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AC Bias Read Write Driver

IP.com Disclosure Number: IPCOM000051667D
Original Publication Date: 1981-Feb-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 42K

Publishing Venue

IBM

Related People

Klaassen, KB: AUTHOR

Abstract

A buried servo disk file is a moving head magnetic disk storage system in which the position of the head is controlled by a prerecorded servo signal that resides in the same portion of the recording medium as the stored data. For continuous servoing from such recorded servo information, using only one head, the head write and read electronics must make it possible to read back the servo information not only while reading data but also while writing data. To make servo reading possible while writing data, the write driver should present a high output impedance to the head, i.e., the write driver should drive the head with a current rather than a voltage.

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AC Bias Read Write Driver

A buried servo disk file is a moving head magnetic disk storage system in which the position of the head is controlled by a prerecorded servo signal that resides in the same portion of the recording medium as the stored data. For continuous servoing from such recorded servo information, using only one head, the head write and read electronics must make it possible to read back the servo information not only while reading data but also while writing data.

To make servo reading possible while writing data, the write driver should present a high output impedance to the head, i.e., the write driver should drive the head with a current rather than a voltage.

To implement this, present write driver designs for non-center-tapped heads use a differential transistor stage with an impedance in both collector leads that is large with respect to the head impedance. The operation of such a differential stage requires the flow of a quiescence current I(q) through these collector impedances. The value of this current must be equal to or larger than half the maximally required peak-to-peak current through the head.

There are different known methods for the realization of the high collector impedances - namely, resistors, inductors and active loads (using transistors).

If resistors R(c) are used as collector impedances, the current I(q) gives a large power dissipation I(q)/2/R(c) in these resistors.

Moreover, a slight mismatch in resistance R(c) of these resistors and a mismatch in I(q) flowing through these resistors give rise to a DC current through the head. This current causes even-order harmonic distortion in the magnetic write process. Thirdly, when the tail current 2I(q) into the driver stage is turned off to disable the driver, the collector voltage of the transistors rises I(q)R(c) volts. This high collector voltage in the off state causes secondary breakdown of the transistors, necessitating additional circuit complexity in the form of a collector voltage switch that, together with a switched tail current, is used to switch the driver on and off.

If inductors are used as collector impedances, the above disadvantages are eliminated. They are traded in for two other disadvantages. The driver exhibits large transients at the instants at which the tail current is switched on and off. This is caused by the large voltage peaks across the collector inductors at the switch on and off of I(q). A second disadvantage is that across the head there is not only a differential-mode voltage, but also a large unwanted common-mode voltage, especially at high frequencies. This common-mode voltage gives rise to extra filter complexity in the read channel. It is caused by the high collector impedance and slight unbalances in the base drive of the write driver, unbalances in the write driver itself and by the finite value of the tail current source impedance.

The main disadvantage of an active load impedance (using active devices suc...