1. Field of the Invention
This invention relates in general to magnetic storage systems, and more particularly to a method and apparatus for providing quadrature biasing for coupled-pair circuits.
2. Description of Related Art
Magnetic recording systems that utilize magnetic disk and tape drives constitute the main form of data storage and retrieval in present-day computer and data processing systems. In the recording process, information is written and stored as magnetization patterns on the magnetic recording medium. Scanning a write head over the medium and energizing the write head with appropriate current waveforms accomplish this recording process. In a read-back process, scanning a read sensor over the medium retrieves the stored information. This read sensor intercepts magnetic flux from the magnetization patterns on the recording medium and converts the magnetic flux into electrical signals, which are then detected and decoded.
In high capacity storage systems, magnetoresistive read sensors using a higher sensitive giant magnetoresistance effect, commonly referred to as giant magnetoresistive (GMR) heads, are the prevailing read sensor because of their capability to read data from a surface of a recording medium at greater track and linear densities. Hereinafter, the term “MR head” or “MR sensor” will be used to refer to a magnetoresistive sensor in the general sense, i.e., any MR sensor including GMR sensors.
An MR sensor detects a magnetic field through the change in resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer. To convert the changes in resistance into a usable voltage signal, MR heads are typically biased with a current that creates a voltage drop across the resistive portion of the MR head. Ideally, the bias current is controlled so that any changes in the voltage drop across the MR head are attributed to changes in the resistance of the head. Thus, when the head is properly biased, the voltage across the head tracks the changes in the magnetic fields, making the voltage useful as a read signal.
Often, the voltage across the MR head is amplified by a differential amplifier. To improve the linearity of the differential amplifier, it is advantageous that the MR head be biased so that the MR head's two terminals have the same voltage magnitude but opposite voltage polarity from each other. When the MR head is biased in this manner, an increase in the resistance of the MR head due to a magnetic field causes an increase in the voltage of one terminal of the head and an equivalent decrease in the voltage of the other terminal of the head.
One type of differential amplifier is a quadrature-type amplifier. Quadrature-type amplifiers are biased/balanced by Quadrature Biasing for Coupled Pair (QBCP) circuits, where said amplifiers have input-coupled pairs which can require relatively large dc-input-voltage offsets. Quadrature-type amplifier circuits have unique biasing challenges. One such biasing challenge is that the four transistors in the quadrature-type amplifier require equivalent biasing conditions for the amplifier's transistors to properly operate in their linear region. Another biasing challenge is that the quadrature-type amplifier, with prior-art bias-control schemes, can have undesirable bias feedback-loop interactions which requires higher-order control schemes.
A prior bias-control scheme for a quadrature-type amplifier 1000 is shown in FIG. 10. FIG. 10 shows a bias-control scheme wherein the bias of transistors Q11010 and Q21012 are made to be equivalent by sensing the voltage between Node C 1020 and Node D 1022. The bias of transistors Q31024 and Q41026 then rely on the Integrated Circuits (IC) process tracking tolerances, which vary depending on the particular process. This bias-control scheme has tightly coupled input-and-output, differential-and-common mode control loops. The bias-control scheme of FIG. 10 creates an accuracy issue with biasing across the MR sensor 1040. The bias-control scheme of FIG. 10 has an undesirable frequency response because of its second order high pass response which therefore causes non-linear phase distortion. Accordingly, the high pass frequency point must be moved lower. The bias-control scheme of FIG. 10 has a longer transient recovery and therefore does not switch as quickly.
FIG. 11 shows the control-signal flow-graph 1100 for the schematic in FIG. 10. A higher-order control scheme is created by sensing the coupled sense points Vout 1110 and VPdiff 1120 (coupled by G3 1130, the voltage transfer to Nodes C and D). However, with this higher-order control scheme, there is no independent common-mode sense point. Furthermore, the analysis, implementation, and modification are more involved due to the interactions among the control loops.
It can then be seen that there is a need for a method and apparatus for ensuring that the biasing through all four transistors in a quadrature biasing for coupled pair circuits is equivalent.
It can then be seen that there is a need for a method and apparatus for quadrature biasing for coupled pair circuits that reduce the feedback-loop interaction to simplify the bias control system.