JPH0430640B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention converts an output signal from a sensor that detects physical quantities such as pressure and temperature into a corresponding current signal, and transmits the signal via a two-wire transmission line. This invention relates to a two-wire transmitter for transmitting data to a remote receiver.

<Prior Art> FIG. 1 is a circuit diagram showing an example of a conventional two-wire transmitter. In the figure, 1 is a sensor circuit that detects a physical quantity such as pressure and generates an electric signal corresponding to the physical quantity. Ea and Eb represent the output voltage of each sensor. These voltages are applied to the two inputs of the operational amplifier 2 via resistors 3 and 4. 5 is a transistor for output current control to which the output of the operational amplifier is applied; 6 is a constant voltage circuit composed of a constant current circuit 6a and a Zener diode 6b. A feedback resistor 7 is connected between the common line C and the power receiving terminal 9, and both ends thereof are connected to the input of the operational amplifier 2 via resistors 10 and 11. One end of each of the transmission lines L1 and L2 is connected to power receiving terminals 8 and 9, and a series circuit of a power source 12 and a load resistor 13 is connected between the other ends of the transmission lines.

With this configuration, when the input voltage of the operational amplifier 2 increases, the current flowing through the transistor 5 increases and the circuit current Io increases. This makes resistor 7
The voltage across the amplifier increases, and this voltage is negatively fed back to the input of the operational amplifier 2 for balance.

By the way, in such a conventional device, the measurement accuracy of the device is determined by the resistors 3, 4, 7, 10, and 11, so expensive high-precision resistors must be used in these parts, and the manufacturing process is slow. It had the disadvantage of increasing costs.

<Objective> An object of the present invention is to realize a two-wire transmitter that does not require such a high-precision feedback resistor and can be manufactured at low cost.

<Configuration of the Invention> The configuration of the present invention receives DC power from a DC voltage source at a remote point via a two-wire transmission line, converts the output signal from the sensor into a corresponding current signal, and converts the output signal from the sensor into a corresponding current signal. In a two-wire transmitter that transmits remotely via a wire transmission line, a signal current from a sensor circuit and a current obtained by shunting the circuit current are applied, and a Norton type amplifier generates an output according to the difference between these current inputs. , comprising current control means for increasing or decreasing the circuit current in response to the output of the Norton type amplifier, and a current mirror circuit for shunting a part of the circuit current and returning it to the input side of the Norton type amplifier,
The present invention is characterized in that the signal current and the shunt current are balanced.

<Embodiment> FIG. 2 is a circuit diagram of an apparatus according to an embodiment of the present invention.
In the figure, the same elements as those in FIG. 1 are given the same reference numerals. The sensor circuit 1 includes, for example, a variable capacitor 1 in which two capacitances change differentially.
a, 1b are used. 14 is a constant voltage circuit 6
An AC oscillator that is supplied with power +V from
The AC signal from here is commonly applied to one end of variable capacitors 1a and 1b. Between the other ends of these variable capacitors and the common line C, there are diodes 15a, 1
5b are connected with the same polarity. Variable capacity 1
Between the other ends of a and 1b and the later-described Norton type amplifier 16, there are diodes 15c and 15 in the opposite direction.
5d is connected. 17a and 17b are capacitors for smoothing the current signals from the variable capacitors 1a and 1b which have been half-wave rectified by the diodes 15c and 15d.

The Norton type amplifier 16 is a current differential amplifier that operates based on a difference in input current. By connecting transistors 16a and 16b having the same characteristics as shown in the figure, these transistors exhibit current mirror characteristics in which the same current flows. Variable capacitance 1a, 1b
When the current inputs 11 and 12 from the current mirror circuit 18 and the feedback current If, which is a shunt of the circuit current Io from the current mirror circuit 18 described later, are applied as shown in the figure, the transistor 16c at the subsequent stage has the following current: {(I1-I2 )−If)
A base current with a difference between the input currents is applied, and a signal voltage related to this current is applied to the output control transistor 5 in the subsequent stage.

The current mirror circuit 18 is composed of, for example, two transistors 18a and 18b with different junction areas, and the circuit current Io is determined based on the junction area ratio.
A part of the current is shunted and applied to the input side of the Norton type amplifier 16 as a feedback current If.

Reference numeral 19 denotes a control unit for controlling the amplitude of the AC oscillator 14 at a constant level. Among these, 19a has a non-inverting input and one power supply connected to the common line C, and an inverting input connects the diode 15a through the common line C. 15b
This is a power supply driven type current amplifier connected to the anode side of the During the negative half cycle of the AC signal, the current applied from the Norton type amplifier 16 through the common line C flows through the power supply section of this amplifier, and from the output section, the current is applied to the resistor R, the diodes 15a and 15b, and the variable The current flows through the capacitors 1a and 1b and returns to the AC oscillator 14. As a result,
The output voltage is (I1+I2)・R, and the signal current I
1 and I2.

19b is an operational amplifier that compares the output from the current amplifier 19a with a reference voltage Eo, and controls the AC oscillator 14 based on the comparison output.

With this configuration, the Norton type amplifier 16
generates an output according to the difference between the current input difference signal (I1+I2) and the feedback current If, thereby controlling the output control transistor 5. As a result,
The circuit current Io is controlled toward the point where the feedback current If from the current mirror circuit 18 and the difference signal (I1+I2) are balanced, and the circuit current Io corresponding to this difference signal is transmitted through the transmission lines L1 and L2. Transmitted to the load side.

<Effects> According to the present invention, since the input signal and the feedback signal are compared as current signals, a high-precision resistor for feedback is not required, and a two-wire transmitter can be manufactured at an extremely low cost. be able to. Additionally, since no external resistor is required, the circuit configuration can be made suitable for IC implementation.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional device, and FIG. 2 is a circuit diagram of a device according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Sensor circuit, 5...Output current control transistor, 8, 9...Power receiving terminal, 12...Power supply, 13...Load, 14...AC oscillator, 16...Norton type amplifier, 18...Current mirror circuit, L1, L2 …
transmission line.

Claims (1)

[Claims] 1 Receiving DC power from a DC voltage source at a remote point via a two-wire transmission line, converting the output signal from the sensor into a corresponding current signal, and transmitting it remotely via the two-wire transmission line. In wire transmitters,
A Norton-type amplifier receives a signal current from a sensor circuit and a current obtained by shunting the circuit current, and generates an output according to the difference between these current inputs; and a Norton-type amplifier that increases or decreases the circuit current in response to the output of the Norton-type amplifier. 1. A two-wire transmitter comprising: current control means for controlling the circuit current; and a current mirror circuit that shunts a part of the circuit current and returns it to the input side of the Norton type amplifier.