Amplifiers – Sum and difference amplifiers
Reexamination Certificate
2001-04-25
2002-06-18
Pascal, Robert (Department: 2817)
Amplifiers
Sum and difference amplifiers
C330S075000, C330S109000
Reexamination Certificate
active
06407631
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a charge-type sensor amplifying circuit for amplifying and outputting the output from a charge-type sensor such as an acceleration sensor.
2. Description of the Related Art
Hitherto, piezoelectric-type acceleration sensing devices, pyroelectric-type infrared sensing devices, and the like have been known as sensing devices that use sensors obtaining the detected outputs in the form of charges. Since the amount of charge which is generated when these sensing devices respond to acceleration, infrared, or the like is minute, e.g. 0.01 to several thousands pC, an amplifying circuit for amplifying the output from the sensor and obtaining the output in the form of a voltage signal is used. The piezoelectric acceleration sensing device is used for sensing activation of an airbag in a vehicle (collision detection), sensing the angular velocity while the vehicle is in motion, sensing a shock against a hard disk drive, and the like.
A known amplifying circuit amplified the output from the sensor using a charge amplifier (Japanese Unexamined Patent Application Publication No. 8-338781).
FIG. 7
shows the construction of the amplifying circuit disclosed in the application. In this amplifying circuit, a resistor (feedback resistor) R
11
is connected between an inverting input terminal and an output terminal of an operational amplifier Amp. A capacitor (feedback capacitor) C
11
is connected in parallel with this resistor R
11
. One end of an acceleration sensor G is connected to the inverting input terminal of the operational amplifier and the other end of the acceleration sensor G is connected to a reference voltage V
ref
. A non-inverting input terminal of the operational amplifier is connected to the reference voltage V
ref
.
In this amplifying circuit, vibration is applied to the acceleration sensor in which charge Q is generated in accordance with the magnitudes of the acceleration and vibration, and the generated charge Q is amplified and output using the operational amplifier. The relationship between the charge Q generated at the acceleration sensor and the output voltage V
o
from the operational amplifier is given by
V
0
(
s
)
=
-
sQ
×
R
11
1
+
sC
11
R
11
[
equation
13
]
V
0
(s) is a Laplace transform function and is an algebraic equation with respect to s.
The frequency characteristic of this circuit is generally expressed by the output voltage and the cut-off frequency in a flat region. The cut-off frequency f
c
is given by
fc
=
1
2
π
C
11
R
11
[
equation
14
]
In order to expand the sensitivities of acceleration and vibration, the cut-off frequency f
c
should be decreased. As is obvious from the above [equation 14], when the capacitance of the capacitor C
11
and the resistance of the resistor R
11
are increased, the cut-off frequency f
c
is decreased. Therefore, the capacitance of the capacitor C
11
and the resistance of the resistor R
11
should be increased in order to increase sensitivity ranges of acceleration and vibration.
However, when the capacitance of the capacitor C
11
is increased, the following problems occur: the circuit behavior becomes unstable, which tends to cause oscillation; and, in addition, since the output voltage in the flat region is decreased, the sensitivity is decreased. Furthermore, there is another problem in that, since a resistance element having high resistance is expensive, when the high-resistance resistance element is used as the resistor R
11
, the cost of the amplifying circuit is increased. Here, the sensitivity means the gain of the operational amplifier.
The amplifying circuit for solving the foregoing problems is already disclosed in Japanese Unexamined Patent Application Publication No. 11-242048.
FIG. 6
shows the construction of a piezoelectric-type sensor amplifying circuit that is disclosed in the application. In this piezoelectric-type sensor amplifying circuit, a capacitor C
21
is connected between the inverting input terminal and the output terminal of the operational amplifier Amp. Two resistors, R
22
and R
23
(function as dividing resistors) are connected in series in this order between the output terminal of the operational amplifier and the reference voltage V
ref
. One terminal of the resistor R
21
is connected to a node between the resistors R
22
and R
23
and the other terminal of the resistor R
21
is connected to the inverting input terminal of the operational amplifier. The non-inverting input terminal of the operational amplifier is connected to the reference voltage. One terminal of the acceleration sensor G sensor is connected to the inverting input terminal of the operational amplifier and the other terminal thereof is connected to the reference voltage V
ref
.
In the above amplifying circuit as well, vibration is applied to the acceleration sensor in which charge Q is generated in accordance with the magnitudes of the acceleration and vibration, and the generated charge Q is amplified and output using the operational amplifier. The relationship between the charge Q generated at the acceleration sensor and the output voltage V
o
from the operational amplifier is given by
V
0
(
s
)
=
-
sQ
×
(
R
21
R
22
+
R
21
R
23
+
R
22
R
23
R
23
)
1
+
sC
21
(
R
21
R
22
+
R
21
R
23
+
R
22
R
23
R
23
)
[
equation
15
]
V
0
(s) is a Laplace transform function in the same manner as the above [equation 13] and is an algebraic equation with respect to s. The cut-off frequency f
c
given by
fc
=
1
2
π
C
21
(
R
21
R
22
+
R
21
R
23
+
R
22
R
23
R
23
)
=
1
2
π
C
21
R
21
(
1
+
R
22
R
23
+
R
22
R
21
)
[
equation
16
]
As is obvious from the above [equation 16], the cut-off frequency f
c
of the amplifying circuit shown in this
FIG. 7
is one part in (1+R
22
/R
23
+R
22
/R
21
) of that of circuit shown in FIG.
6
. By adjusting the resistances of the resistors R
21
to R
23
, the cut-off frequency f
c
can be decreased without decreasing the sensitivity (the sensitivity range can be expanded).
However, the amplifying circuits shown in the above
FIGS. 6 and 7
did not compensate for the temperature characteristic of the charge sensitivity (the temperature characteristic of the amount Q of charge which is generated at the time of responding to acceleration, an infrared, or the like) of a piezoelectric sensor such as an acceleration sensor. Accordingly, there is a problem in that change in the output voltage Vo from an operational amplifier is considerable with respect to variation in the temperature.
Theoretically, when a feedback capacitor C
11
(or C
21
), whose temperature characteristic is the same as the temperature characteristic of the charge sensitivity of the piezoelectric sensor is used, the fluctuation in the output voltage Vo from the operational amplifier with respect to the variation in the temperature can be prevented. This enables a temperature-compensated piezoelectric-type sensor amplifying circuit to be obtained. However, since it is difficult to obtain the piezoelectric sensor and the feedback capacitor that have the same temperature characteristic, implementation of the temperature-compensated piezoelectric-type senor amplifying circuit is difficult.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention is to provide a charge-type sensor amplifying circuit to prevent the change in the output from the circuit with respect to the variation in the temperature in the circuit.
To this end, according to a first aspect of the present invention, there is provided a charge-type sensor amplifying circuit including an operational amplifier having an inverting input terminal thereof connected to a terminal of a charge-type sensor, a voltage divider having two voltage-dividing points which divide the output voltage from the operational ampl
Murata Manufacturing Co. Ltd.
Nguyen Khanh V.
Ostrolenk Faber Gerb & Soffen, LLP
Pascal Robert
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