
           [0019]    The object is achieved by the provision of a current driver circuit which includes a first switching transistor of which one end is connected with a high electric potential power source and of which the other end is connected with a first end of a first resistor, a second switching transistor of which one end is connected with the high electric potential power source and of which the other end is connected with a first end of a second resistor, a third switching transistor of which one end is connected with a low electric potential power source and of which the other end is connected with the first end of the first resistor, a fourth switching transistor of which one end is connected with the low electric potential power source and of which the other end is connected with the first end of the second resistor, and an inductance element of which both ends are connected with a second end of the first resistor and a second end of the second resistor respectively.  
           [0020]    Here, a current supply circuit includes a first group of the first switching transistor and the first resistor or a second group of the second switching transistor and the second resistor, and a current receiving circuit includes a first group of the second resistor and the fourth switching transistor or a second group of the first resistor and the third switching transistor. In this case, the inductance element, the current supply circuit and the current receiving circuit are set so as to match both an output impedance of the current supply circuit and an input impedance of the current receiving circuit with an impedance of the inductance element.  
           [0021]    Accordingly, even though a current flowing though the inductance element is changed at high frequency to accurately write data to a magnetic storing device at high data transfer rate, the occurrence of “undershoot” can be prevented, and data can be accurately written at high data transfer rate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a circuit view showing a current driver circuit according to a first embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is a timing chart showing an operation of the current driver circuit shown in FIG. 1;  
         [0024]    [0024]FIG. 3 is a circuit view showing a current driver circuit according to a second embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a circuit view showing a current driver circuit according to a third embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a waveform view showing a flyback voltage phenomenon which occurs in the current driver circuit shown in FIG. 4;  
         [0027]    [0027]FIG. 6 is a circuit view showing a current driver circuit according to a fourth embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is a timing chart showing an operation of the current driver circuit shown in FIG. 6;  
         [0029]    [0029]FIG. 8 is a circuit view showing a current driver circuit according to a fifth embodiment of the present invention;  
         [0030]    [0030]FIG. 9 is a circuit view showing a conventional current driver circuit;  
         [0031]    [0031]FIG. 10 is a timing chart showing an operation of the conventional current driver circuit shown in FIG. 9; and  
         [0032]    [0032]FIG. 11 shows a waveform view showing an undershoot phenomenon. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Embodiments of the present invention will now be described with reference to the accompanying drawings.  
         [0034]    Embodiment 1  
         [0035]    [0035]FIG. 1 is a circuit view showing a current driver circuit according to a first embodiment of the present invention.  
         [0036]    In FIG. 1, Tr 1  indicates a first switching transistor having a gate terminal A, Tr 2  indicates a second switching transistor having a gate terminal B, Tr 3  indicates a third switching transistor having a gate terminal C, and Tr 4  indicates a fourth switching transistor having a gate terminal D. The switching transistors Tr 1  and Tr 2  disposed on the driver side are formed of PMOS transistors respectively, and the switching transistors Tr 3  and Tr 4  disposed on the receiver side are formed of NMOS transistors respectively. WH indicates a write head (or an inductance element) formed of an inductance coil, R 1  indicates a first resistor having a resistance of Rh, and R 2  indicates a second resistor having a resistance of Rh. An H-type bridge circuit comprises the switching transistors Tr 1  to Tr 4 , the write head WH and the resistors R 1  and R 2 . Vo indicates a power source, V 1  indicates a tracking power source, V 2  indicates a tracking power source, Rv 1  indicates an output resistor of the tracking power source V 1 , and Rv 2  indicates an output resistor of the tracking power source V 2 .  
         [0037]    [0037]FIG. 2 is a timing chart showing an operation of the current driver circuit.  
         [0038]    Next, an operation of the current driver circuit will be described below.  
