Temperature Drift Stabilized Filter Circuit

1. Field of the Invention

The present invention relates to DC component cutoff filter circuits or high-pass filter circuits and, more particularly, to a filter circuit utilizing a capacitance amplifier circuit (C multiplier).

2. Description of the Related Arts

As a capacitance connected to a DC cutoff filter circuit or the like as shown in FIG. 2, a capacitance amplifier circuit can be used, wherein a composite capacitance is formed using an operational amplifier as shown in FIG. 3A. The capacitance amplifier circuit (FIG. 3A) can be considered as a circuit which equivalently amplifies a capacitance by a factor of R1/R3 when resistance R3 is sufficiently smaller than resistance R1. This circuit is widely used because it can be provided utilizing a capacitor having a capacitance smaller than that which is actually needed and it can be made compact as a whole even when it must have a large capacitance. The filter circuit shown in FIG. 2 is constituted by a CR circuit having a composite capacitance and series resistance R4 (the equivalent circuit is shown in FIG. 3B), and its output is current-amplified by a voltage follower of an operational amplifier A2.

However, the capacitance amplifier circuit shown in FIG. 3A has a problem in that even if the operational amplifier A1 used is of a high performance type having a low drift, a drift in the capacitance output voltage is caused by a temperature drift in its offset voltage and offset current, a temperature drift of the capacitor used therein itself and a temperature drift of the resistor used therein itself. This results in a problem that the drift adversely affects the characteristics of the filter, especially the output characteristics when the input signal is very small, thereby reducing the reliability of the device as a whole.

One solution to this problem is to use a resistor having a different drift to compensate for the temperature drift of the capacitor. However, a problem still remains in this case that even though the temperature drift in the capacitor can be canceled, drifts in DC components due to the operational amplifier can not be completely canceled and, as a result, the characteristics of the circuit are not satisfactory especially as those of a filter circuit wherein highly accurate signal processing must be performed.

It is an object of the present invention to provide the output of a filter circuit utilizing a capacitance amplifier circuit with sufficient stability against temperature changes.

In a filter circuit which comprises a CR circuit constituted by a composite capacitance C synthesized by a capacitance amplifier circuit having an input capacitance, an operational amplifier connected to the input capacitance at a non-inverted input thereof, an output resistor and feedback resistors and series resistance R and which cuts off DC components, the arrangement according to the present invention is characterized by a combination of a reference voltage circuit which obtains its reference voltage at a grounding point of the filter circuit and a resistor included in the reference voltage circuit for generating the reference voltage and for generating a temperature drift in the reference voltage which compensates for a temperature drift in the output voltage of the capacitance amplifier circuit.

The resistors forming part of the capacitance amplifier circuit may be resistors of various materials such as metal film resistors, carbon resistors and thermistors and, when the circuit is implemented on an integrated circuit, diffused resistors or thin film resistors on the surface thereof may be used. Each of these resistors has a temperature coefficient specific thereto, and the ratios of the resistance thereof vary at various temperatures depending on the combination of the materials used. This results in a change in voltage division performed by the resistors and, consequently, a temperature drift in the electric potential of the terminal of interest can be compensated. In circuit configurations which are common products excluding special and expensive circuits such as for precise measurement, however, it is not possible to completely eliminate a temperature drift in an output voltage of interest by way of only compensation in this capacitance amplifier circuit. Therefore, according to the present invention, the ground side of the series resistor connected to the output of the capacitance amplifier circuit is regarded as a virtual ground, and an arrangement is made so that a temperature drift appears in a reference voltage which determines the electric potential of the virtual ground. This temperature drift is obtained by the use of a combination of resistors having different temperature coefficients, and the magnitude of the temperature drift is set so that a stable output voltage having no temperature drift is obtained by correcting a temperature drift in the output voltage. For example, if the drift in the output voltage of the capacitance amplifier circuit is a positive temperature characteristic, i.e., if the voltage is increased with an increase in the temperature, the reference voltage is generated by arranging resistors having different temperature coefficients such that a negative temperature drift appears at the reference voltage, i.e., the virtual ground potential is decreased by the same degree with the increase in the temperature. The arrangement of the resistors having different temperature coefficients can be made in various combinations and will be specifically described later with reference to a preferred embodiment of the present invention.

