JPH08204475A - Voltage/current conversion circuit - Google Patents

Abstract

PURPOSE: To change a current made to flow to a load upto 0mA corresponding to input by constituting a differential amplifier in the circuit of an absorption type. CONSTITUTION: A difference between DC input and a reference voltage is amplified in an operational amplifier 6 and inputted to the base of a transistor 7, the emitter of the transistor 7 is connected to ground in the order of a resistor 10, a load resistor 13 and a Zener diode 21 and a voltage generated in the resistor 10 is fed back to the input part of the operational amplifier 6 by feedback resistors 8 and 12. Then, a diode 20 is connected between the emitter and the base of the transistor 7 so as to turn a base side to a cathode side and the Zener voltage value of the Zener diode 21 is turned to a value larger than the sum of the lower limit value of the output saturation voltage of the operational amplifier 6 and the forward voltage of the diode 20. Thus, when the transistor 7 is OFF, the current is absorbed by the operational amplifier 6 through the diode 20 and the current made to flow to the resistor 13 is turned to 0mA.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to various measuring instrument circuits, and more particularly to a voltage-current conversion circuit capable of controlling a current flowing through a load according to an input voltage.

[0002]

2. Description of the Related Art A conventional voltage-current conversion circuit has an operational amplifier 6 used as a differential amplifier, as shown in FIG.
And the base is connected to the output part of the operational amplifier 6, and the emitter and the reference potential are connected in series with the current detection resistor 10 and the load resistor 13 such that the reference potential side is the load resistor 13. Then, the current amplification transistor 7 that controls the current flowing through the load resistor 13 according to the input from the operational amplifier 6, and the resistor 8 and the resistor that feed back the voltage generated in the resistor 10 to the input portion of the operational amplifier 6. 12 and a DC input (V1) is input to the inverting input terminal of the operational amplifier 6 via the input terminal 1 and the input resistor 5, and the non-inverting input terminal of the operational amplifier 6 is the input terminal 2 and the input resistor. 1
The reference voltage (V2) is input via 1 so that a current corresponding to the input difference voltage (| V2-V1 |) can be passed through the load resistor 13 as shown in FIG. .

[0003]

However, when the input differential voltage (| V2-V1 |) approaches 0 V, the emitter current of the transistor 7 also becomes approximately 0 mA, so that the voltage at the output terminal becomes equal to that of the load resistor 13. It becomes almost 0V regardless of the size. At this time, the DC input V1 has a value almost equal to the reference voltage V2, and the DC input V1 causes the current flowing through the load resistor 13 via the resistors 5, 8, and 10 and the resistance due to the reference voltage V2. Since the current flowing through the load resistor 13 via the resistor 11 and the resistor 12 remains, the current flowing through the load resistor 13 cannot be set to 0 mA,
For example, there is a problem that the saturation occurs at about 0.8 mA. According to the present invention, by configuring the differential amplifier as a suction type circuit, the current flowing through the load is reduced to 0 depending on the input voltage.
The purpose is to be able to change to mA.

[0004]

SUMMARY OF THE INVENTION A first invention of the present application, a differential amplifier for amplifying and outputting a difference between a DC input inputted at one end and a reference voltage inputted at the other end, and this differential amplifier. A base is connected to the output part of the, the emitter and the reference potential are connected by a series circuit of a resistor for detecting current and a load, and a current for controlling the current flowing to the load according to the input from the differential amplifier. In a voltage-current conversion circuit comprising a transistor for amplification and a feedback circuit for feeding back the voltage generated in the resistor to the input section of the differential amplifier, a predetermined Zener voltage is provided between the load and a reference potential. A zener diode whose reference potential side is the anode side is inserted, and the diode is connected between the emitter and the base of the transistor so that the base side is the cathode side. A second invention of the present application is characterized in that a Zener voltage of the Zener diode is set to a value larger than a sum of a lower limit value of an output saturation voltage of the differential amplifier and a forward voltage of the diode.

[0005]

The present invention is configured as described above. One end of the load is connected from the emitter of the transistor for current amplification through the resistor for current detection, and the other end of this load is connected to the cathode side of the Zener diode. The anode side of the Zener diode is connected to a reference potential, for example, ground, and the diode is connected between the emitter and the base of the transistor so that the base side is the cathode side. When the input voltage difference (| V2-V1 |) is reduced to lower the current flowing in the load and the potential at the connection point between the current detection resistor and the load becomes less than the Zener potential, the current flowing in the load is almost the same. It becomes 0 mA.

Since the Zener voltage of the Zener diode is set to a value larger than the sum of the lower limit value of the output saturation voltage of the differential amplifier and the forward voltage of the diode, the current flowing from the input section of the differential amplifier through the feedback resistor. Can be passed as a sink current to the differential amplifier through a diode connected between the emitter and the base of the transistor so that the base side becomes the cathode side. Therefore, the current flowing to the load depending on the input voltage is 0 mA. It is possible to change up to.