         [0039]    As shown in FIG. 2, in a time period T 1  and a time period T 3 , the gate terminal A of the transistor Tr 1  is set to a low level of Vl1, the gate terminal B of the transistor Tr 2  is set to a high level of Vh1, the,gate terminal C of the transistor Tr 3  is set to a low level of Vl2, and the gate terminal D of the transistor Tr 4  is set to a high level of Vh2. In this case, the transistors Tr 1  and Tr 4  are turned on, the transistors Tr 2  and Tr 3  are turned off, and a write current Iwc flows through the tracking power source V 1 , the first switching transistor Tr 1 , the first resistor R 1 , the write head WH, the second resistor R 2 , the fourth switching transistor Tr 4  and the tracking power source V 2  in that order. Therefore, the write current Iwc flows through the write head WH in the right direction in FIG. 1. Also, in a time period T 2  and a time period T 4 , the gate terminal A of the transistor Tr 1  is set to the high level of Vh1, the gate terminal B of the transistor Tr 2  is set to the low level of Vl1, the gate terminal C of the transistor Tr 3  is set to the high level of Vh2, and the gate terminal D of the transistor Tr 4  is set to the low level of Vl2. In this case, the transistors Tr 1  and Tr 4  are turned off, the transistors Tr 2  and Tr 3  are turned on, and a write current Iwc flows through the tracking power source V 1 , the second switching transistor Tr 2 , the second resistor R 2 , the write head WH, the first resistor R 1 , the third switching transistor Tr 3  and the tracking power source V 2  in that order. Therefore, the write current Iwc flows through the write head WH in the left direction in FIG. 1.  
         [0040]    In this case, Vl1 is set to a sufficiently low voltage on condition that the channel of each transistor Tr 1  or Tr 2  is strongly changed to an inversion layer only in the linear region, and Vh2 is set to a sufficiently high voltage on condition that the channel of each transistor Tr 3  or Tr 4  is strongly changed to an inversion layer only in the linear region. Therefore, in cases where the switching operation for the transistors Tr 1  to Tr 4  is performed in the time periods T 1  to T 4  in that order, the transistors Tr 1  to Tr 4  are turned on and off in the linear region, and the write current Iwc flows through the write head WH in the right direction and the left direction alternately, a magnetic field is induced by the write current Iwc, the intensity of the magnetic field is changed while changing the flow direction of the write current Iwc, and data indicated by the changing magnetic field is written in a magnetic disk (not shown).  
         [0041]    The voltage values of the tracking power sources V 1  and V 2  are determined so as to set the write current Iwc to a value required of the write head WH. The write current Iwc flowing in the right direction is expressed according to an equation (1). 
           Iwc= ( V   1   +V   2   −Vds   1   −Vds   4 )/(2 Rh+Rv   1  + Rv   2   +Rwh )  (1) 
         [0042]    Here, V 1  and V2 denote the voltage values of the tracking power sources V 1  and V2 respectively, Vds1 denotes an electric potential difference between the drain and the source in the first switching transistor Tr 1 , Vds4 denotes an electric potential difference between the drain and the source in the fourth switching transistor Tr 4 , Rv 1  and Rv 2  denote output resistance values of the tracking power sources V 1  and V 2  respectively and are determined according to the Thevenins theorem, and Rwh denotes a resistance value of the write head WH.  
         [0043]    Because the transistors Tr 1  and Tr 4  are turned on only in the linear region, a drain current Id of the transistor Tr 1  or Tr 4  is expressed according to an equation (2). 
           Id= ( k/ 2)×( W/L )×(2( Vgs−Vth )× Vds−Vds   2 )  (2) 
         [0044]    In cases where the electric potential difference Vds between the drain and the source is sufficiently low in the transistors Tr 1  and Tr 4 , the second degree term Vds 2  can be omitted. Therefore, an output resistance of the transistor Tr 1  or Tr 4  is equivalent to a resistance Ron of a resistive element.  
                   Ron   =     Vds   /   Id                 =     1   /     (     k   ×     (     W   /   L     )     ×     (     Vgs   -   Vth     )       )                     (   3   )                               
 
         [0045]    Because the equation (3) is also satisfied for the transistors Tr 2  and Tr 3  set to the “on” state in the time period T 2  or T 4 , an equivalent resistance of a transistor TrX (representing the transistors Tr 1  to Tr 4 ) is expressed by Ron(TrX). In this case, the write current Iwc expressed according to the equation (1) can be rewritten according to an equation (4). 
