# Patent

## Title
Write driver circuit for magnetic data storage systems

## Abstract

A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes comprises a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between the first input and the first output. A second driver circuit has a second input, a second output that communicates with the second node, and a second feedback path between the second input and the second output. A first write current circuit selectively drives current through the write head in a first direction. A second write current circuit selectively drives current through the write head in a second direction. A first boost circuit decreases a first transition period between current flowing in the first direction and current flowing in the second direction.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 10/392,242 filed on Mar. 18, 2003. The disclosure of the above application is incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to magnetic data storage systems, and more particularly to a write driver circuit for a write head in a magnetic data storage system operating at high data transfer rates.
BACKGROUND OF THE INVENTION
Conventional data storage systems typically write information onto a recording surface of a magnetic storage medium. These systems typically include a write head and a write driver circuit. The magnetic storage medium may be a disk drive of a computer. The write head may be an inductive coil, although other types of write heads may be used.
Information is written to the magnetic storage medium by switching a direction of current flowing through the write head. A magnetic field that is produced by the write head is stored by the magnetic storage medium. One polarity represents one digital value and the opposite polarity represents the other digital value. Data storage rates of these systems are proportional to a rate that the write driver circuit can change the direction of the write current through the write head.
SUMMARY OF THE INVENTION
A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes comprises a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between the first input and the first output. A second driver circuit has a second input, a second output that communicates with the second node, and a second feedback path between the second input and the second output. A first write current circuit selectively drives current through the write head in a first direction. A second write current circuit selectively drives current through the write head in a second direction. A first boost circuit decreases a first transition period between current flowing in the first direction and current flowing in the second direction.
In other features, a second boost circuit decreases a second transition period between current flowing in the second direction and current flowing in the first direction. The first write current circuit includes a first write current source that communicates with the opposite end of the first resistance and a second write current source that communicates with the second node of the write head. The second write current circuit includes a third write current source that communicates with the opposite end of the second resistance and a fourth write current source that communicates with the first node of the write head.
In other features, the first feedback path includes a first resistance having one end that communicates with the output of the first driver circuit. A first transistor has a base that is connected to an opposite end of the first resistance and a collector that is connected to the input of the first driver circuit. The second feedback path includes a second resistance having one end that communicates with the output of the second driver circuit. A second transistor has a base that is connected to an opposite end of the second resistance and a collector that is connected to the input of the second driver circuit. A first common mode (CM) current source communicates with the opposite end of the second resistance. A second common mode (CM) current source communicates with the opposite end of the first resistance.
In other features, the first boost circuit includes a first switch that shorts the base of the first transistor and that turns off the first common mode current source during a first boost period. The second boost circuit includes a second switch that shorts the base of the second transistor and that turns off the second common mode current source during a second boost period. A third resistance has one end that communicates with the output of the first driver circuit and an opposite end that communicates with the first node of the write head. A fourth resistance has one end that communicates with the output of the second driver circuit and an opposite end that communicates with the second node of the write head.
In still other features, the write driver circuit has a substantially constant output impedance during operation. The write driver circuit has balanced differential and common mode resistance. The write driver circuit maintains a substantially constant common mode voltage across the write head during operation.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1A is a schematic diagram of a write driver circuit according to the present invention;
FIG. 1B is a graph showing an exemplary write current waveform;
FIG. 2 illustrates various waveforms associated with common mode current sources, write current sources and switches of FIG. 1A according to the present invention;
FIG. 3 is a partial schematic diagram illustrating the operation of the write driver circuit of FIG. 1A without the write current sources;
FIG. 4 is a partial schematic diagram illustrating the operation of the write driver circuit of FIG. 1A with one write current source connected to one node of a write head;
FIG. 5 is a partial schematic diagram that is similar to FIG. 4 with an additional write current source connected to a base of a feedback transistor;
FIG. 6 is a schematic and functional block diagram of a write driver circuit including current limiters in accordance with the present invention;
FIG. 7 is a schematic of a write driver circuit with exemplary current limiters in accordance with the present invention; and
FIG. 8 is a schematic illustrating a write drive circuit having a two level voltage supply;
FIG. 9 is a schematic illustrating a writer driver circuit with a first charge pump circuit including a capacitor and a diode;
FIG. 10 is a schematic illustrating a write driver circuit with a third charge pump circuit including a capacitor, a diode and an isolation buffer;
