Sensor circuit

This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2013-026905 filed on Feb. 14, 2013, the entire content of which is hereby incorporated by reference.

1. Field of the Invention

The present invention relates to a sensor circuit, and more particularly, to a sensor circuit having a bridge sensor element.

2. Description of the Related Art

An output signal of a sensor element (sensor signal) is generally minute, and thus, is required to be amplified by a sensor circuit in order to be applied to an electronic circuit using a sensor element.

In recent years, miniaturization of electronic equipment is proceeding, and miniaturization of a sensor circuit built therein is also proceeding. Miniaturization of a sensor circuit results in a more minute sensor signal. In order to use a sensor signal which is a minute signal in ordinary electronic equipment, a higher amplification factor is necessary in an amplifier in the sensor circuit. On the other hand, a sensor circuit is required to operate at higher speed. A high amplification factor and high speed operation are generally mutually contradictory. In order to attain those requirements, hitherto, the current consumption of the amplifier is high (see, for example, Japanese Patent Application Laid-open No. 2010-181211).

However, electronic equipment, in particular, a mobile device driven by a battery, has a problem in that high current consumption is difficult to accept in a market.

The present invention has been made in view of those problems, and provides a sensor circuit which can amplify a sensor signal at high speed and with a high amplification factor without increasing the current consumption.

In order to solve the above-mentioned problems, according to one embodiment of the present invention, there is provided a sensor circuit including: a primary amplifier for amplifying in advance a differential output signal which is a current signal of a sensor element; a secondary amplifier for amplifying the amplified differential output signal; a constant voltage generating circuit for maintaining a sensor element driving current to be constant; and a feedback circuit for feeding back a feedback signal to adjust an amplification factor. Most of the currents which pass through the primary amplifier are bias currents of the sensor element.

According to the sensor circuit of the present invention, a sensor signal amplified by the primary amplifier is input to the secondary amplifier, and thus, the sensor circuit can amplify a sensor signal at high speed and with a high amplification factor.

Further, although the primary amplifier is added to the sensor circuit, most of the current consumption of the primary amplifier is used as a sensor element driving current, and thus, the current consumption of the sensor circuit almost does not increase.

FIG. 1 is a circuit diagram illustrating a sensor circuit of a first embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a sensor circuit of a second embodiment of the present invention.

FIG. 3 is a circuit diagram illustrating a sensor circuit of a third embodiment of the present invention.

FIG. 4 is a circuit diagram illustrating a sensor circuit of a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram illustrating a sensor circuit of a fifth embodiment of the present invention.

First Embodiment

FIG. 1 is a circuit diagram illustrating a sensor circuit of a first embodiment of the present invention.

The sensor circuit of the first embodiment includes a sensor element S 1 , a primary amplifier C 1 for amplifying in advance a differential output signal (sensor signal) which is a current signal of the sensor element S 1 , a secondary amplifier A 1 for amplifying the amplified sensor signal, a constant voltage generating circuit C 2 for maintaining a sensor element driving current to be constant, and a feedback circuit C 3 for feeding back a feedback signal to adjust an amplification factor. In this case, most of the currents which pass through the primary amplifier C 1 pass through the sensor element S 1 as a bias current of the sensor element S 1 .

The sensor element S 1 is formed as a bridge sensor element with four resistors R 33 to R 36 having the same resistance value. In other words, the sensor element S 1 is an element represented as four equivalent bridge resistors.

In the primary amplifier C 1 , a gate and a drain of an NMOS transistor M 11 are connected to a node N 13 , and a source of the NMOS transistor M 11 is connected via serially connected resistors R 11 and R 12 to a ground terminal. A constant current source I 1 is provided between a power supply terminal and the node N 13 . A gate and a drain of an NMOS transistor M 21 are connected to a node N 23 , and a source of the NMOS transistor M 21 is connected via serially connected resistors R 21 and R 22 to the ground terminal. A constant current source 12 is provided between the power supply terminal and the node N 23 . A gate of an NMOS transistor M 12 is connected to the node N 13 , a source of the NMOS transistor M 12 is connected to a node N 31 , and a drain of the NMOS transistor M 12 is connected via a resistor R 31 to the power supply terminal. A gate of an NMOS transistor M 22 is connected to the node N 23 , a source of the NMOS transistor M 22 is connected to the node N 31 , and a drain of the NMOS transistor M 22 is connected via a resistor R 32 to the power supply terminal. A node N 331 is a node with the drain of the NMOS transistor M 12 . A node N 332 is a node with the drain of the NMOS transistor M 22 . A node N 11 is the source of the NMOS transistor M 11 . A node N 21 is the source of the NMOS transistor M 21 .

