JPH045578A - Sensitivity adjusting circuit of ceramic type electrostatic sensor - Google Patents

Abstract

PURPOSE:To lead out an extremely fine detection signal with extremely small electrostatic capacity by simple circuit constitution and adjust the gain of an amplifying circuit by applying a DC bias voltage to a detecting circuit part and increasing the level of an AM-modulated wave. CONSTITUTION:In the ceramic type electrostatic sensor circuit 9, the AM signal of a tuning circuit 2 which is controlled by an oscillation circuit 1 of small-sized constitution using a dielectric resonator and a tuning point detecting circuit 3 is detected and outputted by the impedance increasing circuit 8, detecting circuit 4, and operational amplifying circuit 12 of a detecting circuit part and the operational amplifying circuit 14 of a bias circuit 13. An FET 15 is brought under feedback control with this output to control the DC bias current applied to the detecting circuit part, the gain of the amplifying circuit is adjusted automatically by the simple constitution, and even the extremely fine detection signal with the extremely fine electrostatic capacity is led out without being cut off.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a sensitivity adjustment circuit for a ceramic electrostatic sensor that detects minute changes in capacitance.

[Conventional technology]

The electrostatic sensor device that has been very commonly used changes the oscillation frequency by changing the capacitance used in the tank circuit of the oscillation circuit in response to changes in external capacitance. However, the detection sensitivity is low, so
In recent years, more sensitive RCA-type devices (a method of obtaining AM modulated waves by changing the capacitance of a G-tuned circuit with a resonant frequency that slightly deviates from the oscillation frequency of the oscillation circuit) have been used. It has become to.

As shown in FIG. 2, this RCA type electrostatic sensor device includes an oscillation circuit 1, a tuning circuit 2, a detection section 3 that detects a change in capacitance with the detected object, and a detection circuit 4. It consists of an amplifier circuit 5. The oscillation circuit 1 and the tuning circuit 2 each include separate and independent resonators. For example, as shown in FIG.
The resonance frequency r0 of is set at a slightly shifted position, and the minute capacitance change ΔC detected by the detection unit 3
The resonant frequency is shifted by Δf from fo in accordance with
The change in capacitance ΔC is converted into a change in the output voltage Δ■ to create an AM modulated wave using the oscillation frequency signal of the oscillation circuit 1 as a carrier wave, which is detected and amplified and extracted.

Generally, in this type of conventional RCA type electrostatic sensor device, each resonator of the oscillation circuit 1 and the tuning circuit 2 is formed using a strip line.

[Problem to be solved by the invention]

As is well known, a stripline requires at least a length of Z of the oscillation frequency wavelength, which inconveniences the equipment to be large.Also, equipment using this stripline has a low Q, so it is difficult to use in microstatics. The drawback is that changes in capacitance cannot be detected with ultra-high sensitivity.

The present inventors have succeeded in solving the above-mentioned conventional drawbacks by using dielectric resonators for each resonator of the oscillation circuit 1 and the tuning circuit 2. That is, we focused on the fact that by using a dielectric resonator as a resonator, the size of the resonator can be made smaller than in the case of a strip line, and this dielectric was constructed from ceramic. Since the dielectric constant ε of ceramic is 40 to 90, when the oscillation frequency is set to approximately IG7, the shape of the ceramic resonator can be made to have a cross section of 8 to 10 degrees, resulting in significant device miniaturization. In addition, the Q of Ceraminku is 2.
Since the value is as high as 00 to 300, it is possible to achieve ultra-high sensitivity of the device, and it is possible to detect minute capacitances on the order of I Xl0-'PF.

By the way, the above-mentioned prototype device is expected to be useful in industrial applications as a small and ultra-sensitive device.
Recently, for example, there has been a trend toward smaller and more densely packed information storage devices, and when an electrostatic sensor device is used as a host for video equipment, it has to be moved within a very narrow space. Such usage patterns are conceivable, and in order to meet the demands of such usage patterns, it is necessary to further downsize the device.

