Abstract:
A write amplifier circuit in a magnetic storage system has a cross coupling circuit and an active damp circuit to supply an improved write current to the head writing the data onto the media within the magnetic storage system. The inclusion of the cross coupling circuit decreases a rise time and a fall time associated with the write current. The active damp circuit reduces the undershoot and ringing of the write current. Thus, the write amplifier circuit is suitable for high speed data storage writing applications requiring minimal distortion of the data written to a magnetic medium. The write amplifier circuit achieves these improvements in the waveform of the write current by incorporating circuit elements and using both a negative feedback path and a feedforward path. In particular, the cross coupling circuit provides a feedforward path within the write amplifier circuit to a first current which creates a second current that is proportional and greater than the first current such that the second current increases the write current available for the head. Similarly, the active damp circuit provides a negative feedback path from the output terminals of the write amplifier circuit to a third current which creates a fourth current that is proportional and greater than the third current such that the fourth current damps an undershoot and ringing associated with the write current.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) of the co-pending U.S. provisional application Ser. No. 60/160,858 filed on Oct. 21, 1999 and entitled “Architecture For A Hard Disk Drive Write Amplifier Circuit With Damping Control.” The provisional application Ser. No. 60/160,858 filed on Oct. 21, 1999 and entitled “Architecture For A Hard Disk Drive Write Amplifier Circuit With Damping Control” is also hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of write amplifier circuits within magnetic storage systems. More particularly, the present invention relates to write amplifier circuits supplying a write current and having circuitry to improve response time and quality when recording data on a magnetic storage system. 
     BACKGROUND OF THE INVENTION 
     A magnetic storage system, such as a conventional hard disk drive, is generally used for mass storage of data. Typically, the conventional hard disk drive includes a magnetic medium, an inductive element positioned near the surface of the magnetic medium, and a write amplifier circuit which provides a write current to the inductive element. The magnetic medium usually includes one or more disks composed of a metallic material such as an aluminum alloy. A magnetizable coating is deposited on the disk surface and serves as a data medium. 
     Generally, the inductive element comprises a head which writes data onto the disk as small magnetizations in the data medium by converting the write current into a magnetic field which magnetizes the surface area of the disk below the head. The small magnetizations align according to the generated magnetic field and a “1” is written. By inverting the polarity of the magnetic field, the small magnetizations are also aligned, but in another direction, thus a “0” is written. The polarity of the magnetic field is inverted by changing the direction of the write current supplied to the head. The head is generally a ferrite head or a thinfilm head. The thinfilm head typically is smaller and lighter in weight than the ferrite head. The thinfilm head can be positioned closer to the disk surface than the ferrite head, thus requiring a less intense magnetic field to write data to the disk. 
     FIG. 1 illustrates a schematic diagram of the conventional write amplifier circuit  100 . The conventional write amplifier circuit includes differential input signals WDX and WDY, a top switch driver  30 , a bottom switch driver  40 , output terminals HX and HY, and H-switch transistors Q 1 , Q 2 , Q 3 , and Q 4 . The head  50  is coupled to the output terminals HX and HY. 
     In practice, the differential input signals WDX and WDY determine whether the npn transistor Q 3  and the npn transistor Q 4  are turned on or whether the npn transistor Q 1  and the npn transistor Q 2  are turned on. If the transistors Q 3  and Q 4  are turned on, the write current Iw(t) travels from the emitter of the transistor Q 3  to the output terminal HX. From the output terminal HX, the write current Iw(t) enters the head  50  and then returns to the output terminal HY. From the output terminal HY, the write current Iw(t) enters the collector of the transistor Q 4 . In essence, the transistor Q 3  sources the write current Iw(t) while the transistor Q 4  sinks the write current Iw(t). 
     If the transistors Q 1  and Q 2  are turned on, the transistor Q 1  sources the write current Iw(t) while the transistor Q 2  sinks the write current Iw(t). However, the write current Iw(t) enters the head  50  through the output terminal HY and then returns to the output terminal HX. Hence, the direction of the write current Iw(t) through the head  50  is opposite of the direction described above with respect to the situation when the transistors Q 3  and Q 4  are turned on. This change in the direction of the write current Iw(t) facilitates writing data as a “1” and a “0” on the disk surface. 
     The top switch driver  30  defines the DC voltages of the output terminals HX and HY and controls the H-switch transistors Q 1  and Q 3 . The bottom switch driver  40  controls the H-switch transistors Q 2  and Q 4  and determines the DC current of the write current Iw(t). Additionally, the bottom switch driver  40  is coupled to a variable current source Iw/K. 
