JPH0993096A - Comparator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a comparator which has hysteresis characteristics in simple configuration while using an operational amplifier circuit inserting a capacitor composed of a ferroelectric thin film into a feedback circuit, to reduce a current to usually flow into the circuit, and to attain the reduction of power consumption. SOLUTION: A capacitor Cf is inserted into the feedback circuit of this comparator. Besides, a signal is inputted through a ferroelectric capacitor Cferro to an operational amplifier A at the comparator. By inserting and replacing these elements, hysteresis characteristics reflecting the operation of comparator with the characteristics of ferroelectric capacitor Cferro are applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a comparator having an operational amplifier.

[0002]

2. Description of the Related Art When using a semiconductor integrated circuit, it is often necessary to determine the magnitude of a signal voltage with respect to a certain threshold value. However, in reality, the signal voltage itself slightly oscillates due to factors such as noise, power supply voltage fluctuations, and waveform distortion due to the transmission path.When a strict judgment is made using a comparator, the judgment value near the threshold value Becomes extremely unstable. For this reason, a slight positive feedback is applied to the comparator to provide a hysteresis characteristic for stable operation.

FIG. 3 is a conventional example of a circuit technique for applying positive feedback to a comparator. In the circuit of FIG. 3, the input signal (voltage signal) Vin is input to the inverting input (-) of the operational amplifier (differential amplifier) A through the resistor R1 and the reference voltage Vre.
f is input to the non-inverting input (+) of the operational amplifier A via the resistor R2. Also, this non-inverting input (+)
The output Out flows through the resistors R4 and R5 toward the ground point,
The voltage generated at the midpoint between the resistors R4 and R5 is the resistance R3.
Positive feedback is provided via.

In this circuit, the original input signal Vin
If the voltage of V becomes smaller than the reference voltage Vref, the operational amplifier A is turned on to output, thereby raising the reference voltage Vref input to the non-inverting input (+) via the resistor R3. Then, when the voltage of the input signal Vin becomes larger than the raised reference voltage Vref, the comparator A is turned off. As described above, the output Out with respect to the input voltage Vin has hysteresis.

[0005]

However, although the above method has been widely put to practical use, it has the following problems. The circuit constant must be considered from both aspects of the stable operation of the circuit and the accuracy of the determination, and the number of elements constituting the circuit also increases. Further, since the output impedances of the reference voltage source and the signal source must be taken into consideration, it is not easy to change the threshold value and the judgment margin when integrated.

Further, a current constantly flows from the output / reference voltage source toward the ground point, and this current is an obstacle to power saving. Especially, considering that the output is almost equal to the power supply voltage except when the transient operation is performed and the requirement of the output impedance for the reference voltage source can be relaxed, the effect of reducing the steady current is great. As in the two points described above, there is room for improvement in the circuits currently used for the number of elements forming the comparator and power saving.

The present invention has been made in view of the above-mentioned conventional problems, and it is an object of the present invention to provide a comparator which realizes a comparator having a hysteresis characteristic with a small number of elements and whose characteristics can be easily changed. It is an issue. Another object of the present invention is to provide a comparator capable of reducing the current that constantly flows in the circuit and reducing the power consumption.

[0008]

The present invention has the following constitution in order to solve the above problems. The invention of claim 1 is a comparator having an operational amplifier, characterized in that a ferroelectric capacitor is connected to a signal input terminal of the operational amplifier.

According to a second aspect of the present invention, the operational amplifier is provided with a feedback circuit through a capacitor.
The comparator described in 1.

According to the first aspect of the present invention, the comparator is composed of the ferroelectric capacitor connected to the input terminal of the operational amplifier, and its capacitance is changed by the electric field applied to the ferroelectric capacitor. By utilizing the history, it is possible to realize a comparator that has a hysteresis characteristic and operates stably.

According to the second aspect of the present invention, the feedback of the operational amplifier is performed by the capacitor so that the direct current component is not fed back, so that the current that constantly flows in the circuit together with the ferroelectric capacitor. And power consumption can be reduced.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of a comparator according to the embodiment.