FIG. 11 is a schematic illustrating a write driver circuit with a second charge pump circuit including a capacitor, a diode and a replica transistor;
FIG. 12 illustrates a write driver circuit including first and second voltage supplies that supply first and second voltage levels;
FIG. 13 illustrates a write driver circuit including first and second driver circuits with feedback and charge pumps connected to outputs of said first and second drivers;
FIG. 14 illustrates a write driver circuit including first and second driver circuits with feedback and charge pumps; and
FIG. 15 illustrates a write driver circuit including first and second driver circuits with feedback and charge pumps that are connected by buffers to outputs of said first and second drivers.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.
Referring now to FIGS. 1A and 1B, an exemplary write driver circuit 10 in accordance with the present invention is shown. The write driver circuit 10 switches a direction of write current IW (shown in FIG. 1B) flowing through a write head 12 of a magnetic storage device. While the following description describes the write head 12 implemented in the magnetic storage device, the write driver circuit 10 according to the present invention may be useful in other applications.
The write driver circuit 10 according to the present invention achieves several operating requirements. The write driver circuit 10 has substantially constant output impedance at very high operating frequencies. The write head 12 sees an impedance that is predominantly resistive during operation. Second, the write driver circuit 10 balances both differential and common mode resistance. Third, the write driver circuit 10 maintains a common mode voltage level across the write head 12 with minimal fluctuation during operation. The common and differential resistance is also independent of the magnitude of the write current.
Referring to FIGS. 1A and 1B, the write driver circuit 10 is a translinear circuit with symmetric sub-circuits 14-A and 14-B (collectively identified as 14) that are connected to opposite sides of the write head 12. For purposes of illustration, the left side of the symmetric write driver circuit 10 is described further below. Since the sub-circuits 14A and 14B are symmetric, the same reference numbers and/or other identification will be used followed by A to identify components in sub-circuit 14A and B to identify components of sub-circuit 14B.
The sub-circuit 14A includes a transistor 16A operating as a driver circuit. A collector of the transistor 16A is coupled to a voltage source Vdd. An emitter of the transistor 16A communicates with one side of the write head 12. In particular, the emitter of the transistor 16A is coupled to node D, which is coupled through a resistance R3A to head node Wop of the write head 12.
The voltage source Vdd is connected via a resistance R2A to a base of the transistor 16A. The base of the transistor 16A is designated as node E. As will be further described below, a feedback path 18A is provided between the emitter of the transistor 16A and the base of the transistor 16A. The feedback path includes a resistance R1A and a transistor 24A. The emitter of the transistor 16A is connected to one end of the resistance R1A. An opposite end of the resistance R1A is connected to a base of the transistor 24A (designate node F). A collector of the transistor 24A is connected to a one end of a resistance R2A and to the base of the transistor 16A.
A write current source IW—1A, a common mode current source ICM—A and a switch S2A are connected in parallel between node F and ground. The node F is coupled via the resistance R1A to node D. An additional write current source IW—2A is coupled between the write head node Wop of the write head 12 and ground. A symmetric circuit 14B is coupled to the right side of the write head 12 and elements are labeled with subscript “_B”. Note that nodes D, E and F of the sub-circuit 14-A correspond to nodes X, Y, and Z in the sub-circuit 14-B. The current sources may be implemented using a current mirror arrangement as is well known in the art.
As can be appreciated, by switching the write current sources IW—1A and IW—2B off and the write current sources IW—2A and IW—1B on, current flows through the write head 12 from left to right in FIG. 1. To increase the reversal speed of the write current, a boost circuit selectively closes switches S2A and S2B (shown in FIG. 1) as will be described below. The boost circuit also switches the common mode current sources ICM—A and ICM—B off temporarily during transitions.
Referring now to FIGS. 1A and 2, exemplary waveforms depicting the operation of the controlled current sources IW—1A, IW—2A, IW—1B, ICM—A and ICM—B and the switches S2A and S2B are shown. To obtain current flow from Wop to Won, the write current sources IW—1A and IW—2B are turned on and the write current sources IW—2A and IW—1B are turned off. During an initial boost period, the common mode current sources ICM—A and ICM—B are turned off and the switch S2A is turned on. The boost circuit reduces the voltage at node X and increases the voltage at node D quickly and symmetrically. The improved transition speed allows increased data rates while the symmetric transition maintains a relatively constant common mode voltage. To bring the voltage at node X down, the common mode current sources are turned off, which decreases the voltage at node X. To increase the voltage at node D, the transistor 24A is turned off by the switch S2A, which forces Vbe of the transistor 16A to increase. Current flows from Wop to Won.
To obtain current flow from Won to Wop, the write current sources IW—2A and IW—1B are tuned on and the write current sources IW—1A and IW—2B are turned off. During a boost period, the current sources ICM—A and ICM—B are turned off and the switch S2B is turned on. The boost circuit reduces the voltage at node D and increases the voltage at node X both quickly and symmetrically. To bring the voltage at node D down, the common mode current sources ICM are turned off, which decreases the voltage at node D. To increase the voltage at node X, the transistor 24B is turned off, which forces Vbe of the transistor 16B to increase. Current flows from Won to Wop.
Referring now to FIG. 3, operation of the circuit will be described in more detail without the write current sources IW—1A, IW—2A, IW—1B, and IW—2B and the switches S2A and S2B. Based on Kirchoff's voltage law:

Vbe—24A+VDF+VD,Wop=Vbe—24B+VXZ+VX,Won 

If R1A=R1B=R3A=R3B,ICM—A=ICM—B, and Vbe—24A=Vbe—24B,

Then VDF=ICM—AR1A=VXZ=ICM—BR1B, and VXZ=ICM—BR1B.

In addition, VWop=VWon and IW=0.

Therefore, the voltage at the write head is zero and the write current is zero.

Referring now to FIG. 4, introducing the write current source IW—2B into the write circuit of FIG. 3 will force IW—2B/2 across R3A and R3B. In other words, the write current IW is equal to IW—2B/2 and the voltage at nodes of the write head is:

VWop=VWon=R3AIW—2B/2.

Therefore, the write current causes an imbalance, which increases the voltage at the write head and increases the write current.

Referring now to FIG. 5, introducing the write current source IW—2A further increases the write current from IW—2B/2 to IW—2B. More particularly, substituting:

VDF=ICM—AR1A+IW—1AR1A; and VXY=ICM—BR1B 

into Kirchoff's voltage equation yields:

Vbe—24A+(ICM—AR1A+IW—1AR1A)+VD,Wop=Vbe—24B+ICM—BR1B+VX,Won 

If R1A=R1B=R3A=R3B, Vbe—24A=Vbe—24B, and ICM—A=ICM—B,

Then, VD,Wop=IW—1AR3A 

or, VD,Wop=IW—1BR3A 

since IW—1A=−IW—2B and IW=IW—2B.

The speed of the low voltage swing at node D or X is related to the speed of the feedback loops including the resistance R1A or R1B and the transistor 24A or 24B, respectively. The speed of the feedback loop is related to a first pole of the feedback loop. The first pole is:




  
    
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Therefore, increasing R2 or decreasing R1 will increase the speed. However, the speed cannot be increased beyond τf Therefore, R1 and R2 should be selected to be approximately equal. The speed of the high voltage swing at node D or X is maximized when R2 is minimized.