     The conventional write amplifier circuit  100  has a number of deficiencies. The write current Iw(t) supplied by the conventional write amplifier circuit  100  has a large undershoot and a long ringing. The undershoot and the ringing slow down the writing speed of the magnetic storage system, such as a hard disk drive, and distort the written data when the head converts the write current into a magnetic field. Therefore, the undershoot and the ringing affect the speed and the performance of a magnetic storage system including a conventional write amplifier circuit such as illustrated in FIG.  1 . 
     SUMMARY OF THE INVENTION 
     A write amplifier circuit in a magnetic storage system has a cross coupling circuit and an active damp circuit to supply an improved write current to the head writing the data onto the media within the magnetic storage system. The inclusion of the cross coupling circuit decreases a rise time and a fall time associated with the write current. The active damp circuit reduces the undershoot and ringing of the write current. Thus, the write amplifier circuit is suitable for high speed data storage writing applications requiring minimal distortion of the data written to a magnetic medium. The write amplifier circuit achieves these improvements in the waveform of the write current by incorporating circuit elements and using both a negative feedback path and a feedforward path. 
     In particular, the cross coupling circuit provides a feedforward path within the write amplifier circuit to a first current which creates a second current that is proportional and greater than the first current such that the second current increases the write current available for the head. 
     Similarly, the active damp circuit provides a negative feedback path from the output terminals of the write amplifier circuit to a third current which creates a fourth current that is proportional and greater than the third current such that the fourth current damps an undershoot and ringing associated with the write current. 
     In one aspect of the present invention, a cross coupling circuit for decreasing a switching response time of a write current supplied to an inductive element by a write amplifier circuit including a first driving circuit having an output terminal, a second driving circuit having a current amplifier, and a switching circuit coupled to the first and second driving circuits and to the inductive element, wherein the inductive element writes data to a magnetic medium, includes a first terminal coupled to the output terminal for providing a feedforward path to a first current, a second terminal coupled to the current amplifier for supplying the first current to the current amplifier such that a second current which is proportional to and greater than the first current is simultaneously formed in the switching circuit, wherein the switching circuit supplies the write current to the inductive element such that the second current increases the write current available for the inductive element and a feedforward element coupled to the first terminal and to the second terminal for controlling the first current. 
     In another aspect of the present invention, a damping circuit for reducing an undershoot and a settling time of a write current supplied to an inductive element by a write amplifier circuit including a driving circuit having a current amplifier and a switching circuit coupled to the driving circuit and having a first output terminal coupled to the inductive element and a second output terminal coupled to the inductive element, wherein the inductive element writes data to a magnetic medium, includes a first terminal coupled to the first and second output terminals for providing a negative feedback path to a first current, a second terminal coupled to the current amplifier for supplying the first current to the current amplifier such that a second current which is proportional to and greater than the first current is simultaneously formed in the switching circuit, wherein the switching circuit supplies the write current to the inductive element such that the second current damps a ringing associated with the write current and an impedance element coupled to the first terminal and to the second terminal for adjusting a waveform of the first current to enhance a damping function performed by the second current on the write current. 
     In still another aspect of the present invention, a write amplifier circuit for supplying a write current to an inductive element that writes data to a magnetic medium, the write amplifier circuit includes a switching circuit coupled to the inductive element for providing the write current to the inductive element, wherein the switching circuit includes a first output terminal coupled to the inductive element and a second output terminal coupled to the inductive element, a first driving circuit coupled to the switching circuit for driving the switching circuit, wherein the first driving circuit includes an output terminal, a second driving circuit coupled to the switching circuit for driving the switching circuit, wherein the second driving circuit includes a current amplifier and a cross coupling circuit coupled to the first driving circuit and to the second driving circuit for decreasing a switching response time of the write current. 
     In yet another aspect of the present invention, a write amplifier circuit for supplying a write current to an inductive element that writes data to a magnetic medium, the write amplifier circuit includes a switching circuit coupled to the inductive element for providing the write current to the inductive element, wherein the switching circuit includes a first output terminal coupled to the inductive element and a second output terminal coupled to the inductive element, a first driving circuit coupled to the switching circuit for driving the switching circuit, wherein the first driving circuit includes an output terminal, a second driving circuit coupled to the switching circuit for driving the switching circuit, wherein the second driving circuit includes a current amplifier and a damping circuit coupled to the switching circuit and to the second driving circuit for reducing an undershoot and a settling time of the write current, the damping circuit including a feedback input coupled to the first and second output terminals for providing a negative feedback path to a first current, a feedback output coupled to the current amplifier for supplying the first current to the current amplifier such that a second current which is proportional to and greater than the first current is simultaneously formed in the switching circuit, wherein the second current damps a ringing associated with the write current and an impedance element coupled to the feedback input and to the feedback output for adjusting a waveform of the first current to enhance a damping function performed by the second current on the write current. 
