JP2015060267A - Current output circuit and two-wire broadband transmitter having the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current output circuit of a two-wire broadband transmitter, which enables stable control without increasing a frequency band and has an improved power efficiency.SOLUTION: A current output circuit 10 includes: a first current source circuit 11 that outputs a first current which is controlled by a control voltage x that is output from a signal processing circuit; a second current source circuit 12 that outputs a second current which is controlled by the first current; a first shunt voltage supply circuit 14 that generates an internal power supply by the second current; a third current source circuit 13 that generates a third current which is controlled by a predetermined voltage source and a fourth current which is controlled by the second current source circuit 12; and a second shunt voltage source circuit 15 that generates a power supply of the second current source circuit 12 by the third current. The third current and fourth current are consumed by an internal power supply #1; and a predetermined current which is controlled by the control voltage x is output via a transmission line to an external circuit by the first current and second current.

Description

  The present invention relates to a current output circuit that is connected to an external circuit via two transmission lines and outputs a predetermined current signal to the external circuit while using the external circuit as a power source, and a broadband two-wire transmitter having the circuit About.

  A two-wire transmitter is connected to an external circuit such as a DCS (distributed control system) via two transmission lines, and converts a physical quantity acquired from a sensor or the like into a current signal while using the external circuit as a power source. For example, a field device that outputs to an external circuit. Two-wire transmitters are widely used as field devices such as differential pressure / pressure transmission devices and temperature transmitters in plants because they do not require dedicated power supply wiring and can be installed at low cost. This field device converts a physical quantity into a DC current of 4 [mA] to 20 [mA], which is a world standard, as a signal of the field device, and transmits it to an external circuit.

  For example, Patent Document 1 discloses a two-wire transmitter in which an internal voltage can be arbitrarily set by a shunt regulator (shunt voltage source circuit) without using a zener diode and a stable circuit power source can be secured. The invention is disclosed.

  FIG. 3 shows a configuration example of a conventional current output circuit 50 used in the above-described two-wire transmitter. According to FIG. 3, the current output circuit 50 includes a voltage source circuit 51, an operational amplifier 52 (op amp), a voltage / current conversion element 53 (NPN transistor), a current mirror circuit 54, a shunt voltage source circuit 55, A current detection resistor R11, a feedback resistor R12, a band limiting resistor R13, and a band limiting capacitor C11 are included. With the above configuration, the current output circuit 50 generates the internal power supply # 1 of the two-wire transmitter (drive power supply for a sensor or signal processing circuit (not shown)) while outputting the current I1 from the shunt voltage source circuit 55, and the positive electrode A current satisfying Iout = (1 + R12 / R11) * I1 is output between the power supply terminal VP and the negative power supply terminal VN.

JP-A-9-81883

  By the way, according to the current output circuit 50 shown in FIG. 3, the current mirror circuit 54 is inserted into the negative feedback loop (dotted arrow in FIG. 3) from the output terminal of the operational amplifier 52 to the non-inverting input terminal (+). ing. In order to eliminate the inconvenience due to the insertion of the current mirror circuit 54, a band limiting capacitor C11 is connected between the output terminal of the operational amplifier 52 and the inverting input terminal (−) of the operational amplifier 52. The stability of the system from the output of the amplifier 52 to the input is ensured.

  The reason for securing the stability of the system from the output of the operational amplifier 52 to the input by connecting the band limiting capacitor C11 is that the pole of the current mirror circuit 54 is in the vicinity of the band of the operational amplifier 52 and the loop gain is 1. This is based on the fact that the phase rotates in the vicinity of the doubling frequency and a phase margin (margin) cannot be secured. As described above, the current output circuit 50 of the conventional two-wire transmitter includes a circuit element having a low pole like the current mirror circuit 54 in the negative feedback loop of the operational amplifier 52 indicated by a dotted arrow in FIG. As a result of the insertion, the band is narrowed, and the AC input impedance is lowered, resulting in deterioration in noise resistance.