As shown in FIG. 1, the comparator has an operational amplifier (differential amplifier) A. It is desirable that the operational amplifier A has a high input impedance and a small bias current. Reference numeral Cferro is a capacitor made of a ferroelectric substance (for example, a ferroelectric thin film can be used), which is connected to the inverting input (-) terminal of the operational amplifier A, and the capacitor C
An input signal Vin is applied to the inverting input (-) terminal through ferro. Further, Cf is a feedback capacitor which is an ordinary capacitor. Vref is connected to a reference voltage to be compared, and applies the reference voltage to the non-inverting input (+) terminal of the operational amplifier A.

This configuration is more like an inverting amplifier than a comparator in the circuit types currently in use. However, the point that the capacitor is used for the feedback circuit and the element through which the input signal passes, one of which is the ferroelectric capacitor Cferro
Is essentially different in that.

Generally, the electric field E (arbitrary unit) applied to the ferroelectric capacitor Cferro and the stored charge Q (arbitrary unit) are shown in FIG. Therefore, the ferroelectric capacitor Cferro acts as if the electrostatic capacitance had dramatically increased before and after the applied electric field E was inverted, especially immediately after it was inverted. For the ferroelectric capacitor Cferro, a ferroelectric material such as barium titanate, strontium titanate, or lead zirconate titanate can be used, and it is desirable to use a ferroelectric material having a large relative dielectric constant. Further, even with the same ferroelectric substance, a larger relative permittivity allows a larger capacitance to be obtained in the same area (for example, JP-A-6-204843).
No., 5-89691).

This ferroelectric capacitor Cferro is applied to the comparator of the circuit of FIG. Input signal Vin is reference voltage Vre
When it is larger than f and the input signal Vin and the reference voltage Vref are sufficiently close to each other, the comparator acts as an inverting amplifier, and the voltage of the inverting input (-) terminal is almost equal to the feedback of the operational amplifier A. It becomes equal to the reference voltage Vref.
The input signal Vin fluctuates and the input signal Vin and the reference voltage Vref are changed.
Immediately after the magnitude relationship between the two is reversed, the ferroelectric capacitor Cferr
The capacitance of o increases dramatically, and the amplification factor of the comparator also increases accordingly. When the input signal Vin is smaller than the reference voltage Vref and the input signal Vin and the reference voltage Vref are not close to each other, the comparator is saturated and the output Out has a value almost equal to the power supply voltage. The hysteresis characteristic is determined only by the capacitance ratio between the ferroelectric capacitor Cferro and the feedback capacitor Cf and the coercive electric field of the ferroelectric capacitor Cferro.

In the comparator of FIG. 1, the voltage source of the reference voltage Vref is connected only to the non-inverting input (+) of the operational amplifier A, so that it can be operated with an extremely light load. The feedback from the output of the operational amplifier A to the inverting input (-) terminal is also performed by the capacitor Cf, so that no current constantly flows in the circuit. Therefore, power consumption can be reduced.

[0018]

As described above, according to the invention of claim 1, the hysteresis characteristic of the comparator can be realized with a smaller number of elements as compared with the conventional one, and the number of elements to be changed when operating the characteristic is also increased. Can be reduced. According to the second aspect of the present invention, the ferroelectric capacitor and the feedback capacitor are combined to reduce the current that constantly flows in the circuit, so that the power consumption can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a comparator according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of characteristics of a ferroelectric capacitor.

FIG. 3 is a diagram showing an example of a configuration of a conventional comparator having a hysteresis characteristic.

[Explanation of symbols]

 Vin Input signal Vref Reference voltage Out output A Operational amplifier Cferro Ferroelectric capacitor Cf Capacitor Q Charge amount (arbitrary unit) E Electric field (arbitrary unit)

Claims (2)

[Claims] 1. A comparator having an operational amplifier, wherein a ferroelectric capacitor is connected to a signal input terminal of the operational amplifier. 2. The comparator according to claim 1, wherein the operational amplifier is provided with a feedback circuit through a capacitor.