JPH0772939A - Current source circuit - Google Patents

Abstract

PURPOSE:To improve the capacity factor of a power supply voltage by varying an output current from zero linearly by acquiring the output current from the collection electrode of a transistor whose discharge electrode is coupled with a resistor and the inversion input terminals of plural operational amplifiers. CONSTITUTION:The discharge electrode of the transistor 10 is coupled with the resistor 12 and the inversion input terminal of the operational amplifier 14, and the control electrode of the transistor with the output terminal of the operational amplifier 14, and furthermore, a reference voltage source 16 is connected to between the non- inversion input terminal of the operational amplifier 14 and the other terminal of the resistor 12. Also, the output terminal of the operational amplifier 18 is coupled with the other terminal of the resistor 12, and the inversion input terminal of the amplifier with the discharge electrode of the transistor 10, and the non-inversion input terminal receives a prescribed voltage. Since the emitter of the transistor 10 goes to the prescribed voltage in spite of the output current obtained from the collection electrode of the transistor 10 by the negative feedback function of the operational amplifier 18, the capacity factor of the power supply voltage can be improved by increasing the maximum output voltage by setting the prescribed voltage appropriately. Also, the output current can be varied from zero linearly.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current source circuit, and more particularly to a current source circuit capable of linearly changing an output current from 0 and setting a voltage at an output terminal to be high regardless of a change in the output current. .

[0002]

2. Description of the Related Art Current source circuits such as variable current sources and constant current sources include offset circuits,
It is used in various electronic circuits such as ramp wave generation circuits. FIG. 4 shows one conventional example of such a current source circuit. The emitter of the bipolar NPN transistor 10 receives a power supply voltage, for example, −15V via a resistor 12 and is coupled to an inverting input terminal of an operational amplifier (operational amplifier) 14. In addition, the base of the transistor 10 is coupled to the output terminal of the operational amplifier 14, and the collector of the operational amplifier 14 draws an output current Iout. Operational amplifier 14
The reference voltage source 16 is inserted between the non-inverting input terminal of -1 and the voltage line of -15V. Power supply voltage (driving voltage) of operational amplifier
Is -15V and ground voltage. Note that the transistor 10 and the operational amplifier 14 have the same power supply voltage.

In the current source circuit of FIG. 4, the operational amplifier 1
4 controls the base of the transistor 10 so that the voltages at its non-inverting and inverting inputs are equal,
The potential difference across the resistor 12 becomes equal to the voltage of the reference voltage source 16. Therefore, the constant current flowing through the resistor 12 becomes the emitter current of the transistor 10, and this emitter current becomes the collector current, that is, the output current Iout (ignoring the base current). If the reference voltage source 16 is made variable, the output current Iout becomes variable.

By the way, in general, in order to properly operate the operational amplifier, it is necessary to have a margin of 2 V with respect to the power supply voltage. In the case of FIG. 4, since the power supply voltage of the operational amplifier is −15 V and 0 V, its input voltage range is at least −.
Must be between 13V and -2V. Therefore, the voltage of the reference power supply 16 must be 2V to 13V. That is, the voltage difference across the resistor 12 is at least 2V. Therefore, even if the reference voltage source 16 is variable,
There arises a problem that the output current Iout cannot be linearly changed from 0 mA. Due to this problem, when the offset adjustment is performed by the variable current source, fine adjustment cannot be performed, and when the ramp wave is generated, the waveform with a gentle slope cannot be generated.

A conventional current source circuit that solves this problem is shown in FIG. 4 is different from that of FIG. 4 in that the power supply voltage of the operational amplifier 14 is a separate system, the lower end of the resistor 12 and the cathode of the reference voltage source 16 receive the power supply voltage of -12.5V, and the operational amplifier 14 is -15V. This is the point that receives the power supply voltage of. The difference between these two power supply voltages is 2.5V because
This is because 2V has a margin of 0.5V. Since the input voltage range of the operational amplifier 14 is −13V to −2V, the reference voltage source 16 can be adjusted to set the voltage difference across the resistor 12 from 0V. Therefore, the output current Iout
Can be linearly changed from 0 mA.