  For this reason, the applicant filed a patent application on November 12, 2012 for a “current output circuit and a broadband two-wire transmitter having the circuit” shown in FIG. 4 and FIG. 5 (Japanese Patent Application No. 2012). -248042: hereinafter referred to as prior application). The contents will be described below.

  FIG. 4 is a diagram showing a basic configuration of the broadband two-wire transmitter of the prior application. According to FIG. 4, the broadband two-wire transmitter 1 includes a sensor 60, a signal processing circuit 20, and a current output circuit 30, and is connected to two transmission lines L1 and L2, respectively. It is connected to an external circuit 40 such as DCS via a terminal VP and a negative power supply terminal VN.

  The two-wire transmitter 1 is, for example, a field device, receives power from the external circuit 40 via two transmission lines L1 and L2, and converts a physical quantity measured by the sensor 60 into an electrical signal. The signal processing circuit 20 performs signal processing, and the current output circuit 30 outputs a predetermined current of, for example, 4 [mA] to 20 [mA] to the external circuit 40 via the transmission lines L1 and L2.

  The current output circuit 30 includes a current source circuit 31 (first current source circuit), 32 (second current source circuit), 33 (third current source circuit), and a shunt voltage source circuit 34 (first shunt circuit). Voltage source circuit) and 35 (second shunt voltage source circuit). The current source circuit 31 generates a current I1 (first current) controlled by the control voltage (control signal x) output from the signal processing circuit 20 and outputs the current I1 to the current source circuit 32. The current source circuit 32 (second current source circuit) generates a current I2 (second current) controlled by the current I1 and outputs the current I2 to the shunt voltage source circuit 34.

  The shunt voltage source circuit 34 (first shunt voltage source circuit) generates the internal power supply # 1 of the two-wire transmitter 1 (sensor 60 and signal processing circuit 20) from the current I2 output from the current source circuit 32. . The current source circuit 33 generates a current I3 (third current) controlled by the reference voltage Vref. The shunt voltage source circuit 35 generates a power source for the current source circuit 32 based on the current I3 output from the current source circuit 33.

  The current output circuit 30 is connected to an external circuit via transmission lines L1 and L2 by a current I1 generated by the current source circuit 31, a current I2 generated by the current source circuit 32, and a current I3 generated by the current source circuit 33. 40, a predetermined current Iout of 4 to 20 [mA] controlled by a control voltage (control signal x) is output.

  FIG. 5 shows a detailed circuit configuration of each of the current source circuits 31, 32, and 33 constituting the current output circuit 30. In FIG. 5, the current source circuit 31 includes an operational amplifier OP1, a voltage-current conversion element M1 (voltage-current conversion element having a first polarity) made of an N-type MOS-FET, and a current detection resistor R1. The

  In the operational amplifier OP1, a control voltage (control signal x) generated by the signal processing circuit 20 is applied between the non-inverting input terminal (+) and the negative power supply terminal VN connected to the transmission line L2. The voltage-current conversion element M1 has a gate terminal at the output terminal of the operational amplifier OP1, a source terminal at the inverting input terminal (−) of the operational amplifier OP1, and a drain terminal at the current source circuit 32 (a non-inverting input of the operational amplifier OP2 described later). The current I1 generated here (current source circuit 31) is output to the current source circuit 32 via the drain end. The current I1 is connected to the terminal (+) and one end of the voltage / current conversion resistor R7. The current detection resistor R1 has one end connected to the source end of the voltage-current conversion element M1, the other end connected to the negative power supply terminal VN, and the connection point with the source end connected to the inverting input terminal (−) of the operational amplifier OP1. Is done.

  The current source circuit 32 includes an operational amplifier OP2, a voltage / current conversion element M2 (voltage / current conversion element having a second polarity) made of a P-type MOS-FET, a current detection resistor R3, and a voltage / current conversion resistor R7. Consists of.