     In still yet another aspect of the present invention, a magnetic storage system includes a magnetic medium for storing data, an inductive element for writing data to the magnetic medium by converting a write current to a magnetic field and a write amplifier circuit for supplying the write current to the inductive element, the write amplifier circuit including a switching circuit coupled to the inductive element for providing the write current to the inductive element, wherein the switching circuit includes a first output terminal coupled to the inductive element and a second output terminal coupled to the inductive element, a first driving circuit coupled to the switching circuit for driving the switching circuit, wherein the first driving circuit includes an output terminal, a second driving circuit coupled to the switching circuit for driving the switching circuit, wherein the second driving circuit includes a current amplifier and a cross coupling circuit coupled to the first driving circuit and to the second driving circuit for decreasing a switching response time of the write current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic diagram of a write amplifier circuit according to the prior art. 
     FIG. 2 illustrates a plurality of waveforms representing the write current supplied by different write amplifier circuits. 
     FIG. 3 illustrates a schematic diagram of a write amplifier circuit according to the present invention. 
     FIG. 4 illustrates a detailed schematic diagram of the preferred embodiment of the write amplifier circuit of the present invention. 
     FIGS. 5A-C illustrate waveforms representing the write current supplied by a write amplifier circuit of the prior art. 
     FIGS. 6A-C illustrate waveforms representing the write current supplied by a write amplifier circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 illustrates a schematic diagram of a write amplifier circuit  300  according to the present invention. For clarity, unchanged components from the write amplifier of FIG. 1 retain the same labels. As shown in FIG. 3, the write amplifier circuit  300  incorporates a cross coupling circuit  80  and an active damp circuit  90  in addition to the components discussed above and included within the write amplifier circuit of FIG.  1 . 
     The cross coupling circuit  80  preferably includes a first terminal, a second terminal, and a feedforward element  89 . The first terminal is coupled in a differential arrangement to a differential output terminal  33  of the top switch driver  30  and includes the nodes  85  and  86 . The second terminal  87  is coupled in a differential arrangement to the bottom switch driver  40  and includes the nodes  87  and  88 . Within the bottom switch driver  40 , the cross coupling circuit  80  is coupled to a current amplifier (not shown). The feedforward element  89  is coupled to the nodes  85  and  86  of the first terminal and to the nodes  87  and  88  of the second terminal and is configured as a differential circuit. The feedforward element  89  preferably includes a first resistor Rc 1 , a second resistor Rc 2 , a first capacitor Cc 1 , and a second capacitor Cc 2 . The first resistor Rc 1  and the first capacitor Cc 1  are coupled in series and form a first half circuit of the differential feedforward circuit. The second resistor Rc 2  and the second capacitor Cc 2  are coupled in series and form a second half circuit of the differential feedforward circuit. The feedforward element  89  further includes a first voltage buffer  75 A and a second voltage buffer  75 B. The first voltage buffer  75 A is coupled to the node  85  of the first terminal and to the first resistor Rc 1 . The second voltage buffer  75 B is coupled to the node  86  of the first terminal and to the second resistor Rc 2 . 
     The cross coupling circuit  80  is configured to provide additional AC write current to the head  50  during both positive and negative transitions in the write current Iw(t). This additional AC write current gives the write current Iw(t) a compensation current for the parasitic capacitor Cp across the output terminals HX and HY, thus decreasing a rise time and a fall time associated with each transition in the write current Iw(t). 
     The active damp circuit  90  preferably includes a feedback input, a feedback output, and an impedance element  99 . The feedback input includes the nodes  95  and  96  coupled in a differential arrangement to the output terminal HX and the output terminal HY of the write amplifier circuit  300 . Specifically, the node  95  of the feedback input is coupled to the output terminal HX and the node  96  of the feedback input is coupled to the output terminal HY. The feedback output includes the nodes  97  and  98  coupled in a differential arrangement to the bottom switch driver  40  and to the cross-coupling circuit  80 . Specifically, the node  97  of the feedback output is coupled to the bottom switch driver  40  and to the capacitor Cc 2  and the node  98  of the feedback output is coupled to the bottom switch driver  40  and to the capacitor Cc 1 . 