FIG. 6 shows a variable range of the reference voltage Vref from the reference voltage source 16 in the current source circuit of FIG.
A characteristic diagram of the emitter voltage VE and the maximum output voltage (collector voltage) Vout of the transistor 10 in the case of 2V is shown. 6, the vertical axis on the left side represents the power supply voltage, the vertical axis on the right side represents the reference voltage Vref (for setting the output current Iout value), and the horizontal axis represents the reference voltage Vref.
Represents the percentage of the output current Iout with 100% at 2V. The saturation voltage between the collector and the emitter of the transistor 10 is 0.5V.

As can be seen from the characteristic diagram of FIG. 6, the reference voltage Vr
When ef is 0V, it corresponds to the power supply voltage -12.5V, and the emitter voltage VE becomes equal to the reference voltage Vref. Reference voltage V
ref, that is, the output current I in proportion to the emitter voltage VE
out increases. Unless the collector voltage of the transistor 10 is higher than the emitter voltage VE by the collector-emitter saturation voltage (0.5 V), the transistor 10 cannot operate. Therefore, the output current Iout of FIG.
The terminal voltage at the current input of the receiving electronic circuit must be higher than the emitter voltage VE by this saturation voltage 0.5V. Therefore, the maximum output voltage Vout characteristic line is higher by 0.5V than the characteristic line of the emitter voltage VE,
It will be parallel to this straight line. Therefore, the voltage at the current input terminal of the electronic circuit that receives the output current Iout of the current source circuit must be determined in consideration of the worst state when the output current Iout is the maximum, and the electronic circuit that receives the output current Iout. However, there is a problem that the power supply voltage utilization rate of is deteriorated.

Therefore, an object of the present invention is to provide a current source circuit in which the output current can be changed linearly from 0 and the maximum output voltage is increased to improve the power supply voltage utilization rate.

[0009]

According to the current source circuit of the present invention, the radiation electrode of the transistor 10 is coupled to one end of the resistor 12 and the inverting (-) input end of the first operational amplifier 14,
Its control electrode is coupled to the output of the first operational amplifier 14. The reference voltage means 16 is connected between the non-inverting (+) input terminal of the first operational amplifier 14 and the other end of the resistor 12. Second
The output end of the operational amplifier 18 is coupled to the other end of the resistor 12,
Its inverting input is coupled to the radiation electrode of transistor 10, and its non-inverting input receives a predetermined voltage. Output current I
out is obtained from the collecting electrode of the transistor 10. In this specification, the term "transistor" means both a bipolar transistor and a field effect transistor (FET). Therefore, the control electrode corresponds to the base and the gate, the emission electrode corresponds to the emitter and the source, and the collection electrode corresponds to the collector electrode. Corresponds to the collector and drain.

[0010]

Operation: Transistor 10, resistor 12, operational amplifier 1
4 and the reference voltage means (reference voltage source) 16 operate similarly to the conventional current source circuit to generate the output current Iout. Due to the negative feedback effect of the operational amplifier 18, the output current Io
The emitter of the transistor 10 has a predetermined voltage regardless of ut. Therefore, the maximum output voltage is always the predetermined voltage and the saturation voltage between the collector and the emitter of the transistor 10. By appropriately setting the predetermined voltage, the power supply voltage utilization rate of the electronic circuit that receives the output current Iout is improved. Since the absolute value of the power supply voltage of the first operational amplifier 14 is naturally larger than the output voltage of the second operational amplifier 18, the output current Iout can be linearly changed from 0.