  In the operational amplifier OP2, the output of the current source circuit 31 (the drain terminal of the voltage-current conversion element M1) is connected to the non-inverting input terminal (+), and the transmission line L1 is connected to the inverting input terminal (−) via the current detection resistor R3. The positive power supply terminals VP to be connected are respectively connected. The voltage-current conversion element M2 has a gate terminal connected to the output terminal of the operational amplifier OP2, a source terminal connected to the inverting input terminal (−) of the operational amplifier OP2, a drain terminal connected to the shunt voltage source circuit 34, and via the drain terminal. The current I 2 controlled by the current I 1 generated by the current source circuit 31 is generated and output to the shunt voltage source circuit 34. The current detection resistor R3 has one end connected to the source end of the voltage-current conversion element M2, the other end connected to the negative power supply terminal VN, and the connection point with the source end connected to the inverting input terminal (−) of the operational amplifier OP2. Is done. The voltage / current conversion resistor R7 is connected between the positive power supply terminal VP and the non-inverting input terminal (+) of the operational amplifier OP2.

  The current source circuit 33 includes an operational amplifier OP3, a voltage / current conversion element M3 made of a P-type MOS-FET, and a current detection resistor R5.

  In the operational amplifier OP3, a reference voltage is applied between the non-inverting input terminal (+) and the negative power supply terminal VN. The voltage-current conversion element M3 has a gate terminal connected to the output terminal of the operational amplifier OP3, a source terminal connected to the inverting input terminal (−) of the operational amplifier OP3, a drain terminal connected to the shunt voltage source circuit 35, and via the drain terminal. Current I3 is generated and output to the shunt voltage source circuit 35. The current detection resistor R5 is connected between the source end of the voltage-current conversion element M3 and the negative power supply terminal VN, and the connection point between the source end of the voltage-current conversion element M3 is the inverting input terminal (− ).

  Hereinafter, the operation of the current output circuit 30 will be described. First, the sensor 60 converts physical quantities such as pressure and temperature into electrical signals and outputs them to the signal processing circuit 20. The signal processing circuit 20 performs a predetermined process such as distortion correction and noise removal on the electrical signal output from the sensor 60 to generate a control signal x (control voltage), and configures the current source circuit 31. This is applied between the non-inverting input terminal (+) and the negative power supply terminal VN terminal of the operational amplifier OP1.

  The current source circuit 31 generates a current I1 controlled by this control voltage (control signal x). That is, the operational amplifier OP1 controls the voltage between the gate and the source of the voltage-current conversion element M1 so that the voltage applied to both ends of the current detection resistor R1 and the control voltage x have the same voltage value. . As a result, the control voltage (control signal x) is converted into the current I1, and the current I1 is supplied to the current source circuit 32 (the non-inverting terminal of the operational amplifier OP2 and the voltage-current conversion) via the drain terminal of the voltage-current conversion element M1. To one end of the resistor R7.

  Next, the current source circuit 32 generates a current I2 controlled by the current I1 generated by the current source circuit 31. That is, a voltage drop occurs due to the current I1 flowing through the voltage-current conversion resistor R7, and the current I1 is converted into a voltage again. The voltage is between the non-inverting input terminal (+) of the operational amplifier OP2 and the positive power supply terminal VP. To be applied. The operational amplifier OP2 controls the voltage between the gate and the source of the voltage / current conversion element M2 based on the VP reference voltage, and the voltage applied to both ends of the current detection resistor R3 and the VP reference voltage have the same voltage value. Control to become. As a result, the current I2 generated through the drain terminal is output to the shunt voltage source circuit 34. The shunt voltage source circuit 34 uses this current I2 to generate an internal power supply # 1 that drives the sensor 60 and the signal processing circuit 20.

  The main carrier of the voltage-current conversion element (P-type MOS-FET) constituting the current source circuit 32 is a hole (hole), and the voltage input to the gate end is lower than the source end (gate-source) Current) flows from the source to the drain. The current increases as the input voltage is on the negative side, decreases as the input voltage is on the positive side, and becomes zero at a predetermined value.