However, as the device is further miniaturized and the ceramic resonator is made thinner, the Q of the ceramic resonator becomes smaller, and the voltage level of the carrier wave of the AM modulated wave output from the ceramic resonator of the tuned circuit becomes smaller. Become,
When this voltage level becomes smaller than the threshold voltage of the amplifier circuit 5, a problem arises in that the AM modulated wave does not pass through the amplifier circuit 5, and the weak detection signal of the minute capacitance is cut off and cannot be extracted.

Furthermore, even if the electrostatic sensor device is not miniaturized,
If the sensitivity of the sensor is too high, there is a risk that the resonant frequency 10 will deviate greatly due to changes in minute capacitance, and the tuning point will be out of the variable setting range.In this case, it is necessary to consciously lower the Q of the sensor. It is conceivable to design the equipment based on the following. In this case as well, since the Q is made low, it is assumed that the AM modulated wave will become a weak signal, and there is a possibility that a similar problem will occur.

The present invention has been made to solve the above problems, and its purpose is to be able to reliably detect a weak detection signal of minute capacitance without cutting off, and to make it possible to adjust the gain of an amplifier circuit. An object of the present invention is to provide a sensitivity adjustment circuit for a ceramic electrostatic sensor with a simple circuit configuration.

[Means for Solving the Problems] In order to achieve the above object, the present invention is configured as follows. That is, the present invention changes the tuning point between an oscillation circuit that has a built-in resonator and outputs an oscillation frequency signal and the oscillation circuit in response to changes in external capacitance, and responds to changes in the tuning point. A ceramic type including a tuned circuit made of a ceramic resonator that outputs an AM modulated wave, a detection circuit that detects the AM modulated wave output from the tuned circuit, and a first operational amplifier that amplifies the output from the detection circuit. The electrostatic sensor is equipped with a bias circuit that applies a DC bias voltage to the detection circuit.
The present invention is characterized by comprising a second operational amplifier that amplifies the output from the operational amplifier, and a variable resistance circuit using a semiconductor element that constitutes a feedback circuit of the second operational amplifier.

[Effect]

When the voltage level of the AM modulated wave output from the tuning circuit is small, if the DC bias voltage applied to the detection circuit is increased, the voltage level of the AM modulated wave will exceed the threshold voltage of the first operational amplifier, and the voltage level of the AM modulated wave will exceed the threshold voltage of the first operational amplifier. is amplified and extracted.

Further, due to a change in the resistance of the semiconductor element, the magnitude of the feedback of the feedback circuit of the second operational amplifier changes, and the gain adjustment of the second operational amplifier is performed in conjunction with the change in the DC bias voltage.

[Example J Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 shows an embodiment of a sensitivity adjustment circuit for a ceramic electrostatic sensor according to the present invention.

The circuit of this embodiment includes an oscillation circuit 1 including a ceramic resonator, a tuning circuit 2 including a ceramic resonator, a high impedance circuit 8 including a coupling capacitor 6 and an inductance element 7, and a diode 10 and a capacitor 1. A bias circuit 13 is provided in a ceramic type electrostatic sensor circuit 9 which is composed of a detection circuit 4 composed of a semiconductor device and a first operational amplifier 12 as main circuits.

This bias circuit 13 includes a second operational amplifier 14 and a field effect transistor (FET) 15 as a semiconductor element.
The positive terminal (non-inverting input terminal) of the second operational amplifier 14 is connected to the output terminal of the first operational amplifier 12 via a resistor. A first feedback resistor 16 is connected between the output terminal side and the negative input terminal (inverting input terminal) of the second operational amplifier 14,
Furthermore, this negative input terminal is connected to a second feedback resistor 17.
It is connected to the source of FET 15 via. Also,
The source side of the FET 15 is connected to the low voltage side of the inductance element 7 and the capacitor 11. Furthermore, a third feedback resistor 18 is interposed between the positive input terminal of the first operational amplifier 12 and the source of the FET 15.
The source of T15 is connected to the drive power source of FET15 via a voltage dividing resistor 20. Further, the drain side of the FET 15 is connected to ground. The gate of the FET 15 is connected to the output side of the second operational amplifier 14 via a variable resistor 21 for gain adjustment.