As can be appreciated by skilled artisans, the values of the resistances and the magnitude of the current sources can be varied without departing from the present invention. For example, the current sources IW—1A=IW—1B can have a magnitude of IW—2A/k=IW—2B/k and the resistances can be R1A=R1B=k*R3A=k*R3B, where k is a constant. In a preferred embodiment, bipolar transistors such as silicon germanium (SiGe) are employed. Other bipolar transistors, CMOS transistors and/or any other transistor technology may be used.
Referring now to FIG. 6, the write driver sub-circuits 14A and 14B may optionally include current limiters 40A and 40B. A current step of ICM may cause transistors 24A and 42A to momentarily turn off. The current limiter 40A detects when the bases of transistors 24A and 42A are going off and immediately provides extra current to keep them on.
Referring now to FIG. 7, exemplary current limiters 40A and 40B are shown. A first transistor 50A includes an emitter that is connected to the bases of the transistors 24A and 42A. A collector of the transistor 50A is connected to Vdd. A base of the transistor 50A is connected to a base of a transistor 52A. A collector of the transistor 50A is connected to the base of the transistor 52A and to a current source 53A. An emitter of the transistor 52A is connected to a collector and a base of a transistor 54A. An emitter of the transistor 54A is connected to ground or another reference potential. While a specific current limiter 40 is shown, other types of current limiters may be used. A transistor 58A has a base that is connected to node F. An emitter of the transistor 58A is connected to ground or another reference potential. A collector of the transistor 58A is connected to node D. The transistor 58A reduces the impedance seen by the write head node WON.
In some situations, as the voltage at node D goes high, the current that is supplied by the resistor R2 and Vdd may be insufficient. The transistor 16A and/or the transistor 24A may be starved for current, which reduces switching times of the write head. Referring now to FIG. 8, a schematic illustrating a write drive circuit having two voltage supply levels is shown. The resistor R2 is supplied by Vee and the collector of the transistor 16A is supplied by Vdd, where Vee>Vdd. For example, Vee can be approximately 10V and Vdd can be 5V, although other voltage levels are contemplated. The higher voltage potential source Vee provides additional current through the resistor R2 to supply transistors 16A and 24A.
Referring now to FIG. 9, a writer driver circuit with a first charge pump circuit including a capacitor C1A and a diode D1A is shown. The diode D1A has an anode that is connected to a voltage source Vdd. A cathode of the diode D1A is connected to one end of the resistor R2A and to one end of a capacitor C1A. An opposite end of the resistor R2A is connected to the base of the transistor 16A. An opposite end of the capacitor C1A is connected to node D.
When node D is low (current flowing from node X to node D), a voltage difference across the capacitor C1A charges the capacitor C1A. As node D goes from low to high during a transition from current flowing from node X to D to current flowing from node D to node X, the capacitor CiA discharges and provides additional current to the base of the transistor 16A and indirectly to transistor 24A. As a result, the transistors are supplied current during the transition, which increases the speed of the write head transition.
The additional current is provided only during the transitions from low to high. When node D is high, the capacitor C1A is not charged since there is no voltage difference across the capacitor CIA. Therefore, when node D goes from high to low during a transition from current flowing from node D to X to current flowing from node X to node D, the capacitor C1A has nothing to discharge. As a result, current is only provided when needed and power consumption is reduced.