  Next, the current source circuit 33 and the shunt voltage source circuit 35 generate a power source for the current source circuit 32. That is, in the current source circuit 33, the reference voltage Vref is applied between the non-inverting input terminal (+) of the operational amplifier OP3 and the positive power supply terminal VP, and the operational amplifier OP3 is connected between the gate and the source of the voltage-current conversion element M3. By controlling the voltage, the voltage applied to both ends of the current detection resistor R5 is controlled to be the same value as the reference voltage. As a result, the reference voltage Vref is converted into the current I3 and output to the shunt voltage source circuit 35 via the drain terminal of the voltage-current conversion element M3. The shunt voltage source circuit 35 generates a power source for the current source circuit 32 based on the current I3.

  Finally, the current output circuit 30 generates a control voltage (a current I1 generated by the current source circuit 31, a current I2 generated by the current source circuit 32, and a current I3 generated by the current source circuit 33 by the control voltage ( A current Iout of 4 to 20 [mA] controlled by the control signal x) is generated and output to the external circuit 40 such as DCS via the two transmission lines L1 and L2, and the sensor 60 and the signal processing circuit 20 are output. An internal power supply # 1 is generated to drive. Here, if the transfer function of the current source circuit 31 is f (x) and the transfer function of the current source circuit 32 is g (I2), I1 = f (x), I2 = g (f (x)), Iout = F (x) + g (f (x)) + 13.

  According to the current output circuit 30 described above, a current mirror circuit or the like is provided inside the negative feedback loop of the operational amplifiers (OP1 and OP2 respectively) constituting the current source circuits 31 and 32 indicated by solid arrows in the drawing. Since an element having a low station is not included (excluding an element having a slow response), there is no need for band limitation, and therefore, the two-wire transmitter 1 capable of widening the band can be provided. Specifically, the two-wire transmitter 1 transmits an AC digital signal superimposed on a DC analog signal of 4 to 20 [mA] to the external circuit 40. For example, the HART (Highway Addressable Remote Transducer) From a communication waveform having a low carrier frequency, a communication waveform having a high carrier frequency such as Foundation Field BUS can be output in one two-wire transmitter 1 (field device) without changing the constant.

  In addition, the wideband increases the AC input impedance, and the derivative effect that the noise resistance is improved by improving the input impedance is also obtained. It is also possible to generate an internal power supply while outputting current.

  When attention is paid to the constant current source circuit 32 of the current output circuit 30 in FIG. 5, it is necessary to increase the frequency band of the operational amplifier OP2 for stable control. In comparison, the power consumption is relatively large. The current used in the constant current source circuit 32 is determined by the constant current source circuit 33 and is used by being closed in the constant current source circuit 32. If the other circuit blocks 31 and 33 constituting the output circuit 30 can be used, it is preferable because power efficiency is increased.

  The present invention has been made in order to solve the above-described problems, and is capable of stable control without increasing the frequency band, and a current output circuit that improves power efficiency, and a broadband two-wire having the circuit An object of the present invention is to provide a transmitter.

  In order to solve the above-described problems, the present invention receives power supply from an external circuit via two transmission lines, converts a physical quantity measured by a sensor into an electric signal, and performs signal processing by a signal processing circuit. A current output circuit of a two-wire transmitter that generates an internal power supply while outputting a predetermined current to the external circuit via the transmission line, and is controlled by a control voltage output from the signal processing circuit. A first current source circuit that outputs a first current; a second current source circuit that outputs a second current controlled by the first current; and the internal power source is generated by the second current. A first shunt voltage source circuit; a third current source circuit that generates a third current controlled by a predetermined voltage source; and a fourth current controlled by the second current source circuit; The second current source circuit with a current of 3 A second shunt voltage source circuit for generating a power source, and consuming the third current and the fourth current by the internal power source, and by the first current and the second current The predetermined current controlled by the control voltage is output to the external circuit via the transmission line.

