JPH0348715Y2 - - Google Patents

Description

[Detailed description of the invention] [Technical field to which the invention pertains] The invention is applicable to industrial applications where, for example, a movable electrode in a counter electrode is displaced based on the pressure of a fluid, and the capacitance between the two electrodes changes. A displacement conversion device that detects the capacitance value that changes depending on the value of the process quantity, converts it into a voltage signal, and transmits it to the receiving instrument.In particular, the output signal has a high degree of freedom and little error, and is The present invention relates to a configuration in which three-wire wiring can be applied when installing a displacement converter and the receiving instrument.

[Prior art and its problems]

Conventionally, when measuring differential pressure for flow rate measurement, etc.
A movable electrode is placed between two fixed electrodes, the movable electrode is displaced by the differential pressure, and the resulting differential change in capacitance is detected to measure the differential pressure. Sometimes things are done. Figure 2 shows the displacement conversion proposed by the applicant (Utility Model Application Publication No. 140196/1983), which converts capacitance into a voltage signal and outputs it when measuring differential pressure in this way. It is a block diagram of a device.

In FIG. 3, 3a and 3b are power terminals of the displacement converter 1 to which the positive and negative electrodes of the external DC power source 2 are connected, respectively, and the terminal 3b is connected to the reference potential section 5 in the displacement converter 1 in this case.
4 is driven by the DC power supply 2, and a reference potential section 5
This is a constant voltage circuit that generates a low voltage E ₀ with E 0 on the low potential side, and 4a is its high potential side terminal. Reference numeral 6 denotes an oscillation circuit which is driven by applying the output voltage _E0 of the constant voltage circuit 4 through a resistor 7 and outputs an alternating current voltage to the primary coil 8 of the coupling coil, and the resistor 7 is used to change this resistance value. It has a function of adjusting the amplitude of the AC voltage output to the coil 8. 9,
10, in order to detect the difference between two pressures, one movable electrode is arranged between two fixed electrodes, and this movable electrode is displaced according to the pressure difference by a mechanism not shown. , first and second capacitance elements configured such that each of a pair of capacitances between each fixed electrode and a movable electrode changes differentially with respect to each other, and 11 is a capacitance element as will be explained later. This is the third capacitive element in which the following is explained. 12, 13, 1
4 are separate secondary coils, 15, each coupled to the primary coil 8 and generating alternating current voltages of equal amplitude;
16 is a series circuit consisting of a first resistor connected between the secondary coil 12 and the reference potential section 5, a second resistor 17 and a third resistor 18, and the series circuit 16 is connected to the secondary coil 13. and the reference potential section 5 with the third resistor 18 on the reference potential section 5 side, and the secondary coil 12 and the first capacitive element 9 are connected as a diode with the secondary coil 12 side facing the cathode side. The secondary coil 13 and the second capacitive element 10 are connected via a diode 20 such that the secondary coil 13 side is on the cathode side. The electrodes of the capacitive elements 9 and 10 that are not connected to the diodes 19 and 20 are both connected to the reference potential section 5 via a coupling capacitor 21. 2
2 and 23 are the first resistor 15 and the series circuit 1, respectively.
This is a smoothing capacitor connected in parallel to 6.

24 is a field effect transistor whose drain D is connected to the reference potential unit 5 and whose source S is connected to a variable resistor 25 as a current regulator; the gate G of this transistor 24 is connected to the source S through the variable resistor 25; The other end of the secondary coil 14 is connected to the anode of the diode 19 via the diode 27 and to the anode of the diode 20 via the diode 28. In this case, the connections between the diodes 27 and 19 and the connections between the diodes 28 and 20 are both sequential connections. 29 and 30 are smoothing capacitors. A diode 31 has an anode connected to the source S of the transistor 24, and the third capacitor 11 is connected between the cathode of the diode 31 and the anode of the diode 27 or 28. 32 is a buffer amplifier that is driven by a DC voltage obtained by rectifying the AC voltage generated in the secondary coil 14 with a diode 33 and a smoothing capacitor 34; One of the input terminals is connected to the connection point between the first resistor 15 and the secondary coil 12, the other input terminal is connected to the output terminal of this amplifier, and the output terminal of this amplifier 32 is connected in order to the cathode of the diode 31. It is connected to the cathode of a diode 35.