           Iwc= ( V   1 + V   2 )/(2 Rh+Rv   1   +Rv   2   +Rwh+Ron ( Tr   1 )+ Ron ( Tr   4 ))  (4) 
         [0046]    A current supply circuit placed on the driver side comprises the tracking power source V 1 , the first switching transistor Tr 1  and the first resistor R 1 . In cases where output impedances of the tracking power sources V 1  and V2 are expressed by Zv1 and Zv2 respectively, an output impedance Z 1  of the current supply circuit is expressed according to an equation (5). 
           Z   1 = Zv   1   +Rh+Ron ( Tr   1 )  (5) 
         [0047]    Also, a current receiving circuit placed on the receiver side comprises the second resistor R 2 , the fourth switching transistor Tr 4  and the tracking power source V2, and an input impedance Z 2  of the current receiving circuit is expressed according to an equation (6). 
           Z   2   =Zv   2   +Rh+Ron ( Tr   4 )  (6) 
         [0048]    Therefore, in cases where the ratio W/L of the gate width to the gate length in each of the transistors Tr 1  and Tr 4  is appropriately set and the resistance values Rh, Rv1 and Rv2 are appropriately adjusted, the impedance matching between the output impedance Z 1  of the current supply circuit and an input impedance Zwh of the write head WH can be obtained, and the impedance matching between an output impedance Zwh of the write head WH and the input impedance Z 2  of the current receiving circuit can be obtained. Here, the output impedance Z 1  of the current supply circuit denotes an impedance of the current supply circuit seen from a connection point between the first resistor R 1  and the write head WH, and the input impedance Zwh of the write head WH denotes an impedance of the write head WH seen from the connection point. Also, the output impedance Zwh of the write head WH denotes an impedance of the write head WH seen from a connection point between the second resistor R 2  and the write head WH, and the input impedance Z 2  of the current receiving circuit denotes an impedance of the current receiving circuit seen from the connection point.  
         [0049]    Accordingly, a magnetic field can be induced by the write current Iwc alternately changed in the right direction and the left direction at high frequency, and data of a magnetic signal indicated by the magnetic field can be written to a magnetic disk (not shown) in accurate at high data transfer rate.  
         [0050]    Embodiment 2  
         [0051]    [0051]FIG. 3 is a circuit view showing a current driver circuit according to a second embodiment of the present invention. The constituent elements, which are the same as those shown in FIG. 1, are indicated by the same reference numerals as those of the constituent elements shown in FIG. 1, and additional description of those constituent elements is omitted.  
         [0052]    In FIG. 3, Tr 5 , Tr 8  and Tr 9  indicate a fifth transistor, an eighth transistor and a ninth transistor respectively. The transistors Tr 5 , Tr 8  and Tr 9  are formed of NMOS transistors respectively. Tr 6 , Tr 7  and Tr 10  indicate a sixth transistor, a seventh transistor and a tenth transistor respectively. The transistors Tr 6 , Tr 7  and Tr 10  are formed of PMOS transistors respectively. CS 1  and CS 2  indicate current sources respectively. R 3  and R 4  indicate resistors having resistance values R 3  and R 4  respectively.  
         [0053]    Each of the transistors Tr 9  and Tr 10  is formed in a diode connection in which the gate and the drain are short-circuited each other. The source of the ninth transistor Tr 9  is connected to the source of the seventh transistor Tr 7 , and the source of the tenth transistor Tr 10  is connected to the source of the eighth transistor Tr 8 . The gate voltage Vl1 is applied to the gate of the seventh transistor Tr 7 , and the gate voltage Vh2 is applied to the gate of the eighth transistor Tr 8 . Therefore, the transistors Tr 7  and Tr 8  are turned on only in the linear region in the same manner as in the transistors Tr 1  to Tr 4 . In the fifth transistor Tr 5 , the drain is connected with a high potential power source Vcc, the gate is connected with a node at which the gain and the drain of the ninth transistor Tr 9  are connected with each other, and the source is connected to the sources of the transistors Tr 1  and Tr 2 . In the sixth transistor Tr 6 , the drain is connected with a low potential power source Vee, the gate is connected with a node at which the gain and the drain of the tenth transistor Tr 10  are connected with each other, and the source is connected to the sources of the transistors Tr 3  and Tr 4 .  