A non-inverting input terminal of the secondary amplifier A 1 is connected to the node N 332 , an inverting input terminal of the secondary amplifier A 1 is connected to the node N 331 , a non-inverting output terminal of the secondary amplifier A 1 is connected to a non-inverting output terminal NO 2 of the sensor circuit, and an inverting output terminal of the secondary amplifier A 1 is connected to an inverting output terminal NO 1 of the sensor circuit. An output common voltage feedback terminal of the secondary amplifier A 1 is connected to the node N 31 which is a node for supplying a sensor element driving current. An output common voltage input terminal of the secondary amplifier A 1 is connected to a node N 41 which is a node for supplying an output common voltage of the secondary amplifier A 1 .

In the constant voltage generating circuit C 2 , a gate and a drain of an NMOS transistor M 41 are connected to a node N 43 , and a source of the NMOS transistor M 41 is connected via serially connected resistors R 41 and R 42 to the ground terminal. A constant current source I 4 is provided between the power supply terminal and the node N 43 . The node N 41 is the source of the NMOS transistor M 41 .

In the feedback circuit C 3 , a resistor 13 is provided between the inverting output terminal NO 1 of the sensor circuit and a node N 12 . A resistor 23 is provided between the non-inverting output terminal NO 2 of the sensor circuit and a node N 22 .

In this case, the constant current sources I 1 and I 2 cause the same amount of constant currents to pass through. The NMOS transistor M 11 and the NMOS transistor M 21 are of the same size. The NMOS transistor M 12 and the NMOS transistor M 22 are of the same size. The resistors R 11 to R 13 , the resistors R 21 to R 23 , and the resistors R 41 and R 42 are formed of a material which is the same as that of the sensor element S 1 . The resistance values of the resistors R 11 and R 12 and the resistors R 21 and R 22 are the same. The resistance values of the resistors R 41 and R 42 are the same.

Further, the resistance values of the resistors R 11 to R 13 are regarded as R 11 to R 13 , the resistance values of the resistors R 21 to R 23 are regarded as R 21 to R 23 , the resistance values of the resistors R 31 to R 36 are regarded as R 31 to R 36 , and the resistance values of the resistors R 41 and R 42 are regarded as R 41 and R 42 , respectively. The current values of the constant current sources I 1 and I 2 are regarded as I 1 and I 2 , respectively, and the current value of the constant current source I 4 is regarded as I 4 . Then, the size ratio of the NMOS transistor M 11 to the NMOS transistor M 12 is represented as:

1/(R 11 +R 12 ):1/(R 33 +R 34 ),

the size ratio of the NMOS transistor M 21 to the NMOS transistor M 22 is represented as:

1/(R 21 +R 22 ):1/(R 35 +R 36 ),

and the size ratio of the NMOS transistor M 11 and the NMOS transistor M 41 is represented as:

1/(R 11 +R 12 ):1/(R 41 +R 42 ),

and is also represented as:

I 1 :I 4 .

Next, operation of the sensor circuit according to the first embodiment is described.

In this case, the sensor element S 1 outputs, based on the bias current which passes between the node N 31 and the ground terminal and physical quantities such as an applied magnetic force, a differential output signal (sensor signal) which is a current signal of the sensor element S 1 , to the node N 12 and the node N 22 . Further, the constant voltage generating circuit C 2 generates a constant voltage at the node N 41 based on the constant current of the constant current source I 4 and the resistance values of the resistors R 41 and R 42 . The constant voltage is input to the output common voltage input terminal of the secondary amplifier A 1 to be the output common voltage of the secondary amplifier A 1 . The secondary amplifier A 1 controls the voltages at the non-inverting output terminal and the inverting output terminal so that the voltage at the output common voltage input terminal and the voltage at the output common voltage feedback terminal are the same. The output common voltage feedback terminal of the secondary amplifier A 1 is connected to the node N 31 , and thus, the voltage at the node N 31 and the voltage at the node N 41 are the same.