36 are first and second input terminals 36a, 36b
The first input terminal 36a is connected to the connection point between the first resistor 15 and the secondary coil 12, and the second input terminal 36b is connected to the connection point between the second resistor 17 and the secondary coil 13. and furthermore, the high potential side power supply terminal 3
6d is a differential operational amplifier connected to the high potential side output terminal 4a of the constant voltage circuit 4, and the low potential side power supply terminal 36e is connected to the reference potential unit 5; 37 has an adjustable terminal 37a; It is a variable resistor in which one of both ends is connected to a first input terminal 36a and a second input terminal 36b, respectively, and an adjustable terminal 37a is connected to the reference potential unit 5 via a resistor 38.
A fourth resistor 39 has one end connected to the output terminal 36c of the operational amplifier 36 and the other end connected to the connection point between the second resistor 17 and the third resistor 18;
is the output voltage E ₀ of the constant voltage circuit 4 through the sixth resistor 51
_V1 is the potential difference between this potential point and the reference potential section 5, and _V0 is the potential difference between the calculation output terminal 36c and the reference potential section 5. V ₂ is the potential difference between the output terminal 36c and the set potential point 53. The displacement converter 1 is composed of the above-mentioned parts except for the external DC power supply 2.

In order to explain the problems in the displacement converting device 1 below, the operation of this circuit 1 will be explained first. Since the displacement converter 1 is configured as described above, when the DC power supply 2 is connected, the secondary coils 12, 13,
An alternating current voltage of double amplitude E is induced in each of 14,
In FIG. 2, the polarities of these induced voltages are set in the secondary coils 12 and 1 so that they point in the same direction at the same time.
3 and 14 are configured. Therefore, during the positive half-wave period when the voltage induced in the secondary coil 12 is downward, the current due to this voltage flows through the parallel circuit consisting of the secondary coil 12, the first resistor 15 and the capacitor 22, and the coupling The current flows through the capacitor 21, the first capacitive element 9, and the diode 19 in sequence, and the current due to the voltage induced in the secondary coil 13 during the same positive half-wave period flows from the secondary coil 13, the series circuit 16, and the capacitor 23. The current flows sequentially through the parallel circuit, the coupling capacitor 21, the second capacitive element 10, and the diode 20. During the next half-wave period following the above-mentioned positive half-wave, that is, during the negative half-wave period in which the voltage induced in the secondary coil 14 is upward, the current due to this voltage flows into the secondary coil 14.
After passing through the diode 27 and 28, the current flowing through the diode 27 flows into the coupling capacitor 21 via the first capacitive element 9, and the current flows through the diode 2.
The current flowing through the capacitor 8 flows into the coupling capacitor 21 via the second capacitive element 10, and both currents flowing into the coupling capacitor 21 pass through the capacitor 21 and are then connected to the transistor 24 and the capacitor 2.
9 and a parallel circuit consisting of a variable resistor 25 and a capacitor 30. When the voltage induced in the secondary coil 12 becomes downward, the current flowing through the first capacitive element 9 due to this voltage is transferred to the smoothing capacitor 2 in the first resistor 15.
2, it becomes a direct current, so a voltage effect V _a is generated at both ends of the resistor 15 due to this direct current, and at this time, the voltage induced in the secondary coil 13 causes a second
Since the current flowing through the capacitive element 10 becomes a direct current in the series circuit 16 due to the smoothing capacitor 23, a voltage drop occurs at both ends of the series circuit 16 due to this direct current, and the voltage induced in the secondary coil 14 also increases. When the voltage rises, the current flowing through the first and second capacitive elements 9 and 10 due to this voltage is caused by the potential difference between the drain D and gate G of the transistor 24 due to the presence of the smoothing capacitors 29 and 30.
Generate V _b .