         [0054]    A fixed current Iref is supplied from the current source CS 1  to the drain of the ninth transistor Tr 9 , and the fixed current Iref sent to the drain of the tenth transistor Tr 10  is received in the current source CS 2 . One end of the resistor R 3  is connected with the drain of the seventh transistor Tr 7 , and the other end of the resistor R 3  is connected with the power source Vo. One end of the resistor R 4  is connected with the drain of the eighth transistor Tr 8 , and the other end of the resistor R 4  is connected with the power source Vo.  
         [0055]    Next, an operation of the current driver circuit will be described below.  
         [0056]    As shown in FIG. 3, the fixed current Iref flows through the ninth transistor Tr 9 , the seventh transistor Tr 7 , the resistor R 3 , the resistor R 4 , the eighth transistor Tr 8  and the tenth transistor Tr 10  in that order. Therefore, an electric potential difference Vrefp between the drain of the ninth transistor Tr 9  and the other end of the resistor R 3  is expressed according to an equation (7). 
           V refp= R 3 ×I ref+ Vds   7   +Vgs   9   (7) 
         [0057]    Here, VdsX denotes an electric potential difference between the drain and the source in the transistor TrX, VgsX denotes an electric potential difference between gate and source in the transistor TrX, and the transistor TrX (X=1 to 10) represents the transistor Tr 1  to Tr 10 .  
         [0058]    Also, an electric potential difference Vrefn between the other end of the resistor R 4  and the drain of the eighth transistor Tr 8  is expressed according to an equation (8). 
           V refn= R   4   ×I ref+ Vds   8   +Vgs   10   (8) 
         [0059]    Therefore, an electric potential difference between the gates of the transistors Tr 5  and Tr 6  is expressed by Vrefp+Vrefn.  
         [0060]    When the transistors Tr 1  and Tr 4  are set to the “on” state while setting the transistors Tr 2  and Tr 3  to the “off” state, a write current Iwc is expressed according to an equation (9).  
                   Iwc   =              (     Vrefp   +   Vrefn   -   Vgs5   -   Vds1   -   Vds4   -   Vgs6     )     /                          (       2      Rh     +   Rwh     )                 =            (         (     R3   +   R4     )     ×   Iref     +   Vds7   +   Vds8   +   Vgs9   +   Vgs10   -                              Vgs5   -   Vds1   -   Vds4   -   Vgs6     )     /     (       2      Rh     +   Rwh     )                   (   9   )                               
 
         [0061]    In cases where the transistors Tr 1 , Tr 4 , Tr 5 , Tr 6 , Tr 7 , Tr 8 , Tr 9  and Tr 10  are formed so as to satisfy Vds 1 =Vds 7 , Vds 4 =Vds 8 , Vgs 6 =Vgs 10  and Vgs 5 =Vgs 9 , the equation (9) can be rewritten to an equation (10). 
           Iwc= (( R   3   +R   4 )/(2 Rh+Rwh ))× I ref  (10) 
         [0062]    In this case, the voltage at the source terminal of the fifth transistor Tr 5  corresponds to the voltage value V 1  of the tracking power source V 1  shown in FIG. 1, and the voltage at the source terminal of the sixth transistor Tr 6  corresponds to the voltage value V 2  of the tracking power source V 2  shown in FIG. 1. Also, an output impedance Zv 1  of the source follower using the fifth transistor Tr 5  corresponds to the output impedance Rv 1  of the tracking power source V 1 , and an output impedance Zv 2  of the source follower using the sixth transistor Tr 6  corresponds to the output impedance Rv 2  of the tracking power source V 2 . Therefore, the output impedances Zv 1  and Zv 2  are expressed according to equations (11) and (1).  
                   Zv1   =       d        (   Vgs5   )       /     d        (   Id   )                     =       1   /                      (     2                 β                   (   Tr5   )     ×   Iwc     )                     (   11   )                     Zv2   =       d        (   Vgs6   )       /     d        (   Id   )                     =       1   /                      (     2                 β                   (   Tr6   )     ×   Iwc     )                     (   12   )                               
 
         [0063]    Here, β(Tr 5 ) and β(Tr 6 ) denote β for the transistors Tr 5  and Tr 6  respectively, and β is defined in the prior art.  
         [0064]    A current supply circuit placed on the driver side comprises the source follower of the fifth transistor Tr 5 , the first switching resistor Tr 1  and the first resistor R 1 . An output impedance Z 1  of the current supply circuit is expressed according to an equation (13) by referring to the equation (5). 