Referring now to FIG. 10, a write driver circuit with a second charge pump circuit including the capacitor C1A, the diode D1A and an isolation buffer B1A is shown. The diode D1A has an anode that is connected to a voltage source Vdd. A cathode of the diode D1A is connected to the resistor R2A and to one end of a capacitor C1A. The isolation buffer B1A is connected to node D and to another end of the capacitor C1A. The isolation buffer B1A reduces loading on the transistor 16A that would otherwise be caused by the capacitor C1A. Otherwise, the second charge pump circuit provides additional current during the low to high transition at node D in a manner that is similar to the first charge pump circuit.
Referring now to FIG. 11, a schematic illustrating a write driver circuit with a third charge pump circuit including the capacitor C1A, the diode D1A and a replica transistor 80A is shown. The diode D1A has an anode that is connected to a voltage source Vdd. A cathode of the diode D1A is connected to the resistor R2A and to one end of a capacitor C1A. The replica transistor 80A has a base that is connected to node D, a collector that is connected to Vdd, and an emitter that is connected to an opposite end of the capacitor C1A. A current source IS—A is also connected to the emitter of the transistor 80A. The replica transistor 80A reduces loading on the transistor 16A that would otherwise be caused by the capacitor C1A. Otherwise, the third charge pump circuit provides additional current during the low to high transition at node D in a manner that is similar to the first charge pump circuit.
Referring now to FIG. 12, a write driver circuit 100 includes first and second driver circuits 102A and 102B, which are connected to first and second voltage supplies 108A and 110A and 108B and 110B, respectively. The first voltage supplies 108A and 108B provide a higher voltage potential than the second voltage supplies 110A and 110B.
The first and second driver circuits 102A and 102B include feedback circuits 114A and 114B, respectively, that are connected between inputs and outputs of the first and second driver circuit 102A and 102B, respectively. The first and second driver circuits 102A and 102B drive current through a write head 120 in first and second directions to write data to the magnetic storage medium. The higher voltage potentials of the first voltage supplies 108A and 108B are used to increase current that is supplied to the first and second driver circuits 102A and 102B, respectively. The additional current provided during write head transitions increases the speed of the write head transitions.
Referring now to FIG. 13, a write driver circuit 130 includes first and second driver circuits 102A and 102B, respectively. Voltage sources 132A and 132B provide a first voltage potential to the first and second driver circuits 102A and 102B. Charge pump circuits 140A and 140B are connected to outputs of the first and second driver circuits 102A and 102B, respectively. The first and second driver circuits 102A and 102B include feedback circuits 114A and 114B, respectively, that are connected between inputs and outputs of the first and second driver circuit 102A and 102B, respectively. The charge pump circuits 140A and 140B provide additional current to the first and second driver circuits 102A and 102B, respectively. The additional current provided during the write head transitions increases the speed of the write head transitions.
Referring now to FIG. 14, a write driver circuit 148 is shown that is similar to the write driver circuit 130 in FIG. 13 except that charge pump circuits 150A and 150B are connected between the voltage sources 132A and 132B, respectively, (or any other suitable voltage source) and the first and second driver circuits 102A and 102B, respectively. The charge pump circuits 150A and 150B provide additional current to the first and second driver circuits.
Referring now to FIG. 15, a write driver circuit 158 is shown that is similar to the write driver circuit 130 in FIG. 13 except that buffers 160A and 160B are located between outputs of the first and second driver circuits, respectively, and inputs of the charge pumps 160A and 160B. The buffers 160A and 160B isolate the outputs of the first and second driver circuits 102A and 102B, respectively.
Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.