Although the above description does not mention the current flowing through the third capacitive element 11 due to the voltage induced in the secondary coil 14, the mode of this current is as follows. That is, when the voltage induced in the secondary coil 14 is downward, the voltage due to this voltage is applied to the coil 14, the parallel circuit consisting of the variable resistor 25 and the capacitor 30, the diode 31, and the third
When the induced voltage of the secondary coil 14 is upward, the current due to this voltage passes through the capacitive element 11, the third capacitive element 11, and the diode 3.
5. Passes through the buffer amplifier 32, the connection point between the variable resistor 25 and the gate G of the transistor 24 in this order. Therefore, a current that is the sum of the currents flowing through the first and second capacitive elements 9 and 10 flows downward through the secondary coil 14 through the variable resistor 25, and the secondary coil 14
As a result, the current flowing through the third capacitive element 11 flows upward, and as a result, in the variable resistor 25, since this resistor and the transistor 24 are connected as described above, the above-mentioned effect is caused by the transistor 24. The combined current of the downward current and the upward current is automatically controlled to a constant value. Reference numeral 26 denotes a control circuit comprising a transistor 24 and a variable resistor 25, which performs such an operation. Since the control circuit 26 is configured as described above, the variable resistor 25
By changing the resistance value of the displacement converter, the magnitude of the composite current flowing therein can be changed.As a result, in FIG. 2, the total current consumption of the displacement converter is set to a small value by the variable resistor 25. ing.

In the operational amplifier 36, its input and output terminals are connected to each of the above-mentioned parts, so if there is a difference between the voltages input to the first input terminal 36a and the second input terminal 36b, feedback is generated from the output terminal 36c. current
I _f flows to the reference potential section 5 via the fourth resistor 39 and the third resistor 18 in sequence, and as a result, the input terminal 36
When the voltage input to a and the voltage input to the input terminal 36b become equal, the output voltage with respect to the reference potential unit 5 appearing at the output terminal 36c has a magnitude V ₀
It becomes an equilibrium state.

Next, the magnitude of this output voltage V ₀ will be explained. That is, since the displacement converting device 1 is configured as described above, the first capacitive element 9 and the second capacitive element 10 are The voltages applied to both become AC voltages with the same double amplitude, and the magnitude of this double amplitude is E-(V _a +
_Vb ). Further, the voltage applied to the third capacitive element 11 during one period of the alternating current voltage of double amplitude E induced in the secondary coil 14 is E, with the voltage between the drain and source of the transistor 24 being V _ds .
It becomes an AC voltage with a double amplitude of −(V _a + V _ds ),
Since the current flowing through the resistor 25 is small, V _ds ≈V _b , so the double amplitude of the AC voltage applied to the capacitive element 11 can be considered to be equal to E-(V _a +V _b ). Therefore, the magnitude of the DC current flowing through the first resistor 15 and the series circuit 16 by the secondary coils 12 and 13 is set to I ₁ and I ₂ , respectively, and the magnitude of the DC current flowing through the variable resistor 25 by the secondary coil 14 is set upward. Let the magnitude of the direct current flowing through the secondary coils 12, 13, and 14 be I ₃ , the frequency of the voltage induced in the secondary coils 12, 13, and 14 be f, and the capacitance of each of the first to third capacitive elements to be C ₁ ,
When C ₂ and C ₃ are assumed, the currents I ₁ , I ₂ , and I ₃ are respectively expressed as in equation (1).

I ₁ =f・{E−(V _a +V _b )}・C ₁ I ₂ =f・{E−(V _a +V _b )}・C ₂ I ₃ −f・{E−(V _a +V _b ) }・C ₃ ...(1) That is, the DC current flowing through the first resistor 15 has a magnitude _{I 1 proportional} to the capacitance C 1 of the first capacitive element 9.
The direct current flowing through the series circuit 16 has a capacitance C ₂ of the second capacitive element 10.
The second DC currents 41 and 42 having a magnitude I ₂ proportional to 2 first and second current circuits that generate direct current, and a first resistor 1
5 and a capacitor 22, a first current that generates a first DC voltage V _a proportional to the magnitude I ₁ of the first DC current with respect to the reference potential section 5 is connected to both ends of the parallel circuit. In the first circuit means provided in the circuit 41, a parallel circuit consisting of the series circuit 16 and the capacitor 23 has a second DC voltage as a reference, which has a magnitude proportional to the magnitude I ₂ of the second DC current, at both ends thereof. This is a second circuit means provided in the second current circuit 42 that generates the potential in the potential section 5.