           Z   1 = Rh+Ron ( Tr   1 )+ Zv   1   (13) 
         [0065]    Also, a current receiving circuit placed on the receiver side comprises the second resistor R 2 , the fourth switching transistor Tr 4  and the source follower of the sixth transistor Tr 6 , and an input impedance Z 2  of the current receiving circuit is expressed according to an equation (14) by referring to the equation (6). 
           Z   2 = Rh+Ron ( Tr   4 )+ Zv   2   (14) 
         [0066]    Therefore, in cases where the ratios W/L in the transistors Tr 1  to Tr 4  and the values Rh, Zv1 and Zv2 are appropriately set, the relationship Z 1 =Z 2 =Zwh/2 is obtained. As a result, the impedance matching between the output impedance Z 1  of the current supply circuit and the input impedance Zwh of the write head WH can be obtained, and the impedance matching between the output impedance Zwh of the write head WH and the input impedance Z 2  of the current receiving circuit can be obtained.  
         [0067]    Accordingly, data can be written to a magnetic disk (not shown) in accurate at high data transfer rate.  
         [0068]    Embodiment 3  
         [0069]    [0069]FIG. 4 is a circuit view showing a current driver circuit according to a third embodiment of the present invention. The constituent elements, which are the same as those shown in FIG. 3, are indicated by the same reference numerals as those of the constituent elements shown in FIG. 3, and additional description of those constituent elements is omitted.  
         [0070]    In FIG. 4, Tr 11 , Tr 12  and Tr 15  indicate gate-grounded transistors formed of PMOS transistors respectively. A gate electric potential V 3  is applied to the gates of the gate-grounded transistors Tr 11 , Tr 12  and Tr 15 . Tr 13 , Tr 14  and Tr 16  indicate gate-grounded transistors formed of NMOS transistors respectively. A gate electric potential V 4  is applied to the gates of the gate-grounded transistors Tr 13 , Tr 14  and Tr 16 . The source of the gate-grounded transistor Tr 11  is connected with the drain of the first switching transistor Tr 1  to reduce the electric potential difference Vds1 between the drain and the source of the transistor Tr 1 . The source of the gate-grounded transistor Tr 12  is connected with the drain of the second switching transistor Tr 2  to reduce the electric potential difference Vds2 between the drain and the source of the transistor Tr 2 . The source of the gate-grounded transistor Tr 13  is connected with the drain of the third switching transistor Tr 3  to reduce the electric potential difference Vds3 between the drain and the source of the transistor Tr 3 . The source of the gate-grounded transistor Tr 14  is connected with the drain of the fourth switching transistor Tr 4  to reduce the electric potential difference Vds4 between the drain and the source of the transistor Tr 4 . The sources of the gate-grounded transistors Tr 15  and T 16  are connected with the drains of the transistor Tr 7  and Tr 8  respectively to set the relationship of electric potentials of the transistor Tr 7  and Tr 8  symmetrically to the relationship of electric potentials of the transistor Tr 1  and Tr 4  (or Tr 2  and Tr 3 ).  
         [0071]    Next, an operation of the current driver circuit will be described below.  
         [0072]    An impedance of the inductance component L of the write head WH for a frequency component f of the write current Iwc is equal to 2 πfL. Therefore, the higher the frequency f, the higher the impedance. In cases where the flow direction of the write current Iwc is changed at higher frequency to heighten the frequency f, because the write current Iwc is changed according to a very small time constant, the impedance of the write head WH is considerably increased.  
         [0073]    [0073]FIG. 5 is a waveform view showing a flyback voltage phenomenon.  
         [0074]    Because the impedance of the write head WH is considerably increased, as shown in FIG. 5, a high voltage called a flyback voltage is applied between both ends P and Q of the write head WH each time the flow direction of the write current Iwc is changed. Because the flyback voltage is applied to the switching transistors Tr 1  to Tr 4  through the resistors R 1  and R 2 , protective transistors are required for the switching transistors Tr 1  to Tr 4  according to the values Vds of the switching transistors Tr 1  to Tr 4 .  