  In the present invention, the third current source circuit includes an operational amplifier to which a predetermined voltage is applied between a non-inverting input terminal and a negative power supply terminal of one of the two transmission lines, and a gate terminal of the operational amplifier. A second output terminal is connected to the output terminal of the amplifier, the source terminal is connected to the inverting input terminal of the operational amplifier, the drain terminal is connected to the second shunt voltage source circuit, and the third current is output via the drain terminal. A voltage-current conversion element having the following polarity: and a source end of the voltage-current conversion element and the negative power supply terminal connected via the first shunt voltage source, and a source end of the voltage-current conversion element A connection point is connected to a first current detection resistor connected to the inverting input terminal of the operational amplifier and a second current detection resistor included in the second current source circuit, and the fourth current is supplied to the second current detection resistor. An operational amplifier and the power supply A constant current diode or a driving resistor for driving a source, and the operational amplifier controls a voltage between the gate and the source of the voltage-current conversion element and applies the voltage applied to the first current detection resistor and the predetermined current The third current is output to the drain terminal of the voltage-current conversion element by controlling so that the voltage of the voltage source becomes the same.

  The broadband two-wire electric transmitter having the current output circuit of the present invention supplies power through a sensor, a signal processing circuit, and a positive power supply terminal and a negative power supply terminal to which two transmission lines are connected from an external circuit, respectively. A current output circuit that converts a physical quantity measured by the sensor into an electrical signal, processes the signal by the signal processing circuit, and outputs a predetermined current to the external circuit via the transmission line. It is characterized by.

  ADVANTAGE OF THE INVENTION According to this invention, stable control is possible, without enlarging a frequency band, the current output circuit which aimed at the improvement of power efficiency, and the broadband 2-wire transmitter which has the same circuit can be provided.

It is a figure which shows the circuit structure of the current output circuit concerning embodiment of this invention. It is a figure which shows the circuit structure of the modification of the current output circuit concerning embodiment of this invention. It is a figure which shows the structure of the conventional current output circuit. It is a figure which shows the basic composition of the broadband 2-wire type transmitter of a prior application. It is a figure which shows the circuit structure of the current output circuit of a prior application.

  Hereinafter, an embodiment for carrying out the present invention (hereinafter simply referred to as an embodiment) will be described in detail with reference to the accompanying drawings.

(Configuration of the embodiment)
FIG. 1 is a diagram illustrating a circuit configuration of a current output circuit according to the present embodiment. The current output circuit 10 of this embodiment is supplied with power from an external circuit (reference numeral 40 in FIG. 4) via two transmission lines (reference numerals L1 and L2 in FIG. 4), and receives a sensor (reference numeral 60 in FIG. 4). ) Is converted into an electrical signal, signal processing is performed by a signal processing circuit (reference numeral 20 in FIG. 4), and a predetermined current is output to the external circuit 40 via the transmission lines L1 and L2, while the internal power supply ( Used for a broadband two-wire transmitter (reference 1 in FIG. 4) that generates an internal power supply # 1).

  According to FIG. 1, the current output circuit 10 includes a current source circuit 11 (first current source circuit), a current source circuit 12 (second current source circuit), and a current source circuit 13 (third current source). Circuit), a shunt voltage source circuit 14 (first shunt voltage source circuit), and a shunt voltage source circuit 15 (second shunt voltage source circuit).

  The current source circuit 11 generates a current I1 (first current) controlled by the control voltage (control signal x) output from the signal processing circuit 20 and outputs the current I1 to the current source circuit 12. The current source circuit 12 (second current source circuit) generates a current I2 (second current) controlled by the current I1 and outputs the current I2 to the shunt voltage source circuit 14.

  The current source circuit 13 (third current source circuit) generates a current I3 (third current) controlled by a predetermined voltage source and a current I4 (fourth current) controlled by the current source circuit 12. Output to the first current source circuit 11. The shunt voltage source circuit 14 (first shunt voltage source circuit) generates the internal power supply # 1 by the current I2, and consumes the current I3 and the current I4. Further, the shunt voltage source circuit 15 (second shunt voltage source circuit) generates a power source for the current source circuit 12 based on the current I3.