The current flowing through the transistor 24 by the secondary coil 14 is equal to I ₁ +I ₂ because it is the discharge current of the first and second capacitive elements 9 and 10 charged by the currents I ₁ and I ₂ . As a result, the magnitude of the current flowing through the variable resistor 25 becomes I ₁ +I ₂ −I ₃ , and since the control circuit 26 controls the magnitude of this current to a constant value I _k , equation (2) is established, and (1 ) and equation (2) give equation (3). That is, the control circuit 26 forms a composite current of the difference between the sum of the first DC current and the second DC current and the current _I3 , and controls this composite current by the variable resistor 25 serving as a current regulator. This is a circuit that controls to a variable set value I _k .

I ₁ + I ₂ − I ₃ = I _k …(2) I ₁ = I _k・{C ₁ / (C ₁ + C ₂ − C ₃ ) I ₂ = I _k・{C ₂ / (C ₁ + C ₂ − C ₃ ) ...(3) Also, since the operational amplifier 36 has an input/output voltage state as described above, the output voltage V ₀ is in a balanced state, so (4)
Equations and equations (5) hold true. Here R ₁ , R ₂ , R ₃ ,
_R4 are the first resistor 15, the second resistor 17, and
These are the respective resistance values of the third resistor 18 and the fourth resistor 39.

V ₀ = I ₂ · R ₃ + I _f (R ₃ + R ₄ ) …(4) I ₁ · R ₁ = I ₂ · (R ₂ + R ₃ ) + I _f /R ₃ … (5) In Figure 2, R ₁ = R ₂ + R ₃ = R, so equation (6) is obtained from equation (4) and equation (5), and equation (7) is obtained from equation (6) and (3). The formula is obtained.

V ₀ = I ₂ · R ₃ + (R ₃ + R ₄ ) · (R / R ₃ ) · (I ₁ - I ₂ ) ...(6) V ₀ = I ₂ · R ₃ + (R ₃ + R ₄ ) · (R/R ₃ ) ・I _k・C ₁ −C ₂ /C ₁ +C ₂ −C ₃ …(7) Electrostatic capacitances C ₁ and C ₂ are both A, The distance between the movable electrode and the fixed electrode when the movable electrode is at the reference position is d, the displacement of the movable electrode is Δd, and the stray quantity due to the capacitive elements 9 and 10 is C _S1 , respectively.
When C _S2 , it is expressed by equation (8). Here, ε is the dielectric constant of the medium between the fixed electrode and the movable electrode.

C ₁ = {(ε・A)/(d−Δd)}+C _S1 C ₂ = {(ε・A)/(d+Δd)}+C _S2 …(8) In Fig. 2, C _S1 = C _S2 = C Since the capacitance of the third capacitive element 11 is set so that C ₃ =2C _{S ,} equation (9) can be obtained from equations (7) and (8). It will be done.

C _S is generally very small compared to C ₀ . Therefore, as is clear from equation (9), the output voltage V ₀ of the operational amplifier 36 has a value proportional to the mechanical displacement (Δd/d) of the movable electrodes forming the capacitive elements 9 and 10, and this value is hardly affected by the stray capacitance C _S associated with the capacitive elements 9 and 10. Therefore the voltage
By measuring V ₀ it is possible to measure the differential pressure that led to the displacement Δd.

For this reason, conventionally, the displacement converter 1 either directly transmits the voltage V ₀ to the receiving instrument or
The differential pressure is measured by transmitting it as V ₂ and measuring these voltages with this receiving instrument, but the method of converting the differential pressure to V ₂ is to convert the potential at the set potential point 53 to By selecting the respective resistance values of the resistors 51 and 52, the potential of the reference potential unit 5 can be set arbitrarily within the range of magnitude _E0 , resulting in a voltage V corresponding to the starting point of the measured differential pressure. ₂ can be set over a wide range from negative to positive values, and as a result, the voltage corresponding to the starting point of the measurement range may be negative or zero volts. Although this displacement converter has the advantage of being able to be immediately incorporated into a measurement signal system such as the above, in other words, the output signal can have a high degree of freedom, the output voltage E of the constant voltage circuit 4 ₀ fluctuates or resistor 51,
52 changes and the potential at the set potential point 53 fluctuates, there is a problem that the potential fluctuation at the point 53 is included as an error in the output voltage _V2 . This method requires four wires, two wires for supplying power from the DC power source 2 and two wires for transmitting the output voltage _V2 , which is uneconomical. In addition, in the method of directly transmitting the voltage V ₀ , the reference potential of the output voltage V ₀ is the potential of the reference potential section 5, so fluctuations in this potential do not cause errors in the output voltage V ₀ , and the output voltage As a result, one of the electric wires for V ₀ transmission is connected to the reference potential section 5, and as a result, if both the current supplied from the DC power supply 2 to the displacement converter 1 and the resistance of this power supply line are small,
While it is possible to make a three-wire connection in which one power wire and one signal wire are shared, making wiring economical, it is also possible to reduce the voltage V ₀ corresponding to most of the measured differential pressures to zero. For example, the output voltage of the constant voltage circuit 4 E ₀
When V is 10 volts, the output voltage V 0 must be set to 1 volt or more, which reduces the degree of freedom in determining the output voltage V ₀ .

