Recording apparatus provided with a write driver circuit

[0002] This invention relates to a recording apparatus for recording an information signal on a magnetic record carrier, the apparatus comprising:

[0003] an input terminal for receiving the information signal,

[0004] a write driver circuit and

[0005] a magnetic write head, the write driver circuit comprising

[0006] a first voltage driver having an input coupled to the input terminal of the apparatus, and an output coupled to a first terminal of the magnetic write head,

[0007] a second voltage driver having an input coupled to the input terminal of the apparatus and an output coupled to the second terminal of the write head, as well as to a write driver circuit for use in the recording apparatus.

[0008] Write drivers of the voltage drive type are well known in the art. Reference is made in this respect to U.S. Pat. No. 4,853,800 (D1 in the list of related documents that can be found at the end of this description) and DE-A 2,735,498 (D2).

[0009] There is a strong pressure, especially on hard-disk drive manufacturers, to increase bit density and to shorten access times. The current rate of increase in bit density (60 % per annum) is being sustained by a variety of technical improvements with a resulting increase in data rates of 40 % per year. In order to sustain this increase in data rates, the high frequency performance of the media, heads and electronics, in particular the write driver, have to increase at the same rate.

[0010] Conventional write driver techniques, based on current driving, see D3, current steering, see D4, have severe speed limitations. These limitations result not only from poor high-frequency performance inside the driver, but also from limitations imposed by the head-amplifier combination. In addition to the poor high-frequency performance of thin-film write heads, these shortcomings have become a major limitation in the write process for magnetic recording.

[0011] It is an object of the invention to further improve the recording characteristics of the recording apparatus. The recording apparatus in accordance with the invention is characterized in that the first voltage driver circuit comprises an output circuit of a series arrangement of an npn transistor and an impedance, coupled between a point of a first constant potential and the output of the first voltage driver circuit,

[0012] that the second voltage driver circuit comprises an output circuit of a series arrangement of an npn transistor and an impedance, coupled between said point of a first constant potential and the output of the second voltage driver circuit, the apparatus further comprising

[0013] a first current driver having an input coupled to the input terminal and an output coupled to said first terminal of the write head, and

[0014] a second current driver having an input coupled to the input terminal and an output coupled to said second terminal of the write head. The invention is based on the recognition to combine voltage drive and current drive techniques in order to obtain an improved recording apparatus.

[0015] A voltage driver is proposed for the reason that it does not only has a superior internal high-frequency performance, but is also less sensitive to the performance limitations resulting from head-amplifier combinations. Further, by adding a current driver, preferably comprising at least one transistor of the npn type, coupled between its output and the point of a second constant potential, which is the negative supply voltage, an improved behavior can be obtained. Both the current driver as well as the voltage driver have been realized by npn transistors. npn transistors are much faster in their characteristics, so that replacing the pnp transistor normally present in a current driver circuit by a voltage driver having an npn transistor in its output circuit speeds up the circuit significantly.

[0016] The write driver proposed herein has a major advantage in that it can be integrated in a chip area much smaller than that used for previous current drivers. The reduced chip area comes as a result of an improved architecture.

[0017] These and other aspects of the invention will become more apparent in the following figure description, in which:

[0018] FIG. 1 shows a principle diagram of a write driver circuit, provided with voltage drivers, in which the voltage drivers are driven by a differential voltage source,

[0019] FIG. 2 shows the principle diagram of FIG. 1, in which the voltage drivers are driven by a voltage source implemented with a current source,

[0020] FIG. 3 shows an elaborated version of the driver circuit of FIG. 2,

[0021] FIG. 4 shows the essential elements of the driver circuit in accordance with the invention, and

[0022] FIG. 5 a further elaborated version of the driver circuit of FIG. 4.

[0023] The write driver circuit shown in FIG. 1 receives an input signal from a differential voltage source, indicated in FIG. 1 by the two voltage sources 1 and 2 that generate a voltage I_w.R_d/2, with the polarities as shown in FIG. 1. The write driver circuit comprises a first voltage driver 4and second voltage driver 6. FIG. 1 further shows the write head 8 driven by the write driver circuit. The voltage drivers each comprise a voltage follower 10a, 10b and a series resistor R_d/2.

[0024] FIG. 2 shows how the voltage sources 1 and 2 can be implemented with a current source 12 and the resistors 2R_b. The bandwidth of the write driver circuit can be increased by adding capacitors C_bacross the voltage followers 10aand 10band a corresponding resistor R_b, as shown in FIG. 3. This results in an improvement of the step response of the write driver circuit. The addition of the capacitors C_facross the resistors R_d/2 in the output chain of the voltage drivers improves the behavior as well, in that it speeds up the response of the driver circuit.

[0025] The voltage followers 10a and 10b can be realized in various ways, such as in the form of a class A voltage follower, or a class A/B voltage follower.

[0026] FIG. 4 shows the essential elements of the write driver circuit in accordance with the invention. The first voltage driver comprises an npn transistor 40 and the impedance 42 in a series connection between the positive supply voltage (+) and the output 46. A series connection of an impedance 45 and a current source 48 is provided between the positive (+) and negative (−) supply voltages. The current source 48 is in the form of an npn transistor as well. The current driver comprises an npn transistor 52 coupled between the output 46 and the negative supply voltage (−). The current driver 52 could be considered to be coupled in parallel with the current source 48. An equivalent circuit construction is shown for the right hand side of the write driver circuit.

[0027] An input signal supplied to the input terminal 44, which is in the form of a current having either a “high” current or a “low” current, such as a zero current, controls the voltage driver and the current driver in the following way. In response to a high current supplied to the input 44, a current flows through the current source 48, so that the voltage at the node 54 is “low”. As a result, the voltage at the output 46 is low and the current driver 52 receives a current that flows through the head L_H. In response to a “low” current supplied to the input 44, no current flows through the current source 48, so that the voltage at the node 54 is “high”, and thus the voltage at the output 46 is high, resulting in a current running via the output 46 through the head L_H.

[0028] Further, when the input 44 receives a “high” current, the input 56 receives a “low” (no) current. As a result, the current that flows through the head L_Hruns from the positive supply voltage (+), via the transistor 40, through the head L_Hand via the transistor 58 to the negative supply voltage. And, when the input 44 receives a “low” current, the input 56 receives a “high” current. As a result, the current that flows through the head L_Hruns from the positive supply voltage (+), via the transistor 60, through the head L_Hand via the transistor 52 to the negative supply voltage.

[0029] FIG. 5 shows a further elaborated version of the only the left hand side of the write driver circuit of FIG. 4. From FIG. 5, it is clear that npn transistors are used in the output circuits of both the voltage driver and the current driver. This has the advantage over using pnp transistors in that npn transistors can cope with larger currents and are faster than pnp transistors. When using npn transistors, this implies a smaller chip surface area that is required for realizing the writer circuit. Further, the voltage drop across R_d/2 is reduced, for the reason that current through the head L_His now (partly) taken up by the current source 52.

[0030] Whilst the invention has been described with reference to preferred embodiments thereof, it is to be understood that these are not limitative examples. Thus, various modifications may become apparent to those skilled in the art, without departing from the scope of the invention, as defined by the claims.

[0031] Further, the invention lies in each and every novel feature or combination of features.