     Within the bottom switch driver  40 , the active damp circuit  90  is coupled to a current amplifier (not shown). The impedance element  99  of the active damp circuit  90  is coupled to the feedback input and to the feedback output and is configured as a differential circuit. The impedance element  99  preferably includes a first resistor Rd 1 , a second resistor Rd 2 , a first capacitor Cd 1 , and a second capacitor Cd 2 . The first resistor Rd 1  and the first capacitor Cd 1  are coupled in series between the nodes  95  and  97  and form a first half circuit of the differential impedance circuit. The second resistor Rd 2  and the second capacitor Cd 2  are coupled in series between the nodes  96  and  98  and form a second half circuit of the differential impedance circuit. 
     The active damp circuit  90  provides a feedback loop from the output terminals HX and HY. This feedback loop adjusts a damping factor of the write amplifier circuit  300  such that the undershoot and ringing associated with the write current Iw(t) are reduced, thus decreasing a settling time associated with the write current Iw(t). 
     FIG. 4 illustrates a detailed schematic diagram of the preferred embodiment of the write amplifier circuit  300  of the present invention. 
     In the top switch driver  30 , the differential input signals WDX and WDY are coupled to the base of the pnp transistors Q 18  and Q 17 , respectively. The transistors Q 17  and Q 18  are arranged in a current switch configuration with their emitters coupled to each other and to a current source Irl providing a current to the current switch. A “LOW” value on the differential input signals WDX and WDY causes the transistor Q 18  to turn on and the transistor Q 17  to be turned off. When a “LOW” value occurs on the differential input signals WDX and WDY, the input signal WDX is at a logical low voltage level and the input signal WDY is at a logical high voltage level. A “HIGH” value on the differential input signals WDX and WDY causes the transistor Q 17  to turn on and the transistor Q 18  to be turned off. When a “HIGH” value occurs on the differential input signals WDX and WDY, the input signal WDX is at a logical high voltage level and the input signal WDY is at a logical low voltage level. 
     The collector of the npn transistor Q 15  is coupled to the collector of the transistor Q 17  and to the base of the npn transistor Q 13 . The collector of the npn transistor Q 16  is coupled to the collector of the transistor Q 18  and to the base of the transistor Q 14 . Moreover, the base of the transistor Q 15  is coupled to the base of the npn transistor Q 12  and to the emitter of the transistor Q 13 . The base of the transistor Q 16  is coupled to the base of the npn transistor Q 11  and to the emitter of the transistor Q 14 . The emitters of the transistors Q 15  and Q 16  are coupled to each other and to a first terminal of the resistor R 15 . A second terminal of the resistor R 15  is coupled to ground. A first terminal of the resistor R 13  is coupled to the emitter of the transistor Q 13  and to the bases of the transistors Q 15  and Q 12 . A second terminal of the transistor R 13  is coupled to ground. A first terminal of the resistor R 14  is coupled to the emitter of the transistor Q 14  and to the bases of the transistors Q 16  and Q 11 . A second terminal of the transistor R 14  is coupled to ground. The collectors of the transistors Q 13  and Q 14  are coupled to VCC. 
     The emitters of the transistors Q 12  and Q 11  are coupled to each other and to a first terminal of the resistor R 10 . A second terminal of the resistor R 10  is coupled to ground. A first terminal of the resistor R 12  is coupled to VCC. A second terminal of the resistor R 12  is coupled to the collector of the transistor Q 12 . A first terminal of the resistor R 11  is coupled to VCC. A second terminal of the resistor R 11  is coupled to the collector of the transistor Q 11 . Moreover, the differential output terminal  33  of the top switch driver circuit  30  is formed at the collectors of the transistors Q 12  and Q 11 . 
     The transistors Q 15  and Q 12  form a current mirror. When the transistor Q 17  is on, the current mirror formed by the transistors Q 15  and Q 12  sinks current at the collector of the transistor Q 12  and turns off the npn transistors Q 6  and Q 1 , where the bases of the transistors Q 6  and Q 1  are coupled to the collector of the transistor Q 12 . 
     Similarly, the transistors Q 16  and Q 11  form a current mirror. When the transistor Q 18  is on, the current mirror formed by the transistors Q 16  and Q 11  sinks current at the collector of the transistor Q 11  and turns off the npn transistors Q 5  and Q 3 , where the bases of the transistors Q 5  and Q 3  are coupled to the collector of the transistor Q 11 . 
     In the bottom switch driver  40 , the differential input signals WDX and WDY are coupled to the base of the pnp transistors Q 9  and Q 10 , respectively. The transistors Q 9  and Q 10  are arranged in a current switch configuration with their emitters coupled to each other and to a current source Irw providing a current to the current switch. A “LOW” value on the differential input signals WDX and WDY causes the transistor Q 9  to turn on and the transistor Q 10  to be turned off. A “HIGH” value on the differential input signals WDX and WDY causes the transistor Q 10  to turn on and the transistor Q 9  to be turned off. 