The present embodiment is configured as described above, and its operation will be explained below.

When the detection unit 3 detects a minute change in capacitance while the oscillation circuit l is outputting an oscillation frequency signal of an extremely high frequency of IGHz, as shown in FIG. The resonant frequency of the tuning circuit 2 shifts, and in response to the change in the tuning point, an AM modulated wave is created using the oscillation frequency signal of the oscillation circuit 1 as a carrier wave, and this AM
The modulated wave is applied to the detection circuit 4 via the high impedance circuit 8. The detection circuit 4 detects the AM modulated wave, converts it into a signal in a capacitance change detection band, and outputs this as a minute capacitance detection signal. This detection signal is amplified through the first operational amplifier 12 and the second operational amplifier 14, and is applied to a desired signal processing circuit.

By the way, when the minute change in capacitance detected by the detection unit 3 is obtained as a weak AM modulated wave, this weak A
If the voltage level of the M modulated wave is lower than the threshold voltage of the first operational amplifier 12, in other words, if the carrier wave level is low, a problem arises in that the weak detection signal of this minute capacitance is cut off and cannot be extracted. . In this embodiment, a characteristic bias circuit 13 is added to solve this problem.

This characteristic bias circuit 13 applies a DC bias voltage to the weak AM modulated wave, and also applies a DC bias voltage to the first operational amplifier 12.
and the second operational amplifier 14.
C (Automatic Galn Control+
) functions as a circuit. In this bias circuit 13, when the resistance value of the variable resistor 21 is varied, the gate current of the FET 15 changes, and the resistance between the source and drain of the FET 15 changes. When the gate current is reduced, the source-drain current becomes smaller and the resistance between the source and drain increases. On the other hand, when the resistance value of the variable resistor 21 is decreased, the gate current increases, the current flowing between the source and the drain increases, and the resistance between the source and the drain decreases.

When the resistance between the source and drain is increased, a large DC bias voltage is applied to the high impedance circuit 8 and the detection circuit 4, and the weak AM output from the tuning circuit 2 is
The voltage level of the modulated wave is increased. By applying this bias voltage, the voltage level of the weak AM modulated wave exceeds the threshold voltage of the first operational amplifier 12, and the AM modulated wave is amplified by the first operational amplifier 12 and then transferred to the second operational amplifier 14. However, since the signal amplified by the first operational amplifier 12 exceeds the threshold voltage of the second operational amplifier 14, it is further amplified by the second operational amplifier 14 and sent to the desired signal processing circuit. It is added. In this case, the second operational amplifier 14
The amplification degree is given by the resistance 16.17 and the resistance value between the source and drain of the FET. On the other hand, the output signal from the second operational amplifier 14 is applied to the gate of the FET 15 via the variable resistor 21. At this time, the signal applied to the gate is connected to the first operational amplifier 12 and the second operational amplifier. amplifier 1
Since the signal is amplified in two stages by FET1
5 thread, sufficiently exceeds the 2 shhold voltage, FET1
5 continues to apply the DC bias voltage without turning off.

By the way, in the bias application operation, the positive terminal of the first operational amplifier 12 is connected to the second feedback resistor 18.
A feedback current i flows through the FET 11, and this feedback current 11 decreases as the resistance between the source and drain of the FET 15 increases, and increases as the resistance between the source and drain decreases. In this way, the gain of the first operational amplifier 12 is adjusted in response to the change in resistance between the source and drain of the FET 15. Similarly, the second operational amplifier 14
A voltage feedback circuit is configured from the output side of the FET 15 by the first feedback resistor 16 and the second feedback resistor 17. At this time, as the resistance between the source and drain of the FET 15 increases, the amount of feedback increases. becomes smaller, and conversely, when the resistance between the north and drain becomes smaller, the amount of feedback becomes larger, and the gain of the second operational amplifier 14 can be adjusted effectively according to the change in the resistance between the source and drain of the FET 15. It will be done.

