Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1998-09-17
2001-03-27
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S510000
Reexamination Certificate
active
06208176
ABSTRACT:
TECHNICAL FIELD
The present invention relates to semiconductor magnetoresistive (SMR) devices, also known in the art as magneto-resistors (MR), employed in position and speed sensors, and more particularly to a method and apparatus to achieve increased sensitivity of SMR devices at high temperatures and large air gaps.
BACKGROUND OF THE INVENTION
It is well known in the art that the resistance modulation of SMR devices can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456).
The shortcoming of SMR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of SMR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive SMR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the SMR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the SMR device, it is also desirable to make the current of the current source automatically increase with the SMR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the SMR device. It is also known that air gap variations between the SMR device and ferromagnetic materials or objects will affect the resistance of SMR devices with larger air gaps producing less resistance and decreased output signals.
What is needed is a less complicated method and apparatus having the features of a current source and employing an automatically adjustable current to compensate for decreased SMR sensitivity at high temperatures and large air gaps.
A circuit of interest in this regard, well known in the art, is a conventional current mirror circuit
10
(often referred to simply as a current mirror), shown in FIG.
1
. In current mirror circuit
10
, the reference resistor R has a fixed value and, in conjunction with the constant voltage source V
SS
and transistor Q
1
, determines the magnitude of the reference current I
R
. The electronic operation of the current mirror circuit
10
dictates that the current I
0
will have approximately the same magnitude as the reference current I
R
provided that transistors Q
1
and Q
2
are matched. Thus, the reference current I
R
is mirrored to be the collector current I
0
of transistor Q
2
. In
FIG. 1
, I
0
is conventionally called the mirror current or mirrored current and the “mirrored portion” of the current mirror circuit
10
will be designated the mirrored circuit
12
.
Accordingly, it would be desirable if somehow the current mirroring feature of a mirror circuit could be adapted to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit.
SUMMARY OF THE INVENTION
The present invention is an adaptive driver circuit which uses a modified conventional current mirror circuit, as shown in
FIG. 1
, to provide a current source employing an automatically adjustable current to compensate for decreased SMR device sensitivity at higher temperatures and large air gaps without the need for an active feedback circuit. The adaptive driver circuit according to the present invention is a unique modification of the current mirror circuit
10
of
FIG. 1
, in that an SMR device is used as the reference resistor and a fixed resistor in the mirrored circuit
12
of
FIG. 1
to generate an output voltage.
In operation of the adaptive driver circuit according to the present invention, as the resistance of the SMR device decreases due to temperature increases or air gap increases between the SMR device and ferromagnetic materials or objects, the reference current increases. This produces an increase in the mirror current which increases the value of the output voltage across the fixed resistor, thereby maintaining the peak of the output voltage close to a known fixed saturation voltage. This offers a simple fixed threshold approach in converting the output signal into a digital signal.
A modification of the present invention is also possible whereby two adaptive driver circuits are used in a differential mode.
Accordingly, it is an object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage.
It is an additional object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage as the resistance of an SMR device change due to temperature variations of the SMR device.
It is still another object of the present invention to provide an output voltage having a peak value close to a known fixed saturation voltage as the resistance of an SMR device changes due to air gap variations between the SMR device and ferromagnetic materials or objects.
It is yet another object of the present invention to provide an output voltage representing a differential signal voltage between SMR devices.
These, and additional objects, advantages, features, and benefits of the present invention will become apparent from the following specification.
REFERENCES:
patent: 5168244 (1992-12-01), Muranaka
patent: 5402064 (1995-03-01), Eck et al.
patent: 5404102 (1995-04-01), Gokhale et al.
patent: 5570016 (1996-10-01), Schroeder et al.
patent: 5754042 (1998-05-01), Schroeder et al.
patent: 5986839 (1999-11-01), Klaassen et al.
C.J.Savanr,Jr., Martin S. Roden, Gordon L. Carpenter, “Electronic Design Circuits and Systems”Second Edition, 1991, p. 387 (9.3.4 Current Mirrors).
Horowitz et al, “The Art of Electronics”, 2nd Edition 1989, Cambridge University Press, pp. 88-89.
Heremans Joseph Pierre
Schroeder Thaddeus
Dobrowitsky Margaret A.
General Motors Corporation
Tran Toan
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