  The current output circuit 10 uses a current I1 generated by the current source circuit 11 and a current I2 generated by the current source circuit 12 to the external circuit (reference numeral 40 in FIG. 4) via the transmission lines L1 and L2. A predetermined current lout of 4 to 20 [mA] controlled by the control voltage (control signal x) is output.

  1, the difference in configuration from the current output circuit 30 of the prior application shown in FIG. 5 is in the configuration of the current source circuit 13 and the connection form to the internal power supply # 1, and the current source circuit 12 is characteristically different. The current I3 and the current I4 generated by the above are consumed as the current of the internal power supply # 1. Other configurations are the same as those in FIG. For this reason, the following description will focus on the configuration of the current source circuit 13 in order to avoid duplication of explanation.

  The current source circuit 13 includes an operational amplifier OP3, a voltage-current conversion element M3 composed of a P-type MOS-FET, a Zener diode 16 serving as a voltage source, voltage dividing resistors R2 and R4, a constant current diode D1, and a current detection. And a resistor R5.

  A voltage source by a Zener diode 16 is applied to the operational amplifier OP3 between the non-inverting input terminal (+) and the negative power supply terminal VN. The voltage-current conversion element M3 has a gate terminal connected to the output terminal of the operational amplifier OP3, a source terminal connected to the inverting input terminal (−) of the operational amplifier OP3, a drain terminal connected to the shunt voltage source circuit 15, and via the drain terminal. Current I3 is generated and output to the shunt voltage source circuit 15. The current detection resistor R5 is connected between the source end of the voltage / current conversion element M3 and the negative power supply terminal VN via the shunt voltage source circuit 14, and the connection point between the voltage detection element R3 and the source end of the voltage / current conversion element M3 is calculated. It is connected to the inverting input terminal (−) of the amplifier OP3.

  The operational amplifier OP3 controls the gate voltage of the voltage-current conversion element M3 so that the voltage across the current detection resistor R5 becomes a voltage Y obtained by dividing the voltage source such as the Zener diode 16 with the resistors R1 and R2. . By this operation, the current I3 obtained by the voltage Y / current detection resistor R5 flows, and the operational amplifier OP2 and the Zener diode 14 of the current source circuit 12 can be driven.

  At this time, the constant current diode D1 passes a current I4 that drives the operational amplifier OP3 and the Zener diode 16 that is a voltage source. By connecting the constant current diode D1 to the current detection resistor R3 of the current source circuit 12, the current I4 becomes a part of the current I2 controlled by the current source circuit 12. Therefore, the current accuracy and current fluctuation of the constant current diode D1 are absorbed by the operational amplifier OP3, and therefore do not become an error of the output current Iout. Thus, since the currents I3 and I4 can be consumed as the current of the internal power supply # 1, the power efficiency of the entire circuit of the two-wire transmitter 1 is improved.

(Operation of the embodiment)
Hereinafter, the operation of the current output circuit 10 according to the present embodiment will be described in detail. First, the sensor 60 converts physical quantities such as pressure and temperature into electrical signals and outputs them to the signal processing circuit 20. The signal processing circuit 20 performs a predetermined process such as distortion correction and noise removal on the electrical signal output from the sensor 60 to generate a control signal x (control voltage), and configures the current source circuit 11. This is applied between the non-inverting input terminal (+) and the negative power supply terminal VN terminal of the operational amplifier OP1.

  The current source circuit 11 generates a current I1 controlled by this control voltage (control signal x). That is, the operational amplifier OP1 controls the voltage between the gate and the source of the voltage-current conversion element M1 so that the voltage applied to both ends of the current detection resistor R1 and the control voltage x have the same voltage value. . As a result, the control voltage (control signal x) is converted into a current I1, and the current I1 passes through the drain terminal of the voltage-current conversion element M1, and the current source circuit 12 (the non-inverting terminal of the operational amplifier OP2 and the voltage-current conversion). To one end of the resistor R7.