[Purpose of invention]

The present invention solves the problems with conventional transmitter circuits that convert industrial process quantities into mechanical displacements and then converts these displacements into voltage signals via capacitance, thereby achieving a high degree of freedom. and can obtain an exchange signal with less error,
Another object of the present invention is to provide a transmitter circuit that enables three-wire wiring.

[Key points of the idea]

In order to achieve the above object, the present invention includes an oscillation circuit that generates an alternating current voltage, first and second capacitive elements whose capacitances differentially change according to mechanical displacement, and the first capacitor. a first voltage circuit that is driven by the output AC voltage of the oscillation circuit and generates a first DC current proportional to the capacitance of the first capacitance element; and the second capacitance element; a second current circuit that is driven by the output AC voltage of the oscillation circuit and generates a second DC current proportional to the capacitance of the second capacitive element; and a first DC current of the first current circuit. and an amplifier that outputs a voltage according to the difference current from the second direct current of the second current circuit, and extracts an output voltage according to the mechanical displacement from between the output terminal of the amplifier and the reference potential section. In the displacement converter, the negative power supply terminal of the amplifier is connected to a potential point lower than the reference potential section from which the output voltage is extracted, so that a voltage signal generated between the output terminal of the operational amplifier and the reference potential section This provides a high degree of freedom and less error, and also enables three-wire wiring for this displacement converter.

[Example of idea]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of an embodiment of the present invention. The main differences between FIG. 1 and FIG. 2 are the presence of the fifth fixed resistor 54 and the connection state of the low potential side power supply of the operational amplifier 36. Two points. In FIG. 1, the fifth resistor 54
is connected between the reference potential unit 5 and the drain D of the transistor 24, and the low potential power supply terminal 36e of the operational amplifier 36 is connected to the connection point between the resistor 54 and the drain D. 55 is the DC power supply 2 in Figure 1.
A displacement conversion device consisting of each part except for 55
a, 55b are the amplifier output terminals 36c,
This is an output terminal of the displacement converter 55 connected to the reference potential section 5. Output terminal 55 as shown
The voltage signal V ₀ is transmitted to the outside from a and 55b.

In the displacement converter 55, the configuration and operation of each part other than the above-mentioned parts are the same as those of the displacement converter 1 shown in FIG. 2, so a detailed explanation thereof will be omitted.
In this case, the high potential side power source is supplied from the output terminal 4a of the constant voltage circuit, and the low potential side power source is supplied from the fifth resistor 5.
4 and the transistor 24. Therefore, in this case, the potential of the high potential side power supply with respect to the reference potential section 5 is almost stabilized, but the potential of the low potential side power supply with respect to the reference potential section 5 depends on the current flowing through the fifth resistor 54. fluctuate. However, in this case, the resistor value of the resistor 54, the value of the current flowing through the resistor 54, The specifications etc. of the amplifier 36 are selected. Displacement conversion device 5
5, the sum current I ₁ +I ₂ flows downward through the fifth resistor 54, and if the consumption current of the operational amplifier 36 is I _CC , this current flows upward through the resistor 54.
In this case, since (I ₁ + I ₂ )>I _CC is set, the potential at the connection point between the resistor 54 and the transistor 24 is lower than the potential at the reference potential section 5.
Therefore, the voltage amplitude signal V ₀ output from the amplifier 36 whose low potential side power supply terminal is connected to such a low potential point is within the differential pressure range that is converted into a voltage signal by the displacement converter 55. voltage corresponding to the starting point
It becomes possible to set the value of V ₀ appropriately over a wide range from negative to positive with respect to the reference potential section 5,
As a result, the output voltage of such displacement converter 55
V ₀ becomes an output signal with a high degree of freedom. Further, the output voltage V ₀ in this case is a voltage with little conversion error because the reference potential that defines this voltage is the potential of the reference potential unit 5. Furthermore, since the output voltage V ₀ is based on the potential of the reference potential unit 5 as described above, in such a displacement converter 55, the current supplied from the DC power supply 2 to the displacement converter 55 and the power supply are If the electrical resistance is low, three-wire wiring can be used easily, making installation work simple and economical.