[0007]

1 is a circuit diagram showing an embodiment of the voltage-current conversion circuit of the present invention. In the figure, the same parts as those shown in FIG. 3 are designated by the same reference numerals. The operational amplifier 6 is used as a differential amplifier, and the DC input (V1) from the input terminal 1 is input to the inverting input terminal of the operational amplifier 6 via the input resistor 5, and the reference voltage (V
2) is input to the non-inverting input terminal of the operational amplifier 6 via the input resistor 11. The transistor 7 is used for current amplification, the output of the operational amplifier 6 is input to the base, and the collector is connected to the power source + V via the resistor 9,
Output terminal 3 through emitter 10 through resistor 10 for current detection
The output terminal 4 is connected to a reference potential, for example, ground via the Zener diode 21, and the Zener diode 21 is arranged so that the cathode side thereof faces the output terminal 4 side.

A load resistor 13 is connected between the output terminal 3 and the output terminal 4, and the current flowing through the transistor 7 is detected by the resistor 10 and converted into a voltage which is used as a feedback voltage. Input from the connection portion to the inverting input terminal of the operational amplifier 6 via the feedback resistor 8, and input from the connection portion of the resistor 10 and the output terminal 3 to the non-inverting input terminal of the operational amplifier 6 via the feedback resistor 12. ing. The diode 20 is connected between the emitter and the base of the transistor 7 such that the base side is the cathode side, and when a surge voltage is input to the output terminal 3, the surge voltage is applied to the operational amplifier 6 so that the transistor 7 is not damaged. So that it is protected.

FIG. 2 is an explanatory diagram of conversion characteristics of the voltage-current conversion circuit of the present invention. For example, the reference voltage V2 is about 10
When the DC input V1 is about 5V, the load resistor 13
The constants of each part of the voltage-current conversion circuit are set so that the current flowing through the input terminal will be about 10 mA.
2−V1 |) is decreased, the output voltage from the operational amplifier 6 decreases and the base voltage of the transistor 7 decreases, so that the current flowing through the transistor 7 decreases as shown in FIG. The output current flowing through the device decreases.

When the potential of the output terminal 3 becomes less than the Zener potential, the current flowing through the load resistor 13 becomes almost 0 mA.
At this time, since the Zener voltage of the Zener diode 21 is set to a value larger than the sum of the lower limit value of the output saturation voltage of the operational amplifier 6 and the forward voltage of the diode 20, the current from the DC input (V1) is applied to the resistor 5 and the resistor. It can be made to flow into the operational amplifier 6 via the resistor 8 and the diode 20 as a current, and the current from the reference voltage (V2) is applied to the resistor 11, the resistor 12, the resistor 10 and the diode 20.
Since it can be made to flow into the operational amplifier 6 as a sink current via, it is possible to change the current flowing to the load resistor 13 to 0 mA according to the DC input (V1).

[0011]

As described above, according to the present invention,
Insert a Zener diode between the load and the reference potential (eg ground) so that the load side is the cathode side, and make sure that the operational amplifier side is the cathode side between the emitter of the current amplification transistor and the output section of the operational amplifier. A diode is inserted, and the Zener voltage of the Zener diode is set to a value larger than the sum of the lower limit value of the output saturation voltage of the operational amplifier and the forward voltage of the diode.
When the voltage applied to the Zener diode becomes equal to or lower than the Zener voltage, the current that flows from the input section of the operational amplifier through the feedback resistor can be made to flow as the sink current of the operational amplifier through the diode. Therefore, it flows to the load according to the input voltage. It is possible to provide a voltage-current conversion circuit capable of changing the current to 0 mA.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a voltage-current conversion circuit of the present invention.

FIG. 2 is an explanatory diagram of conversion characteristics of the voltage-current conversion circuit of the present invention.

FIG. 3 is a circuit diagram of a conventional voltage-current conversion circuit.

FIG. 4 is an explanatory diagram of conversion characteristics of a conventional voltage-current conversion circuit.

[Explanation of symbols]

 1 Input Terminal 2 Input Terminal 3 Output Terminal 4 Output Terminal 5 Resistor 6 Op Amp 7 Transistor 8 Resistor 9 Resistor 10 Resistor 11 Resistor 12 Resistor 13 Resistor 20 Diode 21 Zener Diode

Claims (2)

[Claims] 1. A differential amplifier that amplifies and outputs a difference between a DC input input at one end and a reference voltage input at the other end, and a base connected to an output portion of the differential amplifier, and an emitter. The reference potential is connected to a current detection resistor and a load in a series circuit, and a current amplification transistor that controls a current flowing through the load according to an input from the differential amplifier and the resistor are generated. In a voltage-current conversion circuit comprising a feedback circuit for feeding back the voltage to the input part of the differential amplifier, a zener diode having a predetermined zener voltage between the load and a reference potential and having the reference potential side as the anode side. And a diode is connected between the emitter and the base of the transistor such that the base side is the cathode side. 2. The voltage according to claim 1, wherein the Zener voltage of the Zener diode is larger than the sum of the lower limit value of the output saturation voltage of the differential amplifier and the forward voltage of the diode. Current conversion circuit.