         [0075]    As shown in FIG. 4, an electric potential difference Vrefp between the drain of the ninth transistor Tr 9  and the other end of the resistor R 3  is expressed according to an equation (15). 
           V refp= R   3   ×I ref+ Vds   7   +Vgs   9   +Vds   15   (15) 
         [0076]    Here, Vds15 denotes an electric potential difference between the drain and the source in the transistor Tr 15 .  
         [0077]    Also, an electric potential difference Vrefn between the other end of the resistor R 4  and the drain of the eighth transistor Tr 8  is expressed according to an equation (16). 
           V refn= R   4   ×I ref +Vds   8   +Vgs   10   +Vds   16   (16) 
         [0078]    Here, Vds16 denotes an electric potential difference between the drain and the source in the transistor Tr 16 .  
         [0079]    When the switching transistors Tr 1  and Tr 4  are set to the “on” state while setting the switching transistors Tr 2  and Tr 3  to the “off” state, a write current Iwc is expressed according to an equation (17) similar to the equation (9).  
                   Iwc   =            (     Vrefp   +   Vrefn   -   Vgs5   -   Vds1   -   Vds4   -   Vgs6   -                              Vds11   -   Vds14     )     /     (       2      Rh     +   Rwh     )                 =            (         (     R3   +   R4     )     ×   Iref     +   Vds7   +   Vds8   +   Vgs9   +   Vgs10   -                            Vgs5   -   Vds1   -   Vds4   -   Vgs6   -   Vds11   -   Vds14   +                            Vds15   +   Vds16     )     /     (       2      Rh     +   Rwh     )                   (   17   )                               
 
         [0080]    In cases where the transistors Tr 1 , Tr 4 , Tr 5 , Tr 6 , Tr 7 , Tr 8 , Tr 9 , Tr 10 , Tr 11 , Tr 14 , Tr 15  and Tr 16  are formed so as to satisfy Vds1=Vds7, Vds4=Vds8, Vgs6=Vgs10, Vgs5=Vgs9, Vds11=Vds15 and Vds14=Vds16, the equation (17) can be rewritten to an equation (18). 
           Iwc= (( R   3   +R   4 )/(2 Rh+Rwh ))× I ref  (18) 
         [0081]    The equation (18) is the same as the equation (10). A current supply circuit placed on the driver side comprises the source follower of the fifth transistor Tr 5 , the first switching resistor Tr 1 , the transistor Tr 11  and the first resistor R 1 . Therefore, an output impedance Z 1  of the current supply circuit is expressed according to an equation (19) similar to the equation (13). 
           Z   1   =Rh+Ron ( Tr   1 )+ Ron ( Tr   11 )+ Zv 1  (19) 
         [0082]    Also, a current receiving circuit placed on the receiver side comprises the second resistor R 2 , the transistor Tr 14 , the fourth switching transistor Tr 4  and the source follower of the sixth transistor Tr 6 . Therefore, an input impedance Z 2  of the current receiving circuit is expressed according to an equation (20) similar to the equation (14). 
           Z   2   =Rh+Ron ( Tr   4 )+ Ron ( Tr   14 )+ Zv   2   (20) 
         [0083]    Therefore, in cases where the ratios W/L in the transistors Tr 1  to Tr 4  and Tr 11  to Tr 14  and the values Rh, Zv1 and Zv2 are appropriately set, the relationship Z 1 =Z 2 =Zwh/2 is obtained. As a result, the impedance matching between the output impedance Z 1  of the current supply circuit and the input impedance Zwh of the write head WH can be obtained, and the impedance matching between the output impedance Zwh of the write head WH and the input impedance Z 2  of the current receiving circuit can be obtained.  
         [0084]    Accordingly, data can be written to a magnetic disk (not shown) in accurate at high data transfer rate.  
         [0085]    Embodiment 4  
         [0086]    [0086]FIG. 6 is a circuit view showing a current driver circuit according to a fourth embodiment of the present invention. The constituent elements, which are the same as those shown in FIG. 3, are indicated by the same reference numerals as those of the constituent elements shown in FIG. 3, and additional description of those constituent elements is omitted.  