When there is no physical quantity such as a magnetic force applied to the sensor element S 1 , the resistance values of the resistors R 11 and R 12 vary depending on the temperature, and thus, the voltage at the node N 11 also varies. Similarly, the resistance values of the resistors R 33 to R 36 vary, and thus, the voltage at the node N 31 also varies. However, the resistors R 11 and R 12 are formed of a material which is the same as that of the resistors R 33 to R 36 of the sensor element S 1 , and thus, the temperature-resistance characteristics thereof are the same. Therefore, the variation amounts of the resistance values of the resistors R 11 and R 12 and the resistors R 33 to R 36 with respect to the temperature are the same. Then, it follows that the voltage at the node N 11 and the voltage at the node N 31 are the same, which are I 1 ×(R 11 +R 12 ). Similarly, the voltage at the node N 11 , the voltage at the node N 21 , the voltage at the node N 31 , and the voltage at the node N 41 are all the same. In this case, the voltage at a node between the resistor R 33 and the resistor R 34 and the voltage at the node between the resistor R 11 and the resistor R 12 are the same, and thus, a current is not caused to pass therebetween. Similarly, the voltage at a node between the resistor R 35 and the resistor R 36 and the voltage at the node between the resistor R 21 and the resistor R 22 are the same, and thus, a current is not caused to pass therebetween.

Note that, when the resistance values of the resistor R 12 and the resistor R 22 vary depending on the temperature, the output common voltages of the secondary amplifier A 1 at the nodes NO 1 and NO 2 also vary by the variations in resistance.

When there is a physical quantity such as a magnetic force applied to the sensor element S 1 , the sensor signal is output to the node N 12 to be caused to pass to the resistor R 12 . The sensor signal is also output to the node N 22 to be caused to pass to the resistor R 22 . Therefore, a voltage difference ΔV 12 is caused between the node N 12 and the node N 22 . The level of the voltage difference ΔV 12 is shifted and the shifted level propagates to the node N 13 and the node N 23 to be input to the gates of the NMOS transistor M 12 and the NMOS transistor M 22 , respectively.

Due to the voltage difference ΔV 12 , a current which passes through the resistor R 31 and a current which passes through the resistor R 32 vary in accordance with transconductance gm3 between the NMOS transistor M 12 and the NMOS transistor M 22 . Therefore, a voltage difference ΔV 331 is caused between the node N 331 and the node N 332 . The voltage difference ΔV 331 is (gm3×R 31 ) times as much as the voltage difference ΔV 12 . Generally, (gm3×R 31 )≈10 is secured with ease, and the amplitude of a signal which is input to the secondary amplifier A 1 becomes about 10(R 12 )/(R 12 +R 34 ) times as much due to the presence of the primary amplifier C 1 .

The voltage difference ΔV 331 is amplified by the secondary amplifier A 1 , and is fed back to the node N 12 and the node N 22 by the resistor R 13 and the resistor R 23 , respectively. In this case, the resistor R 13 and the resistor R 23 feed back the feedback signal to adjust the amplification factor, and the primary amplifier C 1 and the secondary amplifier A 1 operate so as to cancel out the variations in sensor signal. After the feedback, when the sensor signal stands still, the state of operation of the primary amplifier C 1 becomes the same as that when there is no sensor signal.

Note that, the differential output signal (sensor signal) which is a current signal of the sensor element S 1 depends on the resistance values of the resistors R 33 to R 36 , and thus, when these resistance values vary depending on the temperature, the sensor signal also varies. However, the resistor R 13 , the resistor R 23 , and the resistors R 33 to R 36 are formed of the same material, and thus, the temperature-resistance characteristics thereof are the same. Therefore, the variation amounts of the resistance values of the resistor R 13 , the resistor R 23 , and the resistors R 33 to R 36 with respect to the temperature are the same. In other words, the ratio of the resistance values of these resistors does not vary. Further, due to the dependence of these resistance values on temperature, the current signal of the sensor element S 1 varies. Therefore, the voltages at the nodes NO 1 and NO 2 do not vary and do not depend on the temperature. The necessity of a temperature compensating circuit for compensating for the dependence on temperature is eliminated, which results in reduced scale of the sensor circuit and reduced current consumption.