With the arrangement according to the present invention, a temperature drift at the capacitance amplifier circuit is compensated by a temperature drift at the reference voltage circuit. Thus, the capacitance amplifier circuit provides very stable apparent output and, therefore, the filter circuit exhibits stable and highly accurate operations. Therefore, the present invention can be advantageously applied to filters for very small signals and the like.

These and other objects and advantages of this invention will become more apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a circuit diagram of a filter utilizing a capacitance amplifier circuit wherein temperature compensation is performed at a virtual ground thereof;

FIG. 2 is a circuit diagram of a filter utilizing a related capacitance amplifier circuit;

FIG. 3A is a circuit diagram of a capacitance amplifier circuit;

FIG. 3B is a circuit diagram equivalent to the circuit diagram in FIG. 3A;

FIG. 4 is a circuit diagram showing a second example of the temperature compensation method;

FIG. 5 is a circuit diagram showing a third example of the temperature compensation method;

FIG. 6 is a circuit diagram showing a fourth example of the temperature compensation method;

FIG. 7 is a circuit diagram showing a fifth example of the temperature compensation method; and

FIG. 8 is a circuit diagram showing a sixth example of the temperature compensation method,

The present invention will now be described with reference to a preferred embodiment thereof.

FIG. 1 is a circuit diagram of a DC cutoff filter circuit according to an embodiment of the present invention wherein a capacitance amplifier circuit is formed by an operational amplifier A1 (designated by reference numeral 1 in FIG. 1), a capacitor C1 (designated by reference numeral 5 in FIG. 1), feedback resistors R1 and R2 and an output resistor R3 to provide a composite capacitance C.sub.o. One of the ends of the capacitor C1 serves as an input terminal 2 and the other end is connected to a non-inverted input of the amplifier A1. The output of the amplifier A1 is fed back from the output resistor R3 through the feedback resistor R1 to the non-inverted input. The resistor R2 is a negative feedback resistor having the same value as the resistor R1. The output resistor R3 is connected to a series resistor R4. The other end of the resistor R4 is grounded to a reference voltage V.sub.R (designated by reference numeral 4 in FIG. 1) of a virtual ground to be described later. The series resistor R4 forms a CR circuit in combination with the synthesized capacitance C.sub.o. A capacitance voltage output V.sub.0 ' (designated by reference numeral 10 in FIG. 1) is input to an operational amplifier A2 which constitutes a voltage follower as a current booster, and the output 3 of the amplifier A2 appears as an output V.sub.0. A reference voltage circuit generating the reference voltage V.sub.R is constituted by a voltage follower wherein a divided voltage 9 established by resistors r1, R6 and R5 connected to a constant voltage source V.sub.CC is coupled to a non-inverted input of an operational amplifier A3 to output the reference voltage V.sub.R in a stable manner. This electric potential serves as a virtual ground potential of the above-described CR circuit and provides a DC bias voltage. The resistors represented using "r" are resistors having a temperature coefficient which is different from that of the resistors represented using "R".

The operation of the DC cutoff filter circuit is as follows. First, when an input signal Vin is input to an input terminal 2 of this capacitance amplifier circuit, the voltage at the input terminals of the amplifier A1 provides the same electric potential at both terminals. So, the output resistor R3 and feedback resistor R1 divide the output current in accordance with the ratio between the values of these resistors, and the amplifier A1 operates, i.e., amplifies the apparent charge and discharge of the capacitor 5 (C1) as if they are large capacitance values. In other words, the amplified capacitance C.sub.0, in combination with the series resistor R4, forms a CR circuit having a large time constant and a cutoff frequency f.sub.P of 1/(2.pi.C.sub.0 R4) which performs the function of a filter circuit. The operation of this capacitance amplifier circuit is well known (see FIG. 3A and FIG. 3B).

Various types of capacitors may be used as the capacitor 5 (C1) depending on purposes, and they have different capacitance temperature coefficients which may be positive or negative. Therefore, the combination of the temperature coefficients of the resistors in the capacitance amplifier circuit can vary. If an ideal operational amplifier which is perfect and has no drift is assumed, drifts in the entire circuit can be reduced. However, such a perfect amplifier does not actually exist, the performance of the capacitance amplifier circuit depends on how much the drift can be suppressed.