## Claims

1. A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes, comprising:
a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between said first input and said first output;
a second driver circuit having a second input, a second output that communicates with the second node, and a second feedback path between said second input and said second output;
a first write current circuit that selectively drives current through the write head in a first direction;
a second write current circuit that selectively drives current through the write head in a second direction; and
a first boost circuit that decreases a first transition period between current flowing in said first direction and current flowing in said second direction,
wherein said first feedback path includes:
a first resistance having one end that communicates with said output of said first driver circuit; and
a first transistor having a base that is connected to an opposite end of said first resistance and a collector that is connected to said input of said first driver circuit.

2. The write driver circuit of claim 1 wherein said second feedback path includes:
a second resistance having one end that communicates with said output of said second driver circuit; and
a second transistor having a base that is connected to an opposite end of said second resistance and a collector that is connected to said input of said second driver circuit.

3. The write driver circuit of claim 2 further comprising:
a first common mode (CM) current source that communicates with said opposite end of said second resistance; and
a second common mode (CM) current source that communicates with said opposite end of said first resistance.

4. The write driver circuit of claim 3 wherein said first boost circuit includes a first switch that shorts said base of said first transistor and that turns off said first common mode current source during a first boost period.

5. The write driver circuit of claim 3 further comprising a second boost circuit that includes a second switch that shorts said base of said second transistor and that turns off said second common mode current source during a second boost period.

6. The write driver circuit of claim 2 further comprising:
a first current limiter that communicates with a base of said first transistor; and
a second current limiter that communicates with a base of said second transistor.

7. A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes, comprising:
a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between said first input and said first output;
a second driver circuit having a second input, a second output that communicates with the second node, and a second feedback path between said second input and said second output;
a first write current circuit that selectively drives current through the write head in a first direction;
a second write current circuit that selectively drives current through the write head in a second direction;
a first boost circuit that decreases a first transition period between current flowing in said first direction and current flowing in said second direction;
a first resistance that has one end that communicates with said output of said first driver circuit and an opposite end that communicates with the first node of the write head; and
a second resistance that has one end that communicates with said output of said second driver circuit and an opposite end that communicates with the second node of the write head.

8. The write driver circuit of claim 7 wherein said write driver circuit has a substantially constant output impedance during operation.

9. A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes, comprising:
a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between said first input and said first output;
a second driver circuit having a second input, a second output that communicates with the second node, and a second feedback path between said second input and said second output;
a first write current circuit that selectively drives current through the write head in a first direction;
a second write current circuit that selectively drives current through the write head in a second direction; and
a first boost circuit that decreases a first transition period between current flowing in said first direction and current flowing in said second direction,
wherein said write driver circuit has balanced differential and common mode resistance.

10. The write driver circuit of claim 9 further comprising a second boost circuit that decreases a second transition period between current flowing in said second direction and current flowing in said first direction.

11. The write driver circuit of claim 10 wherein said first feedback path includes a first resistance and said first write current circuit includes a first write current source that communicates with said opposite end of said first resistance and a second write current source that communicates with the second node of the write head.

12. The write driver circuit of claim 11 wherein said second feedback path includes a second resistance and said second write current circuit includes a third write current source that communicates with said opposite end of said second resistance and a fourth write current source that communicates with the first node of the write head.

13. The write driver circuit of claim 9 wherein said write driver circuit maintains a substantially constant common mode voltage across the write head during operation.

14. The write driver circuit of claim 9 further comprising a first charge pump circuit that provides additional current to said input of said first driver circuit during a first transition period between current flowing through the write head in a first direction and current flowing through the write head in a second direction.

15. The write driver circuit of claim 14 wherein said first charge pump circuit includes:
a first voltage source;
a first diode having an anode that communicates with said first voltage source; and
a first capacitor having one end that communicates with said output of said first driver circuit and an opposite end that communicates with a cathode of said first diode.

16. The write driver circuit of claim 15 further comprising a first replica transistor having a base that communicates with said output of said first driver circuit and emitter that communicates with said one end of said first capacitor.

17. The write driver circuit of claim 15 further comprising an isolation buffer that communicates with said output of said first driver circuit and with said one end of said first capacitor.

18. The write driver circuit of claim 9 wherein said first driver circuit includes first and second voltage inputs, wherein said second voltage input is greater than said first voltage input.

19. The write driver circuit of claim 9 further comprising a charge pump circuit that has an output that communicates with said input of said first driver circuit.

20. The write driver circuit of claim 19 wherein said charge pump circuit has an input that communicates with said output of said first driver circuit.

21. The write driver circuit of claim 20 further comprising a buffer that is arranged between said output of said first driver circuit and said input of said charge pump circuit.

22. A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes, comprising:
a first driver circuit having a first input, a first output that communicates with the first node, and a first feedback path between said first input and said first output;
a second driver circuit having a second input, a second output that communicates with the second node, and a second feedback path between said second input and said second output;
a first write current circuit that selectively drives current through the write head in a first direction;
a second write current circuit that selectively drives current through the write head in a second direction; and
a first boost circuit that decreases a first transition period between current flowing in said first direction and current flowing in said second direction,
wherein said first driver circuit includes a first transistor having a base, a collector and an emitter and further comprising:
a first voltage source that outputs a first voltage potential to said collector of said first transistor;
a second voltage source that outputs a second voltage potential that is higher than said first voltage potential; and
a first resistance having one end that communicates with said second voltage source and an opposite end that communicates with said base of said first transistor.