         [0087]    In FIG. 6, Tr 17 , Tr 18 , Tr 21 , Tr 22 , Tr 25  and Tr 27  indicate transistors formed of PMOS transistors respectively, and Tr 19 , Tr 20 , Tr 23 , Tr 24 , Tr 26  and Tr 28  indicate transistors formed of NMOS transistors respectively. CS 3  and CS 4  indicate current sources respectively.  
         [0088]    A current source circuit has the transistors Tr 17 , Tr 18 , Tr 21 , Tr 22 , Tr 25  and Tr 27  disposed on the high electric potential side and the transistors Tr 19 , Tr 20 , Tr 23 , Tr 24 , Tr 26  and Tr 28  disposed on the low electric potential side. On the high electric potential side, the drain and the gate of the transistor Tr 25  are connected with each other in the diode connection. The gates of the transistors Tr 17 , Tr 18  and Tr 25  are connected with each other to be set to a common voltage. Therefore, a current mirror circuit comprises the transistors Tr 17 , Tr 18  and Tr 25 . Also, the gates of the transistors Tr 17 , Tr 18  and Tr 25  are connected with the current source CS 3  so that the transistors Tr 17 , Tr 18  and Tr 25  are set to the “on” state. The drains of the transistors Tr 21 , Tr 22  and Tr 27  are connected with the sources of the transistors Tr 17 , Tr 18  and Tr 25  respectively, and the sources of the transistors Tr 21 , Tr 22  and Tr 27  are connected with the high potential power source Vcc. A fixed low voltage V 13  is always applied to the gate of the transistor Tr 27  to always set the transistor Tr 27  to the “on” state. Therefore, the transistor Tr 27  functions as a resistive element.  
         [0089]    On the low electric potential side, the drain and the gate of the transistor Tr 26  are connected with each other in the diode connection. The gates of the transistors Tr 19 , Tr 20  and Tr 26  are connected with each other to be set to a common voltage. Therefore, a current mirror circuit comprises the transistors Tr 19 , Tr 20  and Tr 26 . Also, the gates of the transistors Tr 19 , Tr 20  and Tr 26  are connected with the current source CS 4  so that the transistors Tr 19 , Tr 20  and Tr 26  are set to the “on” state. The drains of the transistors Tr 23 , Tr 24  and Tr 28  are connected with the sources of the transistors Tr 19 , Tr 20  and Tr 26  respectively, and the sources of the transistors Tr 23 , Tr 24  and Tr 28  are connected with the low potential power source Vee. A fixed high voltage Vh4 is always applied to the gate of the transistor Tr 28  to always set the transistor Tr 28  to the “on” state. Therefore, the transistor Tr 28  functions as a resistive element.  
         [0090]    [0090]FIG. 7 is a timing chart showing an operation of the current driver circuit.  
         [0091]    Next, an operation of the current driver circuit will be described below.  
         [0092]    As shown in FIG. 6, the current source circuit comprising the transistors Tr 17  to Tr 28  is added to the current mirror circuit shown in FIG. 3 for the purpose of shortening the changing time (that is, the rise time Tr and the fall time Tf) required to change the flow direction of the write current Iwc. The current source circuit is operated only when the flow direction changing operation of the write current Iwc is started.  
         [0093]    As shown in FIG. 7, when the flow of the write current Iwc in the right direction in FIG. 6 is started in each of the time periods T 1  and T 3 , the transistors Tr 21  and Tr 23  are turned on and are set to the “on” state for a short time. Therefore, the transistor Tr 17  set to the “on” state functions as a current source, a boost current (or a pulse current) is supplied from the transistor Tr 17  to a node P between the resistor R 1  and the write head WH (or the left end of the write head WH in FIG. 6), a portion of the boost current is used in the write head WH to heighten the change of the write current Iwc from the left flow direction to the right flow direction, and the other portion of the boost current is received in the transistors Tr 19  and Tr 23  respectively set to the “on” state. Also, when the flow of the write current Iwc in the left direction in FIG. 6 is started in each of the time periods T 2  and T 4 , the transistors Tr 22  and Tr 24  are turned on and are set to the “on” state for a short time. Therefore, the transistor Tr 18  set to the “on” state functions as a current source, a boost current (or a pulse current) is supplied from the transistor Tr 18  to a node Q between the resistor R 2  and the write head WH (or the eight end of the write head WH in FIG. 6), a portion of the boost current is used in the write head WH to heighten the change of the write current Iwc from the right flow direction to the left flow direction, and the other portion of the boost current is received in the transistors Tr 20  and Tr 24  respectively set to the “on” state.  