The operation of the reference voltage circuit connected to the virtual grounding point of the above-described CR filter circuit will now be described. This circuit is also a voltage follower, the basic operation of which is well known. The output V.sub.R of operational amplifier A3 is determined by an electric potential which is determined by a reference electric potential at an non-inverted input terminal 9 thereof. Since the output electric potential V.sub.R is determined by the resistors r1, R5 and R6 which determine the electric potential at the non-inverted input terminal 9, the resistance temperature coefficients of these resistors can be selected so that the temperature characteristics of the reference voltage circuit including the temperature characteristics of the amplifier A3 results in a temperature coefficient which is the reverse of the temperature coefficient of the capacitance amplifier circuit. Since the temperature characteristics of the capacitance amplifier circuit are determined by the amplifier A1, capacitor C1, resistors R1, R2, R3 and the like, the coefficient can be either positive or negative. Therefore, if resistors represented using "r" having different temperature coefficients are disposed at the upper side of the divided voltage, an increase in the temperature causes a change in the ratio between the resistors, i.e., a change in the voltage division ratio which causes the output voltage V.sub.R to change. In the present embodiment, it is shown in the circuit diagram that the amplifier used is of a single power supply type, and the virtual ground is not 0 V and also has a function of supplying the reference voltage V.sub.R to the output of the capacitance amplifier circuit as a bias voltage. In other words, a certain bias value is given as the virtual ground, and the temperature coefficient of the bias value is used to compensate for the temperature coefficient of the output of the filter circuit, especially the output V.sub.o ' of the capacitance amplifier circuit. Therefore, for a circuit utilizing an amplifier of a two power supply type, it is of course possible to provide the reference voltage circuit with a temperature coefficient even if the reference voltage is generated at the mid point between the positive and negative voltages and the virtual ground point is thereby set exactly at 0 V.

The output voltage V.sub.0 at the operating point (the value of the output voltage when the input frequency f.sub.IN =0) in the circuit diagram in FIG. 1 is given by the following equation.

where V.sub.OS represents offset voltage of the amplifier A1 and I.sub.OS represents offset current.

V.sub.OS and I.sub.OS in the second term of this equation have temperature characteristics determined by temperature drifts in h.sub.fe and V.sub.BE of the transistors in the amplifier. Then, the change caused by a temperature T can be obtained by differentiating the above equation by T as follows. ##EQU1## where .alpha. is a differentiation sign and k, R' and I.sub.i (i=1,3,4) represent coefficients.

Therefore, it will be understood that if .alpha.V.sub.R /.alpha.T is selected so that Equation 2 results in 0, compensation will be performed by canceling the temperature drift in the capacitance amplifier circuit. Since the temperature coefficients are actually given in only rough values, the resistance is set so that the temperature coefficient at the output is minimized by appropriately selecting the types of the resistors used.

If the R-type resistors in FIG. 1 are diffused resistors on an integrated circuit, the temperature coefficient of the resistance is approximately 2000 ppm/.degree. C. while, if the thin film resistors are used as the r-type resistors, their temperature coefficient is approximately 0 ppm/.degree. C. As a result, the division ratio of the voltage Vcc changes with an increase in the temperature to slightly increase the electric potential at the terminal 9 and to increase the electric potential V.sub.R at the virtual ground accordingly. Since the output V.sub.0 ' of the capacitance amplifier circuit tends to slightly decrease with the temperature rise, fluctuation in the total output V.sub.o can be consequently avoided despite the temperature rise. An example with specific values for calculation will now be given, wherein the output V.sub.0 ' of the capacitance amplifier circuit is decreased by 1% with an increase in the temperature of 30.degree. C. Then, the voltage at the terminal 9 is calculated on the assumption that, in FIG. 1, Vcc is 12 V; r1 is 5 K.OMEGA., 0 ppm/.degree. C.; R6 is 15 K.OMEGA., 2000 ppm/.degree. C.; and R5 is 10 K.OMEGA., 2000 ppm/.degree. C. The result is that the voltage is 4.00 V if there is no influence of the temperature coefficients of the resistance while a temperature increase of 30.degree. C. results in 4.038 V, i.e., an increase in the voltage of approximately 1%. It will now be understood that the voltage V.sub.o ' can be substantially compensated by the use of the configuration of the virtual ground having those constants.