     The collector of the npn transistor Q 7  is coupled to the collector of the transistor Q 9  and to the base of the npn transistor Q 19 . The collector of the npn transistor Q 8  is coupled to the collector of the transistor Q 10  and to the base of the npn transistor Q 20 . Moreover, the base of the transistor Q 7  is coupled to the base of the npn transistor Q 2  and to the emitter of the transistor Q 19 . The base of the transistor Q 8  is coupled to the base of the npn transistor Q 4  and to the emitter of the transistor Q 20 . The emitters of the transistors Q 7  and Q 8  are coupled to each other and to a first terminal of the resistor R 7 . A second terminal of the resistor R 7  is coupled to ground. A first terminal of the resistor R 8  is coupled to the emitter of the transistor Q 19  and to the bases of the transistors Q 7  and Q 2 . A second terminal of the transistor R 8  is coupled to ground. A first terminal of the resistor R 9  is coupled to the emitter of the transistor Q 20  and to the bases of the transistors Q 8  and Q 4 . A second terminal of the transistor R 9  is coupled to ground. The collectors of the transistors Q 19  and Q 20  are coupled to VCC. 
     The transistors Q 7  and Q 2  form a current mirror. When the transistor Q 9  is on, the current mirror formed by the transistors Q 7  and Q 2  sinks current at the collector of the transistor Q 2 . 
     Similarly, the transistors Q 8  and Q 4  form a current mirror. When the transistor Q 10  is on, the current mirror formed by the transistors Q 8  and Q 4  sinks current at the collector of the transistor Q 4 . 
     As described above, the write current Iw(t) to the head  50  is provided by the transistors Q 3  and Q 4  or by the transistors Q 1  and Q 2 , depending on the value of the differential input signals WDX and WDY. The output terminal HX is coupled to the emitter of the transistor Q 3  and to the collector of the transistor Q 2 . The output terminal HY is coupled to the emitter of the transistor Q 1  and to the collector of the transistor Q 4 . Moreover, the collectors of the transistors Q 3  and Q 1  are coupled to VCC. The emitters of the transistors Q 2  and Q 4  are coupled to each other and to a first terminal of the resistor R 4 . A second terminal of the resistor R 4  is coupled to ground. 
     As shown in FIG. 4, the npn transistor Q 5  and the resistor R 5  form the first voltage buffer  75 A of the cross coupling circuit  80  while the npn transistor Q 6  and the resistor R 6  form the second voltage buffer  75 B of the cross coupling circuit  80 . A first terminal of the resistor R 5  is coupled to the emitter of the transistor Q 5 . A second terminal of the resistor R 5  is coupled to ground. A first terminal of the resistor R 6  is coupled to the emitter of the transistor Q 6 . A second terminal of the resistor R 6  is coupled to ground. The collectors of the transistors Q 5  and Q 6  are coupled to VCC. The first and second voltage buffers  75 A and  75 B provide an appropriate bias voltage to the feedforward element  89 . 
     Within the cross-coupling circuit  80 , a first terminal of the resistor Rc 1  is coupled to the emitter of the transistor Q 5  and to the first terminal of the resistor R 5 . A second terminal of the resistor Rc 1  is coupled to a first terminal of the capacitor Cc 1 . A second terminal of the capacitor Cc 1  is coupled to the base of the transistor Q 20 . A first terminal of the resistor Rc 2  is coupled to the emitter of the transistor Q 6  and to the first terminal of the resistor R 6 . A second terminal of the resistor Rc 2  is coupled to a first terminal of the capacitor Cc 2 . A second terminal of the capacitor Cc 2  is coupled to the base of the transistor Q 19 . 
     In practice, the cross coupling circuit  80  provides a feedforward path to an AC current that forms at the differential output terminal  33  of the top switch driver  30 . The values of the first resistor Rc 1  and the first capacitor Cc 1  within the first half of the feedforward circuit control the features of the AC current, such as magnitude and delay, which is generated at the collector of the transistor Q 11  and the bases of the transistors Q 5  and Q 3 . The values of the second resistor Rc 2  and the second capacitor Cc 2  within the second half circuit of the feedforward circuit control the features of the AC current, such as magnitude and delay, which is generated at the collector of the transistor Q 12  and the bases of the transistors Q 6  and Q 1 . Generally, the AC current forms at the differential output terminal  33  only while the write current Iw(t) makes either a positive transition or a negative transition. The AC current generated at the collector of the transistor Q 11  is directed to the collector of the transistor Q 8  through the first half circuit of the feedforward circuit and the transistor Q 20 . The AC current generated at the collector of the transistor Q 12  is directed to the collector of the transistor Q 7  through the second half circuit of the feedforward circuit and the transistor Q 19 . 