  Next, the current source circuit 12 generates a current I2 controlled by the current I1 generated by the current source circuit 11. That is, a voltage drop occurs due to the current I1 flowing through the voltage-current conversion resistor R7, and the current I1 is converted into a voltage again. The voltage is between the non-inverting input terminal (+) of the operational amplifier OP2 and the positive power supply terminal VP. To be applied. The operational amplifier OP2 controls the voltage between the gate and the source of the voltage / current conversion element M2 based on the VP reference voltage, and the voltage applied to both ends of the current detection resistor R3 and the VP reference voltage have the same voltage value. Control to become. As a result, the current I2 generated through the drain terminal is output to the shunt voltage source circuit 14. The shunt voltage source circuit 14 generates an internal power supply # 1 that drives the sensor 60 and the signal processing circuit 20 by the current I2.

  The main carrier of the voltage-current conversion element (P-type MOS-FET) constituting the current source circuit 12 is a hole (hole), and the voltage input to the gate end is lower than the source end (gate-source) Current) flows from the source to the drain. The current increases as the input voltage is on the negative side, decreases as the input voltage is on the positive side, and becomes zero at a predetermined value.

  Next, the operational amplifier OP3 constituting the current source circuit 13 has a voltage so that the voltage at both ends of the current detection resistor R5 becomes a voltage Y obtained by dividing the voltage source such as the Zener diode 16 by the resistors R1 and R2. The gate voltage of the current conversion element M3 is controlled. By this operation, the current I3 obtained by the voltage Y / resistor R5 flows, and the operational amplifier OP2 and the Zener diode 14 of the current source circuit 12 are driven. At this time, the constant current diode D1 passes a current I4 that drives the operational amplifier OP3 and the Zener diode 16 that is a voltage source. By connecting the constant current diode D1 to the current detection resistor R3 of the current source circuit 12, the current I4 becomes a part of the current I2 controlled by the current source circuit 12. Therefore, the current fluctuation of the constant current diode D1 is absorbed by the operational amplifier OP3. Therefore, the accuracy and current fluctuation of the constant current diode D1 do not become an error of the output current Iout.

  The current source circuit 13 and the shunt voltage source circuit 15 generate a power source for the current source circuit 12. That is, in the current source circuit 13, a voltage by the Zener diode 16 serving as a voltage source is applied between the non-inverting input terminal (+) of the operational amplifier OP3 and the positive power supply terminal VP, and the operational amplifier OP3 has a voltage-current conversion element. By controlling the gate-source voltage of M3, the voltage applied to both ends of the current detection resistor R5 is controlled to be the same value as the reference voltage. As a result, the current is converted into I3 and output to the shunt voltage source circuit 15 through the drain terminal of the voltage-current conversion element M3. The shunt voltage source circuit 15 generates a power source for the current source circuit 13 based on the current I3.

  Finally, the current output circuit 10 generates a control voltage (current voltage I1 generated by the current source circuit 11, current I2 generated by the current source circuit 12, and current I3 generated by the current source circuit 13 by the control voltage ( A current Iout of 4 to 20 [mA] controlled by the control signal x) is generated and output to the external circuit 40 such as DCS via the two transmission lines L1 and L2, and the sensor 60 and the signal processing circuit 20 are output. An internal power supply # 1 is generated to drive.

  FIG. 2 shows a configuration of a modified example. The difference from the embodiment shown in FIG. 1 is that the constant current diode D1 constituting the constant current circuit 13 is replaced with a drive resistor R6 that drives an operational amplifier OP3 and a Zener diode 16 that is a voltage source. Also in this case, the fluctuation of the current I4 is absorbed by the operational amplifier OP3, so that it does not cause an error of the output current Iout.

(Effect of embodiment)
As described above, according to the current output circuit 10 according to the present embodiment, the current I3 generated by the current source circuit 13 and the current I2 generated by the current source circuit 12 are converted into the internal power supply #. By using the configuration of consuming 1, it is possible to provide the current output circuit 10 with improved power efficiency and the two-wire transmitter 1 having the circuit.