That is, in a case where the temperature drift in the output voltage of the capacitance amplifier circuit has a negative coefficient, to make the temperature drift in the reference voltage positive coefficient, resistors in the reference voltage circuit are arranged such that a resistor having a small temperature coefficient is disposed on the side of the dividing resistance closer to the constant voltage source. On the other hand, in a case where the temperature drift in the output voltage of the capacitance amplifier circuit has a positive coefficient, to make the temperature drift in the reference voltage negative coefficient, resistors in the reference voltage circuit are arranged such that a resistor having a small temperature coefficient is disposed on the side of the dividing resistance closer to the ground.

FIG. 4 through FIG. 8 show examples of several combinations of the configuration of the capacitance amplifier circuit and the configuration of the resistors in the reference voltage circuit. These figures show modifications such as changes in the combination of the temperature coefficients of the resistor at the portion connected to the virtual ground and the resistors determining the reference voltage of the amplifier A3 and the use of a diode. The letters R and r indicates resistors having different temperature coefficients of resistance, and it is apparent that there are various possible combinations for compensation of the temperature coefficients.

The circuit shown in FIG. 4 shows a case wherein the temperature coefficient of the capacitance value of the capacitor C1 is positive and the temperature coefficient of the output voltage V.sub.o ' of the capacitance amplifier circuit is negative. First, it is assumed that the resistor r2 is not connected to V.sub.o ' yet Then, the ratio between the resistance values of the resistors r4, r3 and R3 changes with an increase in the temperature and there is a decrease in the impedance including Vin. This results in a decrease in the electric potential V.sub.o '. Although the current flowing through the amplifier A1 which amplifies the capacitance is limited by the resistor R3, the resistance of the resistor R3 is increased as the temperature increases to decrease the apparent capacitance. However, since the change in the capacitance is larger, the temperature coefficient of the electric potential V.sub.o ' remains negative. Then, the resistors r1, R5 and R6 are arranged as shown in FIG. 4 so that the temperature characteristics of the virtual ground will be the reverse of the temperature coefficient of the electric potential V.sub.o '. As a result, the electric potential V.sub.R of the virtual ground increases with an increase in the temperature. Then, the resistor r2 is connected to electric potential V.sub.o ' to compensate for the value of the electric potential V.sub.o ', which eliminates fluctuation associated with the temperature.

The circuits shown in other drawings are based on the same idea. FIG. 5 and FIG. 6 show cases wherein the temperature coefficient of the capacitance of the capacitor C1 is negative, and the temperature coefficient of the electric potential V.sub.o ' is negative for both cases In FIG. 5, compensation is performed by resistors in the same combination as that in the reference voltage circuit in FIG. 4. In FIG. 6, forward resistance of a diode is used instead of a resistor. Since this resistance decreases with an increase in the temperature, the reference voltage is also increased and the electric potential V.sub.R of the virtual ground is increased to perform compensation. On the contrary, FIG. 7 shows a situation wherein the temperature coefficient of the electric potential V.sub.o ' is positive which can occur depending on the type of the capacitor and amplifiers used. In this case, the resistor r1 is disposed on the side of the ground contrary to the case shown in FIG. 4 to decrease the electric potential V.sub.R at the virtual ground, whereby the temperature drift in the electric potential V.sub.o ' is compensated. FIG. 8 shows an alternative to the combination shown in FIG. 5.

As described above, the output of a filter circuit utilizing a capacitance amplifier circuit is sufficiently stabilized to improve the operational reliability of the circuit by compensating a temperature drift in the output of the capacitance amplifier circuit with a temperature drift in the output of a reference voltage circuit thereof. Specifically, according to the present invention, a reference voltage is coupled to a virtual ground point connected to the output of the capacitance amplifier circuit, and the reference voltage is generated by combining resistors having different temperature characteristics so that the temperature drift in the output of the capacitance amplifier circuit is canceled. Therefore, a highly accurate and stable filter circuit can be provided as described above, and a filter circuit utilizing an amplifier of a single power supply type as in the present embodiment can be used for purposes including processing of very small sensor signals such as for applications on vehicles. In addition, this circuit configuration is not limited to implementation on a substrate utilizing discrete components, and the same advantage can be obtained by forming this filter circuit in a part of an integrated circuit.

Various details of the invention may be changed without departing from its spirit nor its scope. Furthermore, the foregoing description of the embodiment according to the present invention is provided for the purpose of illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.