In the embodiment described above, the fifth resistor 54 was provided between the reference potential section 5 and the transistor 24, and the negative power supply was supplied to the operational amplifier 36 from the connection point between this resistor and the transistor 24. Fifth
It is obvious that a constant voltage diode may be used in place of the resistor 54, without further explanation. Furthermore, although the above explanation has been about a displacement converter that converts differential pressure into a voltage signal, the present invention is not limited to any other device that can differentially change the capacitance of capacitive elements 9 and 10. The present invention can also be applied to a displacement converter that converts an industrial process quantity into a voltage signal.

In addition, in the displacement converter 55, the capacitive element 1
1 and its related members, the capacitive elements 9, 1
Although the conversion error based on the stray capacitances C _S1 and C _S2 associated with 0 is not included in the output signal V ₀ , the stray capacitance C When _S1 and C _S2 are negligibly small, the capacitive element 11, diodes 31, 33, 35, amplifier 32, and capacitor 34 can be omitted from the displacement converter according to the present invention. Therefore, in this case, the control circuit 26 calculates the sum of the first DC current and the second DC current (I ₁ +
I ₂ ), and serves as a control circuit that controls this sum current to a variable set value.

[Effect of idea]

As described above, the present invention includes an oscillation circuit that generates an alternating current voltage, first and second capacitive elements whose capacitances differentially change according to mechanical displacement, and the first capacitive element. , the oscillation circuit includes a first current circuit driven by the output AC voltage of the oscillation circuit to generate a first DC current proportional to the capacitance of the first capacitance element, and the second capacitance element; a second current circuit driven by an output AC voltage to generate a second DC current proportional to the capacitance of the second capacitive element; a first DC current of the first current circuit and the second current; A displacement conversion device comprising an amplifier that outputs a voltage according to a difference current from a second direct current of the circuit, and extracting an output voltage caused by the mechanical displacement from between an output terminal of the amplifier and a reference potential section. , the negative power supply terminal of the amplifier is connected to a potential point lower than the reference potential section for extracting the output voltage, and the voltage signal generated between the output terminal of the operational amplifier and the reference potential section has a degree of freedom. This has the effect of increasing and reducing the transmission voltage, and furthermore, it is possible to perform three-wire wiring for the transmitter circuit, and the displacement converting device can be installed easily and economically.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a block diagram of a conventional transmitter circuit. 1, 55... Displacement conversion device, 2... DC power supply,
4... Constant voltage circuit, 4a... High potential side output terminal,
5... Reference potential section, 6... Oscillation circuit, 9, 10...
...First and second capacitive elements, 25... Variable resistor as a current regulator, 26... Control circuit, 36... Operational amplifier, 36c... Output terminal, 36d... High potential side power supply terminal, 36e... ...Low potential side output power supply terminal, 41...First current circuit, 42...Second current circuit, 54...Fixed resistor as an impedance member.

Claims (1)

[Scope of utility model registration request] an oscillation circuit that generates an alternating current voltage, first and second capacitive elements whose capacitances differentially change according to mechanical displacement, and the first capacitive element; and a first current circuit that is driven to generate a first direct current proportional to the capacitance of the first capacitive element, and the second capacitive element, and is driven by the output alternating current voltage of the oscillation circuit. a second capacitance proportional to the capacitance of the second capacitive element.
a second current circuit that generates a direct current; a first direct current of the first current circuit; and a second current circuit of the second current circuit;
and an amplifier that outputs a voltage according to a difference current from a direct current, and extracts an output voltage according to the mechanical displacement from between an output terminal of the amplifier and a reference potential section, A displacement conversion device characterized in that a negative side power supply terminal is connected to a potential point lower than a reference potential section for extracting the output voltage.