  Further, according to the current output circuit 10 according to the present embodiment, a low station such as a current mirror circuit is provided in the negative feedback loop of the operational amplifiers (OP1 and OP2 respectively) constituting the current source circuits 11 and 12. Since no elements are included (excluding elements with slow response), there is no need for band limitation, and therefore a two-wire transmitter 1 capable of widening the bandwidth can be provided. In addition, the wideband increases the AC input impedance, and the derivative effect that the noise resistance is improved by improving the input impedance is also obtained. It is also possible to generate an internal power supply while outputting current.

  As mentioned above, although preferred embodiment of this invention was explained in full detail, it cannot be overemphasized that the technical scope of this invention is not limited to the range as described in the said embodiment. It will be apparent to those skilled in the art that various modifications or improvements can be added to the above-described embodiments. Further, it is apparent from the scope of the claims that the embodiments added with such changes or improvements can be included in the technical scope of the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Broadband 2-wire transmitter, 10 ... Current output circuit, 11 ... 1st current source circuit, 12 ... 2nd current source circuit, 13 ... 3rd current source circuit, 14 ... 1st shunt voltage source Circuit 15, second shunt voltage source circuit 16, Zener diode 20, signal processing circuit 40, external circuit 60, sensor, OP 1, OP 2, OP 3, operational amplifier, R 2, R 4, voltage dividing resistor, R1, R3, R5 ... current detection resistor, R6 ... drive resistor, R7 ... voltage-current conversion resistor, D1 ... constant current diode, M1, M2, M3 ... voltage-current conversion element (MOS-FET)

Claims (3)

Power is supplied from an external circuit via two transmission lines, a physical quantity measured by a sensor is converted into an electric signal, signal processing is performed by a signal processing circuit, and a predetermined amount is supplied to the external circuit via the transmission line. A current output circuit for a two-wire transmitter that generates an internal power supply while outputting current,
A first current source circuit that outputs a first current controlled by a control voltage output from the signal processing circuit;
A second current source circuit for outputting a second current controlled by the first current;
A first shunt voltage source circuit for generating the internal power supply by the second current;
A third current source circuit that generates a third current controlled by a predetermined voltage source and a fourth current controlled by the second current source circuit;
A second shunt voltage source circuit for generating a power source for the second current source circuit by the third current,
The third current and the fourth current are consumed by the internal power source, and controlled by the control voltage via the transmission line to the external circuit by the first current and the second current. A current output circuit for outputting the predetermined current. The third current source circuit includes:
An operational amplifier in which a predetermined voltage is applied between a non-inverting input terminal and a negative power supply terminal of one of the two transmission lines;
The gate terminal is connected to the output terminal of the operational amplifier, the source terminal is connected to the inverting input terminal of the operational amplifier, the drain terminal is connected to the second shunt voltage source circuit, and the third current is connected via the drain terminal. A voltage-current conversion element having a second polarity for outputting
The source terminal of the voltage-current conversion element and the negative power supply terminal are connected via the first shunt voltage source, and the connection point with the source terminal of the voltage-current conversion element is the inverting input terminal of the operational amplifier A first current sensing resistor connected to
A constant current diode or a driving resistor connected to a second current detection resistor included in the second current source circuit and configured to flow the fourth current and drive the operational amplifier and the voltage source;
The operational amplifier is
The voltage between the gate and the source of the voltage-current conversion element is controlled so that the voltage applied to the first current detection resistor is the same as the voltage of the predetermined voltage source. The current output circuit according to claim 1, wherein the third current is output to a drain terminal of the conversion element. A sensor,
A signal processing circuit;
Power is supplied from a positive power supply terminal and a negative power supply terminal to which two transmission lines are respectively connected from an external circuit, and a physical quantity measured by the sensor is converted into an electrical signal, and signal processing is performed by the signal processing circuit. And a current output circuit according to claim 1 or 2, wherein a predetermined current is output to the external circuit via the transmission line;
A broadband two-wire transmitter characterized by comprising:

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