23. A write driver circuit for a magnetic storage medium that communicates with a write head having first and second nodes, comprising:
first driver means for providing a first output voltage and having a first input, a first output that communicates with the first node, and a first feedback path between said first input and said first output;
second driver means for providing a second output voltage and having a second input, a second output that communicates with the second node, and a second feedback path between said second input and said second output;
first write current means for selectively driving current through the write head in a first direction;
second write current means for selectively driving current through the write head in a second direction; and
first boost means for decreasing a first transition period between current flowing in said first direction and current flowing in said second direction.

24. The write driver circuit of claim 23 further comprising second boost means for decreasing a second transition period between current flowing in said second direction and current flowing in said first direction.

25. The write driver circuit of claim 24 wherein first feedback path includes a first resistance and said first write current means includes:
a first write current source that communicates with said opposite end of said first resistance; and
a second write current source that communicates with the second node of the write head.

26. The write driver circuit of claim 25 wherein said second feedback path includes a second resistance and said second write current means includes:
a third write current source that communicates with said opposite end of said second resistance; and
a fourth write current source that communicates with the first node of the write head.

27. The write driver circuit of claim 23 wherein said first feedback path includes:
a first resistance having one end that communicates with said output of said first driver means; and
a first transistor having a base that is connected to an opposite end of said first resistance and a collector that is connected to said input of said first driver means.

28. The write driver circuit of claim 27 wherein said second feedback path includes:
a second resistance having one end that communicates with said output of said second driver means; and
a second transistor having a base that is connected to an opposite end of said second resistance and a collector that is connected to said input of said second driver means.

29. The write driver circuit of claim 28 further comprising:
a first common mode (CM) current source that communicates with said opposite end of said second resistance; and
a second common mode (CM) current source that communicates with said opposite end of said first resistance.

30. The write driver circuit of claim 29 wherein said first boost means includes first switching means for shorting said base of said first transistor and for turning off said first common mode current source during a first boost period.

31. The write driver circuit of claim 29 further comprising second boost means for decreasing a second transition period between current flowing in said second direction and current flowing in said first direction, wherein said second boost means includes second switching means for shorting said base of said second transistor and for turning off said second common mode current source during a second boost period.

32. The write driver circuit of claim 28 further comprising:
first current limiting means for limiting current at a base of said first transistor; and
second current limiting means for limiting current at a base of said second transistor.

33. The write driver circuit of claim 23 further comprising:
a third resistance that has one end that communicates with said output of said first driver means and an opposite end that communicates with the first node of the write head; and
a fourth resistance that has one end that communicates with said output of said second driver means and an opposite end that communicates with the second node of the write head.

34. The write driver circuit of claim 23 wherein said write driver circuit has a substantially constant output impedance during operation.

35. The write driver circuit of claim 23 wherein said write driver circuit has balanced differential and common mode resistance.

36. The write driver circuit of claim 23 wherein said write driver circuit maintains a substantially constant common mode voltage across the write head during operation.

37. The write driver circuit of claim 23 wherein said first driver means includes a first transistor having a base, a collector and an emitter and further comprising:
a first voltage source that outputs a first voltage potential to said collector of said first transistor;
a second voltage source that outputs a second voltage potential that is higher than said first voltage potential; and
a fifth resistance having one end that communicates with said second voltage source and an opposite end that communicates with said base of said first transistor.

38. The write driver circuit of claim 23 further comprising first charge pump means for providing additional current to said input of said first driver means during a first transition period between current flowing through the write head in a first direction and current flowing through the write head in a second direction.