     As explained above, the transistors Q 7  and Q 2  form a current mirror having the resistor R 7  coupled to the emitter of the transistor Q 7  and the resistor R 4  coupled to the emitter of the transistor Q 2 . However, the transistor Q 2  preferably has an emitter-base junction area that is  40  times as large as the emitter-base junction area of the transistor Q 7 . Hence, the AC current that reaches the collector of the transistor Q 7  appears as an amplified AC current at the collector of the transistor Q 2 , where the current gain is  40 . Using a current gain that is higher than  40  would require taking into consideration a DC current of the write current Iw(t) to avoid performance degradation of the write amplifier circuit  300 . Additionally, a resistance ratio between the resistor R 7  and the resistor R 4  is preferably  40 , which is equivalent to the current gain of the current amplifier implemented as a current mirror by the transistors Q 7  and Q 8  and the resistors R 7  and R 4 . 
     Similarly, the transistors Q 8  and Q 4  form a current mirror having the resistor R 7  coupled to the emitter of transistor Q 8  and the resistor R 4  coupled to the emitter of transistor Q 4 . However, the transistor Q 4  preferably has an emitter-base junction area that is  40  times as large as the emitter-base junction area of the transistor Q 8 . Hence, the AC current that reaches the collector of the transistor Q 8  appears as an amplified AC current at the collector of the transistor Q 4 , where the current gain is  40 . 
     The amplified AC current formed at the collectors of the transistors Q 2  and Q 4  increases the write current Iw(t) available for the head  50  during the positive and negative transitions of the write current Iw(t). This results in a shorter rise time and a shorter fall time for the write current Iw(t). In terms of a transfer function of the write amplifier circuit  300 , the cross coupling circuit  90  adds a zero to the transfer function. 
     Within the active damp circuit  90 , a first terminal of the resistor Rd 1  is coupled to the output terminal HY, to the collector of the transistor Q 4  and to the emitter of the transistor Q 1 . A second terminal of the resistor Rd 1  is coupled to a first terminal of the capacitor Cd 1 . A second terminal of the capacitor Cd 1  is coupled to the second terminal of the capacitor Cc 1  and to the base of the transistor Q 20 . A first terminal of the resistor Rd 2  is coupled to the output terminal HX, to the collector of the transistor Q 2  and to the emitter of the transistor Q 3 . A second terminal of the resistor Rd 2  is coupled to a first terminal of the capacitor Cd 2 . A second terminal of the capacitor Cd 2  is coupled to the second terminal of the capacitor Cc 2  and to the base of the transistor Q 19 . 
     In practice, the active damp circuit  90  provides a feedback path to a second AC current from the output terminal HX and the output terminal HY. The first resistor Rd 1  and the first capacitor Cd 1  control the features of the second AC current, such as magnitude, delay, and waveform, which is generated at the output terminal HY. The second resistor Rd 2  and the second capacitor Cd 2  control the features of the second AC current, such as magnitude, delay, and waveform, which is generated at the output terminal HX. The second AC current generated at the output terminal HY is directed to the collector of the transistor Q 8  through the transistor Q 20 . The second AC current generated at the output terminal HX is directed to the collector of the transistor Q 7  through the transistor Q 19 . More importantly, the second AC current generated at the output terminal HY is amplified by the current mirror formed by the transistors Q 8  and Q 4  while the second AC current generated at the output terminal HX is amplified by the current mirror formed by the transistors Q 7  and Q 2 , as described above. 
     The amplified second AC current damps a ringing associated with the write current Iw(t) and reduces an undershoot associated with the write current Iw(t). In terms of a transfer function of the write amplifier circuit  300 , the active damp circuit  90  reduces a damping factor associated with the transfer function. 