Second Embodiment

FIG. 2 is a circuit diagram illustrating a sensor circuit of a second embodiment of the present invention.

In the sensor circuit of the second embodiment, an output common voltage adjustment circuit C 4 is added to the sensor circuit of the first embodiment. A reference voltage generating circuit V 41 for generating a desired reference voltage is connected to a node N 42 via a resistor R 43 . The resistance value of the resistor R 43 is, for example, R 13 ×(1+R 12 /R 34 ). In the sensor circuit of the first embodiment, the output common voltages of the secondary amplifier A 1 at the nodes NO 1 and NO 2 are (I 1 ×R 12 ). However, in the sensor circuit of the second embodiment, the output common voltages may be a desired voltage.

Third Embodiment

FIG. 3 is a circuit diagram illustrating a sensor circuit of a third embodiment of the present invention.

In the sensor circuit of the third embodiment, the resistor R 13 and the resistor R 23 in the feedback circuit C 3 are connected differently. In the sensor circuit of the first embodiment, the resistor R 13 and the resistor R 23 are connected to the node N 12 and the node N 22 , respectively. However, in the sensor circuit of the third embodiment, the resistor R 13 and the resistor R 23 may be connected to the resistor N 11 and the resistor N 21 , respectively. When there is no physical quantity such as a magnetic force applied to the sensor element S 1 , in the sensor circuit of the first embodiment, the output common voltages of the secondary amplifier A 1 at the nodes NO 1 and NO 2 are voltages at the node N 12 and at the node N 22 , respectively, which are lower than the power supply voltage. However, in the sensor circuit of the third embodiment, the output common voltages are voltages at the node N 11 and at the node N 21 , respectively, which are higher than those in the case illustrated in FIG. 1 . Enlargement of the amplitudes of the sensor signals at the nodes NO 1 and NO 2 is enabled, which leads to a higher amplification factor of the sensor signals accordingly.

Fourth Embodiment

FIG. 4 is a circuit diagram illustrating a sensor circuit of a fourth embodiment of the present invention.

In the sensor circuit of the fourth embodiment, the resistors R 31 and R 32 in the primary amplifier C 1 are replaced by diode connected PMOS transistors M 31 and M 32 , respectively.

In the sensor circuit of the first embodiment, when the resistance value of the sensor element S 1 becomes higher depending on the temperature, the voltage at the node N 31 also becomes higher, and the drain-source voltages of the NMOS transistor M 12 and the NMOS transistor M 22 become lower. In this case, when, due to a physical quantity such as a magnetic force applied to the sensor element S 1 , the voltage at the node N 12 or the node N 22 is lowered and the voltage at the node N 331 or the node N 332 is lowered, the drain-source voltage of the NMOS transistor M 12 or the NMOS transistor M 22 becomes further lower. Then, the NMOS transistor M 12 or the NMOS transistor M 22 cannot operate normally.

In the sensor circuit of the fourth embodiment, owing to a physical quantity such as a magnetic force applied to the sensor element S 1 , the voltages at the nodes N 331 and N 332 are less liable to vary. Therefore, the drain-source voltages of the NMOS transistor M 12 and the NMOS transistor M 22 are less liable to be lowered.

Fifth Embodiment

FIG. 5 is a circuit diagram illustrating a sensor circuit of a fifth embodiment of the present invention.

As in the sensor circuit of the fifth embodiment, by reversely connecting the structural elements to the power supply terminal and the ground terminal and replacing the NMOS transistors with PMOS transistors, effects similar to those of the other embodiments can be obtained. In the sensor circuit of the fifth embodiment, the structural elements of the sensor circuit of the first embodiment are reversely connected to the power supply terminal and the ground terminal, but the same applies to the other embodiments.