As described above, according to this embodiment, the level of the AM modulated wave is smaller than the operating voltage of a normal transistor (0.4V, which is also the threshold voltage of the operational amplifier 12). Also, by applying a DC bias voltage, the weak modulated wave detection signal can be reliably extracted without being cut off by the first operational amplifier 12 and the second operational amplifier 14, allowing the device to be ultra-compact. This makes it possible to reliably detect minute capacitances even in usage situations where the level of the carrier wave decreases.

Note that the present invention is not limited to the above-mentioned embodiments, and can take various embodiments. For example, in the above example,
Although the field effect transistor as a semiconductor element is constructed using an FET, it is also possible to construct it using a MOS FET.

Further, if necessary, the positive terminal of the first operational amplifier 12 may be connected to ground via a capacitor. In this case, depending on circuit conditions, the resistor 18 may be disconnected from the feedback circuit and grounded.

(Effects of the Invention) The present invention has a bias circuit that applies a DC bias voltage to the entire detection circuit of a ceramic electrostatic sensor. Even if a drop occurs, the bias circuit can add a DC bias voltage to raise the level of the AM modulated wave and perform the desired signal processing, making it possible to reliably extract the weak detection signal of minute capacitance. becomes.

Furthermore, since a DC bias voltage is applied by a bias circuit, gain control without a threshold is possible, which increases the degree of freedom in circuit design and enables various circuit developments.

Furthermore, if the configuration is such that the feedback current of the first operational amplifier is controlled as necessary, the gain control of the first operational amplifier can be effectively performed in conjunction with the adjustment of the bias voltage.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of the variable sensitivity circuit of a ceramic electrostatic sensor according to the present invention, Fig. 2 is a block diagram of a general electrostatic sensor of the RCA type, and Fig. 3 is a block diagram of a general electrostatic sensor of the RCA type. FIG. 3 is an explanatory diagram showing the operation of detecting minute capacitance of an electrostatic sensor. DESCRIPTION OF SYMBOLS 1... Oscillation circuit, 2... Tuning circuit, 3... Detection part, 4... Detection circuit, 5... Amplification circuit, 6-... Coupling capacitor, 7... Inductance element, 8
... High impedance circuit, 9... Ceramic electrostatic sensor circuit, 10... Diode, 11... Capacitor, 12... First operational amplifier, 13... Bias circuit, 14... ...Second operational amplifier, 15...
FET, 16...first feedback resistor 17...second feedback resistor, 18...third feedback resistor, 20...
Voltage dividing resistor, 21...variable resistor. Applicant Murata Manufacturing Co., Ltd. Representative Patent Attorney Kiyoshi Igarashi Kaizu

Claims (1)

[Claims] An oscillation circuit that includes a built-in resonator and outputs an oscillation frequency signal, and a tuning point between the oscillation circuit and the oscillation circuit is changed in response to changes in external capacitance, and an AM modulated wave corresponding to the change in the tuning point is output. In a ceramic electrostatic sensor that has a tuning circuit made of a ceramic resonator, a detection circuit that detects an AM modulated wave output from the tuning circuit, and a first operational amplifier that amplifies the output from the detection circuit, The circuit includes a bias circuit that applies a DC bias voltage, and this bias circuit uses a second operational amplifier that amplifies the output from the first operational amplifier and a semiconductor element that constitutes a feedback circuit for the second operational amplifier. A sensitivity adjustment circuit for a ceramic electrostatic sensor, characterized in that it is constructed from a variable resistance circuit.