     Referring to FIG. 4, when the value of the differential input signals WDX and WDY transitions from a “HIGH” to a “LOW” within the top switch driver  30 , the transistor Q 18  is turned on while the transistor Q 17  is turned off. Since the transistor Q 18  is turned on, current from the current source Irl is sourced by the collector of the transistor Q 18  to the collector of the transistor Q 16  and to the base of the transistor Q 14 , turning on the transistor Q 14 . Thus, the current mirror formed by the transistors Q 16  and Q 11  is turned on and the transistor Q 14  provides a base current to the bases of the transistors Q 16  and Q 11 , causing the collector of the transistor Q 11  to sink current. The current at the collector of the transistor Q 11  causes a voltage drop across the resistor R 11  which turns off the transistors Q 5  and Q 3 , where the emitter of the transistor Q 3  stops sourcing the write current Iw(t) to the head  50  through the output terminal HX. Since the transistor Q 17  is turned off, current from the current source Ir 1  is no longer sourced by the collector of the transistor Q 17  to the collector of the transistor Q 15  and to the base of the transistor Q 13 , turning off the transistor Q 13 . Thus, the current mirror formed by the transistors Q 15  and Q 12  is turned off and the transistor Q 13  does not provide the base current to the bases of the transistors Q 15  and Q 12 , causing the collector of the transistor Q 12  to stop sinking current. Due to the lack of current at the collector of the transistor Q 12  there is no voltage drop across the resistor R 12  which accordingly turns on the transistors Q 6  and Q 1 . The emitter of the transistor Q 1  sources the write current Iw(t) to the head  50  through the output terminal HY. 
     Referring to FIG. 4, when the value of the differential input signals WDX and WDY transitions from a “HIGH” to a “LOW” within the bottom switch driver  40 , the transistor Q 9  is turned on while the transistor Q 10  is turned off. Since the transistor Q 9  is turned on, current from the current source Irw is sourced by the collector of the transistor Q 9  to the collector of the transistor Q 7  and to the base of the transistor Q 19 , turning on the transistor Q 19 . Thus, the current mirror formed by the transistors Q 7  and Q 2  is turned on and the transistor Q 19  provides a base current to the bases of the transistors Q 7  and Q 2 , causing the collector of the transistor Q 2  to sink current. The current at the collector of the transistor Q 2  sinks the write current Iw(t) from the head  50  through the output terminal HX. Since the transistor Q 1 O is turned off, current from the current source Irw is no longer sourced by the collector of the transistor Q 1 O to the collector of the transistor Q 8  and to the base of the transistor Q 20 , turning off the transistor Q 20 . Thus, the current mirror formed by the transistors Q 8  and Q 4  is turned off and the transistor Q 20  does not provide the base current to the bases of the transistors Q 8  and Q 4 , causing the collector of the transistor Q 4  to stop sinking the write current Iw(t) to the head  50  through the output terminal HY. 
     During this activation and deactivation of the transistors in the top switch driver  30 , a collector current at the collector of the transistor Q 11  includes an AC current component. Additionally, during this activation and deactivation of the transistors in the top switch driver  30 , the write current Iw(t) makes a negative transition. The AC current component is directed to the collector of the transistor Q 7  via the transistor Q 6 , the second resistor Rc 2 , and the second capacitor Cc 2 , such that the AC current appears as an amplified AC current at the collector of the transistor Q 2 , as described above. 
     Moreover, an AC feedback current at the output terminal HX is directed to the collector of the transistor Q 7  via the second resistor Rd 2  and the second capacitor Cd 2  such that the AC feedback current appears as an amplified AC feedback current at the collector of the transistor Q 2 , as described above. 
     Referring to FIG. 4, when the value of the differential input signals WDX and WDY transitions from a “LOW” to a “HIGH” within the top switch driver  30 , the transistor Q 17  is turned on while the transistor Q 18  is turned off. Since the transistor Q 17  is turned on, current from the current source Irl is sourced by the collector of the transistor Q 17  to the collector of the transistor Q 15  and to the base of the transistor Q 13 , turning on the transistor Q 13 . Thus, the current mirror formed by the transistors Q 15  and Q 12  is turned on and the transistor Q 13  provides a base current to the bases of the transistors Q 15  and Q 12 , causing the collector of the transistor Q 12  to sink current. The current at the collector of the transistor Q 12  causes a voltage drop across the resistor R 12  which turns off the transistors Q 6  and Q 1 , where the emitter of the transistor Q 1  stops sourcing the write current Iw(t) to the head  50  through the output terminal HY. Since the transistor Q 18  is turned off, current from the current source Ir 1  is no longer sourced by the collector of the transistor Q 18  to the collector of the transistor Q 16  and to the base of the transistor Q 14 , turning off the transistor Q 14 . Thus, the current mirror formed by the transistors Q 16  and Q 11  is turned off and the transistor Q 14  does not provide the base current to the bases of the transistors Q 16  and Q 11 , causing the collector of the transistor Q 11  to stop sinking current. Due to the lack of current at the collector of the transistor Q 11  there is no voltage drop across the resistor R 11  which accordingly turns on the transistors Q 5  and Q 3 . The emitter of the transistor Q 3  sources the write current Iw(t) to the head  50  through the output terminal HX. 