39. The write driver circuit of claim 38 wherein said first charge pump means includes:
a first voltage source;
a first diode having an anode that communicates with said first voltage source; and
a first capacitor having one end that communicates with said output of said first driver means and an opposite end that communicates with a cathode of said first diode.

40. The write driver circuit of claim 39 further comprising a first replica transistor having a base that communicates with said output of said first driver means and an emitter that communicates with said one end of said first capacitor.

41. The write driver circuit of claim 39 further comprising an isolation buffer that communicates with said output of said first driver means and with said one end of said first capacitor.

42. The write driver circuit of claim 23 wherein said first driver means includes first and second voltage inputs, wherein said second voltage input is greater than said first voltage input.

43. The write driver circuit of claim 23 further comprising charge pump means for providing additional current to said input of said first driver means.

44. The write driver circuit of claim 43 wherein said charge pump means has an input that communicates with said output of said first driver means.

45. The write driver circuit of claim 44 further comprising buffer means that isolates said output of said first driver means and said input of said charge pump circuit.

46. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node, wherein said first feedback path includes a first resistance having one end that communicates with an output of said first driver circuit and a first transistor having a base that is connected to an opposite end of said first resistance and a collector that is connected to an input of said first driver circuit.

47. The method of claim 46 wherein said second feedback path includes a second resistance having one end that communicates with an output of said second driver circuit and a second transistor having a base that is connected to an opposite end of said second resistance and a collector that is connected to an input of said second driver circuit.

48. The method of claim 47 further comprising:
decreasing a first transition period between current flowing in said first direction and current flowing in said second direction; and
decreasing a second transition period between current flowing in said second direction and current flowing in said first direction.

49. The method of claim 48 wherein said first write driver sub-circuit includes a first common mode (CM) current source and said second write driver sub-circuit includes a second common mode (CM) current source.

50. The method of claim 49 wherein said decreasing said first transition includes:
shorting a base of said first transistor during a first boost period; and
turning off said first common mode current source during said first boost period.

51. The method of claim 49 wherein said decreasing said second transition includes:
shorting a base of said second transistor during a second boost period; and
turning off said second common mode current source during said second boost period.

52. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node, wherein said write driver circuit has a substantially constant output impedance during operation.

53. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node, wherein said write driver circuit has balanced differential and common mode resistance.

54. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node, wherein said write driver circuit maintains a substantially constant common mode voltage across the write head during operation.

55. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node, wherein said first driver circuit includes a first transistor and further comprising:
biasing a collector of said first transistor with a first voltage potential; and
biasing a base of said first driver circuit using a second voltage potential that is higher than said first voltage potential.

56. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node; and
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node;
providing additional current to said input of said first driver circuit during a first transition period between current flowing through the write head in a first direction and current flowing through the write head in a second direction.

57. The method of claim 56 wherein said additional current is provided by a charge pump circuit.

58. The method of claim 57 further comprising using a first replica transistor between said output of said first driver circuit and said charge pump circuit.

59. The method of claim 57 further comprising using an isolation buffer between said output of said first driver circuit and said charge pump circuit.

60. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node;
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node;
supplying first and second voltage inputs to said first driver circuit, wherein said second voltage input is greater than said first voltage input.

61. A method for operating a write driver circuit for a magnetic storage medium including a write head having first and second nodes, comprising:
connecting a first write driver sub-circuit including a first driver circuit with a first feedback path to the first node;
connecting a second write driver sub-circuit including a second driver circuit with a second feedback path to the second node;
turning on a first current source associated with said first write driver sub-circuit and a second current source associated with said second write driver sub-circuit to drive current in a first direction from the first node to the second node;
turning on a third current source associated with said first write driver sub-circuit and a fourth current source associated with said second write driver sub-circuit to drive current in a second direction from the second node to the first node; and
using a charge pump circuit to supply current to said input of said first driver circuit.

62. The method of claim 61 further comprising connecting an input of said charge pump circuit to said output of said first driver circuit.

63. The method of claim 62 further comprising isolating said output of said first driver circuit and said input of said charge pump circuit.