Dynamic magnetic information storage or retrieval – General recording or reproducing – Specifics of the amplifier
Reexamination Certificate
2000-10-31
2003-10-07
Holder, Regina N. (Department: 2651)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Specifics of the amplifier
C360S046000, C360S100100
Reexamination Certificate
active
06631044
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to improvements in mass data storage devices, and more particularly to frequency compensated amplifiers and methods for providing frequency compensation to amplifiers thereof to reduce a second order frequency response of a magneto-resistive preamplifier used therein.
2. Relevant Background
Mass data storage devices include tape drives, as well as hard disk drives that have one or more spinning magnetic disks or platters onto which data is recorded for storage and subsequent retrieval. Hard disk drives may be used in many applications, including personal computers, set top boxes, video and television applications, audio applications, or some mix thereof. Many applications are still being developed. Applications for hard disk drives are increasing in number, and are expected to further increase in the future.
Recently, hard disk drive manufacturers have begun to use magneto-resistive (MR) heads to provide the required sensitive magnetic transducer for reading data from the spinning disk of the drive. MR heads efficiently convert medium magnetization changes into sufficiently high current or voltage with a minimum amount of noise, detect signals at high densities with a negligible loss in signals, and are cost-effective. Moreover, MR-sensor technology is extendable to very high disk drive densities. The term “magneto-resistive” refers to the change in resistivity of metals in the presence of a magnetic field.
Although in the past, MR heads were thought of as having a very low inductance, recently the inductance of the heads have become a concern as the data densities of the drive have increased. The MR heads, including their inductive components have been modeled in various ways. For example, a circuit model
5
having a differential magneto-resistive head (MR)
10
and its associated preamplifier
12
for use in a hard-disk drive application is shown in FIG.
1
. The MR recording head
10
is represented by a resistor
14
designated as R
mr
. In the circuit model of
FIG. 1
, the MR head is biased by a bias voltage source
16
, and the parasitic inductive elements
18
and
20
are shown in series between the resistor
14
and bias voltage source
16
. The resistor
14
is capacitively coupled by parasitic capacitors
22
and
24
to the differential preamplifier
26
to provide the differential output voltage V
out
.
A similar circuit model
28
that is biased by a current source
30
is shown in FIG.
2
. In the circuit
28
, the resistor
14
of the MR head is connected in parallel with the current source
30
, and is capacitively coupled by the parasitic capacitances
22
and
24
to the differential amplifier by op-amps
32
and
34
. The output of the op-amps
32
and
34
provide the differential input to the differential preamplifier
12
. Other combinations of the head bias and sense schemes are known. For example, V-bias/I-sense and I-bias/V-sense schemes can easily be created.
Using the differential V-bias/V-sense amplifier configuration
5
of
FIG. 1
for the following illustration, the circuit can be described by the equations
Δ
V
S
Δ
V
B
=
Δ
R
mr
R
mr
EQ
(
1
)
and
V
out
=A×&Dgr;V
s
EQ(2)
where a change in R
mr
(&Dgr;R
mr
) generates a signal voltage of &Dgr;V
s
.
However, as mentioned, in reality, the MR head is not a pure resistor. The head contains the parasitic inductances (L
p
), and the parasitic capacitances (C
p
) in addition to the intended resistance R
mr
.
The MR heads, themselves, have been modeled in industry. For example, an electrical schematic diagram of a full model
38
of a magneto-resistive head of the type used in mass data storage devices is shown in
FIG. 3
, and an electrical schematic diagram of a simple model
40
of a magneto-resistive head of the type used in mass data storage devices is shown in FIG.
4
. The full model
38
is essentially modeled as a transmission line of inductors
42
-
45
and capacitors
47
-
49
, with the resistor
14
of the head at one end and the connection to the preamplifier at the other end. The simple model
40
includes only two lumped inductors
50
and
52
and a single lumped capacitor
54
.
With reference to the simple model
40
of
FIG. 4
, it can easily be shown that
Δ
Vs
′
Δ
Vs
=
1
2
×
L
p
×
C
p
s
2
+
(
s
×
R
mr
2
×
L
p
)
+
1
2
×
L
p
×
C
p
EQ
(
3
)
where &Dgr;V
s
is the signal voltage generated by &Dgr;R
mr
, while &Dgr;Vs′ is the resultant signal voltage applied to the preamplifier input.
Note that EQ(3) describes a second-order system with
ω
n
=
1
2
×
L
p
×
C
p
EQ
(
4
)
and
Q
=
1
R
mr
×
2
×
L
p
C
p
EQ
(
5
)
Therefore,
V
out
=
A
×
Δ
V
S
′
EQ
(
6
)
=
A
×
1
2
×
L
p
×
C
p
s
2
+
(
s
×
R
mr
2
×
L
p
)
+
1
2
×
L
p
×
C
p
×
Δ
V
S
EQ
(
7
)
According to EQ(7), the frequency response of Vout/&Dgr;Vs is not only determined by the preamplifier frequency response, but it is exhibiting an additional second-order system response given by the characteristic &ohgr;
n
and Q of a second-order system.
The present trend in the industry is to use small values of R
mr
. Unfortunately, as R
mr
is reduced, the Q given by EQ(5) increases. This effect shows up as an increasing magnitude peaking at about the bandwidth edge of the V
out
/&Dgr;V
s
frequency response, as can be seen in the gain vs. frequency curve of FIG.
5
.
One conventional technique of reducing this peaking is to add a differential capacitor array inside one gain stage
12
of the preamplifier
82
, as shown in
FIG. 6
, to create a pole to suppress the peaking. As shown in
FIG. 6
, the preamplifier gain-stage
12
is a differential amplifier with emitter degeneration. Capacitors
58
and
60
, of value C
BWR
, are connected across the load resistors
62
and
64
to provide poles to compensate the frequency peaks shown in FIG.
5
. The pole frequency is given by
Freq
=
1
2
×
π
×
R
L
×
C
BWR
EQ
(
8
)
and can be varied by varying C
BBR
to tailor to different R
mr
values. For example, smaller R
mr
(and thus more peaking) requires a lower frequency pole. This peak-suppression method is commonly known as the “Bandwidth Reduction Technique.”
An undesirable side effect of this Bandwidth Reduction Technique is the presence of some frequency-response drooping at some frequencies below the passband edge. Ideally, the passband “ripple” should be minimal and well below ±1 dB. The drooping becomes worse as more C
BWR
is employed to reduce larger peaking. This phenomenon can be seen in
FIG. 5
as C
BWR
increases, as denoted by arrow
66
.
What is needed, therefore, is a compensation technique that can reduce the peaking, with reduced frequency-response drooping at some frequencies below the passband edge.
SUMMARY OF THE INVENTION
In light of the above, therefore, it is an object of the invention to provide a compensation technique that can reduce the frequency response peaking of a magneto-resistive head, with reduced frequency-response drooping at some frequencies below the passband edge.
Thus, according to a broad aspect of the invention, a method is presented for reducing a frequency response peaking of a magneto-resistive head. The method includes providing an amplifier stage to amplify a signal that varies in response to an electrical characteristic of the head, and creating at least two poles in a frequency response of the amplifier to compensate for the frequency response peaking. Preferably, the poles have substantially identical pole locations. In a preferred embodiment, the amplifier stage comprises at least a pair of capacitor compensated differential amplifiers, and in another, the amplifier stage comprises three capacitor compensated differential amplifiers.
In
Brady W. James
Holder Regina N.
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
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