     Referring to FIG. 4, when the value of the differential input signals WDX and WDY transitions from a “LOW” to a “HIGH” within the bottom switch driver  40 , the transistor Q 10  is turned on while the transistor Q 9  is turned off. Since the transistor Q 10  is turned on, current from the current source Irw is sourced by the collector of the transistor Q 10  to the collector of the transistor Q 8  and to the base of the transistor Q 20 , turning on the transistor Q 20 . Thus, the current mirror formed by the transistors Q 8  and Q 4  is turned on and the transistor Q 20  provides a base current to the bases of the transistors Q 8  and Q 4 , causing the collector of the transistor Q 4  to sink current. The current at the collector of the transistor Q 4  sinks the write current Iw(t) from the head  50  through the output terminal HY. Since the transistor Q 9  is turned off, current from the current source Irw is no longer sourced by the collector of the transistor Q 9  to the collector of the transistor Q 7  and to the base of the transistor Q 19 , turning off the transistor Q 19 . Thus, the current mirror formed by the transistors Q 7  and Q 2  is turned off and the transistor Q 19  does not provide the base current to the bases of the transistors Q 7  and Q 2 , causing the collector of the transistor Q 2  to stop sinking the write current Iw(t) to the head  50  through the output terminal HX. 
     During this activation and deactivation of the transistors in the top switch driver  30 , a collector current at the collector of the transistor Q 12  includes an AC current component. Additionally, during this activation and deactivation of the transistors in the top switch driver  30 , the write current Iw(t) makes a positive transition. The AC current component is directed to the collector of the transistor Q 8  via the transistor Q 5 , the first resistor Rc 1 , and the first capacitor Cc 1 , such that the AC current appears as an amplified AC current at the collector of the transistor Q 4 , as described above. 
     Moreover, an AC feedback current at the output terminal HY is directed to the collector of the transistor Q 8  via the first resistor Rd 1  and the second capacitor Cd 1  such that the AC feedback current appears as an amplified AC feedback current at the collector of the transistor Q 4 , as described above. 
     FIG. 2 illustrates a plurality of waveforms representing the write current supplied by different write amplifier circuits. 
     Waveforms  210  and  220  represent the write current supplied by a write amplifier circuit of the prior art, which does not have the cross coupling circuit and the active damp circuit of the present invention. Waveform  210  shows the undershoot and the ringing of the write current caused by a positive transition in the write current. Waveform  220  shows the undershoot and the ringing of the write current caused by a negative transition in the write current. 
     Additionally, waveforms  250  and  260  represent the write current supplied by a write amplifier circuit having only the active damp circuit. This illustrates the reduction in undershoot and ringing associated with the write current that is achieved by the active damp circuit. However, the rise and fall times of waveforms  250  and  260  are worse than for the waveforms  210  and  220 , representing the write current supplied by a write amplifier of the prior art. 
     In contrast, waveforms  230  and  240  represent the write current supplied by a write amplifier circuit of the present invention, having the cross coupling circuit and the active damp circuit. Waveforms  230  and  240  illustrate an improvement in the fall and rise times associated with the write current and an improvement in the undershoot and ringing associated with the write current. 
     FIGS. 5A-C illustrate waveforms representing the write current supplied by a write amplifier circuit of the prior art. The write current is supplied to a 33nH inductor and two 8.2 ohm resistors, representing a model for a head. From FIGS. 5B and 5C, it can be observed that the write amplifier circuit of the prior art provides a write current having a 2 ns rise time t r  and a 2 ns fall time t f . 
     FIGS. 6A-C illustrate waveforms representing the write current supplied by a write amplifier circuit of the present invention, having the cross coupling circuit and the active damp circuit. The write current is supplied to a 33nH inductor and two 8.2 ohm resistors, representing a model for a head. From FIGS. 6B and 6C, it can be observed that the write amplifier circuit of the present invention provides a write current having a 1.64 ns rise time t r , a 1.64 ns fall time t f , and reduced ringing and undershoot. As illustrated by these waveforms, the write amplifier circuit of the present invention provides a marked improvement over the performance of the write amplifier circuit of the prior art. 
     The above figures are merely intended to illustrate a particular implementation of the present invention, but are not intended to limit the scope of the present invention to this particular implementation. 
     The present invention has been described in terms of specific embodiments incorporating details to facilitate the understanding of the principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. While the preferred embodiment of the present invention has been illustrated and described as a circuit using bipolar transistors, it will be apparent to a person of ordinary skill in the art that the circuit of the present invention may be implemented using another device technology such as CMOS, MOS, or any other appropriate device technology. It will be apparent to those skilled in the art that modifications may be made in the embodiments chosen for illustration without departing from the spirit and scope of the invention.