Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Amplitude control
Reexamination Certificate
1999-04-27
2001-07-17
Lam, Tuan T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Amplitude control
C327S362000
Reexamination Certificate
active
06262619
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to power amplifiers, and more particularly to nulling output offset current in power amplifiers.
BACKGROUND OF THE INVENTION
Power amplifiers have been widely used as a final stage in multistage amplifiers, such as in audio amplifiers and radio transmitters, to deliver maximum power to a load, rather than maximum voltage gain, for a given percentage of distortion. One particular application of a power amplifier is in a disk drive device. In essence, a power amplifier acts as a transconductance amplifier and takes an input voltage and converts it into an output current to drive a voice coil motor (VCM).
FIG. 1
, a disk drive
10
is shown, which contains one or more magnetic disks
12
that spin about a spindle
14
. Data is written with onto disks
12
by a read/write head
16
, and information is read back using the same read/write head
16
. Read/write head
16
is attached to the bottom of a suspension
18
which is part of an actuator
20
that rotates about a pivot point
22
. Actuator
20
is moved by current through a voice-coil motor
24
. Crash stops (
26
and
28
) limit the travel of actuator
20
.
FIG. 2
shows a first close-up view of a load and unload ramp.
FIG. 3
shows a second close-up view of the load and unload ramp.
A close-up view of load/unload ramp
30
is shown in
FIGS. 2 and 3
. While only a single disk and two heads are shown, multiple disks and multiple sets of heads are also possible. Disk
12
is mounted on spindle
22
and is rotated by a spindle motor
40
. Read/write heads
16
are attached to suspensions
18
that contain tabs
32
. Before disk
12
is stopped, tabs
32
are pushed onto load/unload ramp
30
which is attached to base plate
42
of disk drive
10
by a mounting screw
44
or other fastening means. Heads
16
are lifted off disk
12
by tab
32
on the end of suspension
18
that travels up load/unload ramp
30
.
FIG. 3
is a top view of the arrangement shown from the side in FIG.
2
and illustrates the direction
46
which tab
32
moves when the tip of suspension
18
is pushed onto load/unload ramp
30
before disk
12
is stopped.
FIG. 4
illustrates a portion of a disk drive device controller, e.g., a load/unload controller, that illustrates a power amplifier
110
coupled to a load comprising a sensing resistor
112
, Rs, and a VCM
114
. When a control output signal having a zero voltage value is transmitted to the power amplifier
110
after conversion to analog form through a digital-to-analog (D/A) converter
116
, an output current from the power amplifier may be non-zero due to the intrinsic offsets of the components forming the power amplifier
110
. The non-zero output current is generally known as the output offset current.
The problem with the output offset current is that it can be large in high-performance disk drives due to the low resistance of the current sensing resistor
112
. For example, a power amplifier
110
, whose current sensing amplifier
118
has 5 mV (millivolts) of input offset voltage and uses a 200 milliohm sensing resistor
112
, can have an output offset current on the order of 25 miilliamperes. In a high performance disk drive with relatively large torque constants, an offset current of that value is enough to generate sufficient torque to move the actuator of the disk drive, which can be a problem in drives with a ramp for head loading, if the force of the heads on the ramp and the ramp's coefficient of friction are low enough to be overcome by this torque. This is especially true if the actuator is not normally mechanically latched when the heads are in the unloaded position. The result could be accidental head loading or unloading due to the output offset current alone if it is not sufficiently reduced or eliminated.
Accordingly, what is needed is a system and method for nulling the output offset current in a power amplifier. The present invention addresses such a need.
SUMMARY OF THE INVENTION
Method and system aspects for nulling output offset current in an amplifier are described. In an exemplary method aspect, the method includes determining at least one offset value with a power amplifier in at least one mode. The at least one offset value is then utilized to identify an output offset current value in the power amplifier. An adjustment to an input signal to the power amplifier occurs until the output offset current value is substantially nulled to identify a power amplifier offset value. Further included is supplying the power amplifier offset value to the power amplifier with a zero value control output voltage to ensure substantially zero output offset current in the power amplifier.
Through the present invention, a power amplifier is calibrated and the results of the storage of the calibration allows use for future reference or as an initial starting point for subsequent offset adjustments. The digital nature of the adjustment allows for fast and easy customization of offset compensation in a manufacturing environment. These and other advantages of the aspects of the present invention will be more fully understood in conjunction with the following detailed description and accompanying drawings.
REFERENCES:
patent: 4821125 (1989-04-01), Christensen et al.
patent: 4969059 (1990-11-01), Volz et al.
patent: 5061900 (1991-10-01), Vinn et al.
patent: 5191297 (1993-03-01), Penman et al.
patent: 5262907 (1993-11-01), Duffy et al.
patent: 5451895 (1995-09-01), Lim
patent: 5654840 (1997-08-01), Patton et al.
patent: 5677809 (1997-10-01), Kadlec
patent: 5781362 (1998-07-01), Bang
patent: 5883749 (1999-03-01), Park
McGreer Brooke
Rowan Bryan Scott
Bluestone Randall J.
International Business Machines - Corporation
Lam Tuan T.
Nguyen Hiep
Sawyer Law Group LLP
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