         [0094]    Therefore, the changing time (that is, the rise time Tr or the fall time Tf) of the flow direction of the write current Iwc can be shortened as compared with that in the current driver circuit of the second embodiment shown in FIG. 3. Accordingly, the change of the flow direction of the write current Iwc can be heightened, and data can be accurately written to a magnetic disk (not shown) at a data transfer rate higher than that in the current driver circuit of the second embodiment.  
         [0095]    Here, in dependence on the resistance value Rh of the resistors R 1  and R 2 , the output impedance of the current mirror circuit is considerably larger than that of the source follower. Therefore, the impedance matching between the output impedance Z 1  of the current supply circuit and the input impedance Zwh of the write head WH obtained in the second embodiment can be preferably maintained, and the impedance matching between the output impedance Zwh of the write head WH and the input impedance Z 2  of the current receiving circuit obtained in the second embodiment can be preferably maintained.  
         [0096]    Embodiment 5  
         [0097]    [0097]FIG. 8 is a circuit view showing a current driver circuit according to a fifth embodiment of the present invention. The constituent elements, which are the same as those shown in FIG. 6, are indicated by the same reference numerals as those of the constituent elements shown in FIG. 6, and additional description of those constituent elements is omitted.  
         [0098]    In FIG. 8, Tr 29  and Tr 30  indicate gate-grounded transistors formed of PMOS transistors respectively, and Tr 31  and Tr 32  indicate gate-grounded transistors formed of NMOS transistors respectively. Vg 1  and Vg 2  indicate power sources respectively. The gates of the gate-grounded transistors Tr 29  and Tr 30  are connected with the power source Vg 1 , and the gates of the gate-grounded transistors Tr 31  and Tr 32  are connected with the power source Vg 2 . The source of the gate-grounded transistor Tr 29  is connected with the drain of the transistor Tr 17 , and the drain of the gate-grounded transistor Tr 29  is connected with the node P between the resistor R 1  and the write head WH (or the left end of the write head WH in FIG. 8). The source of the gate-grounded transistor Tr 30  is connected with the drain of the transistor Tr 18 , and the drain of the gate-grounded transistor Tr 30  is connected with the node Q between the resistor R 2  and the write head WH (or the right end of the write head WH in FIG. 8). The source of the gate-grounded transistor Tr 31  is connected with the drain of the transistor Tr 19 , and the drain of the gate-grounded transistor Tr 31  is connected with the left end of the write head WH. The source of the gate-grounded transistor Tr 32  is connected with the drain of the transistor Tr 20 , and the drain of the gate-grounded transistor Tr 32  is connected with the right end of the write head WH.  
         [0099]    Next, an operation of the current driver circuit will be described below.  
         [0100]    As is described in the third embodiment, the flyback voltage is applied between both ends P and Q of the write head WH each time the flow direction of the write current Iwc is changed. Because the flyback voltage is applied to the transistors Tr 17  to Tr 20 , protective transistors are required for the transistors Tr 17  to Tr 20  to reduce values Vds of the transistors Tr 17  to Tr 20 . Therefore, the transistors Tr 29  to Tr 32  are disposed as the protective transistors, and the electric potential difference Vds between the drain and the gate in each of the transistors Tr 17  to Tr 20  is reduced. Accordingly, adverse influence of the flyback voltage on the transistors Tr 17  to Tr 20  can be prevented.  
         [0101]    Here, because the drains of the transistors Tr 29  to Tr 32  are connected to the ends of the write head WH, output resistances of the transistors Tr 29  to Tr 32  are almost the same as that of the current source circuit comprising the transistors Tr 17  to Tr 28  and are considerably larger than an output impedance of a source follower. Therefore, the transistors Tr 29  to Tr 32  hardly influence on both the impedance matching between the output impedance Z 1  of the current supply circuit and the input impedance Zwh of the write head WH maintained according to the fourth embodiment and the impedance matching between the output impedance Zwh of the write head WH and the input impedance Z 2  of the current receiving circuit maintained according to the fourth embodiment.