[0011]

1 is a circuit diagram of a preferred embodiment of the present invention. The same elements as in FIGS. 4 and 5 are designated with the same reference numbers. In this embodiment, an NPN transistor is used as the transistor 10, but it should be noted that the transistor can be a PNP transistor or a field effect transistor, and the present invention can be implemented by appropriately setting the power supply voltage and the bias voltage. I want to be done.

As described above, the transistor 10 and the resistor 1
2, operational amplifier 14 and reference voltage source (reference voltage means) 1
The connection relationship of 6 is the same as that of the conventional current source circuit of FIG. The output terminal of the operational amplifier 18 is coupled to the lower end of the resistor 12 and the cathode of the reference voltage source 16, the inverting (-) input terminal is coupled to the emitter of the transistor 10, and the non-inverting (+) input terminal is the power supply voltage-. Receives 12.5V. The operational amplifiers 14 and 18 are driven by the ground voltage and -15V. The predetermined voltage supplied to the non-inverting input terminal of the operational amplifier 18 is set to -12.5 V because the input voltage is at least 2 V with respect to the power supply voltage in order for the operational amplifier to operate properly. This is because there must be a margin of, and there is more margin.

Similar to the conventional current source circuit, the first operational amplifier 14 has the transistor 10 so that the voltage at its non-inverting input terminal and that at its inverting input terminal become equal due to the negative feedback effect.
Control the base of. Further, since the resistor 12 and the cathode of the reference voltage source 16 are commonly connected, the potential difference across the resistor 12 becomes equal to the reference voltage Vref from the reference voltage source 16. Therefore, a current determined by the resistance value of the resistor 12 and the reference voltage value flows from the emitter of the transistor 10 into the resistor 12, and this current is the output current Iout.
Becomes The output current Iout varies linearly from 0 by varying the reference voltage Vref from 0.

On the other hand, the second operational amplifier 18 generates the output voltage VN by the negative feedback action so that the voltages at its inverting input terminal and non-inverting input terminal become equal. Therefore, regardless of the output voltage of the operational amplifier 14, that is, the output current Iou
The emitter voltage of the transistor 10 becomes the predetermined voltage -12.5V regardless of t. Therefore, the transistor 1
The maximum value of the collector voltage of 0, that is, the maximum voltage Vout at the current output terminal is always -12.5V,
0 collector-emitter saturation voltage (eg 0.5
It becomes a voltage (-12V) obtained by adding V). As a result, the power supply voltage utilization rate of the electronic circuit that receives the output current Iout is improved.

As can be seen from the above, when the reference voltage Vref is changed from 0V to 2V in the current source circuit of FIG. 1, its characteristic diagram is as shown in FIG. 2, the left vertical axis represents the power supply voltage, the right vertical axis represents the reference voltage Vref, and the horizontal axis represents the output current Iout (the output current when the reference voltage is 2V is 100V). %). Comparing FIG. 2 and FIG. 6, according to the present invention, the output current Io
It can be seen that the maximum output voltage Vout can be large regardless of ut.

FIG. 3 is a more specific circuit diagram of the current source circuit of the present invention. Elements corresponding to those of FIG. 1 are designated with the same reference numbers. In FIG. 3, the second operational amplifier 18 is composed of an operational amplifier 20 and an NPN transistor 22 which is its output inverting amplifier. Therefore, the second operational amplifier 1
The inverting input terminal and the non-inverting input terminal of 8 correspond to the non-inverting input terminal and the inverting input terminal of the operational amplifier 20, respectively. The predetermined voltage supplied to the non-inverting input terminal of the operational amplifier 18 (the non-inverting input terminal of the operational amplifier 20) is generated by dividing the power supply voltage -15V by the resistors 24 and 26.

The reference voltage means inserted between the non-inverting input terminal of the first operational amplifier 14 and the lower end of the resistor 12 comprises a constant current source 28 and a resistor 30. That is, the resistor 30 is inserted between the non-inverting input terminal of the operational amplifier 14 and the lower end of the resistor 12, a constant current is passed through the resistor 30, and the voltage drop thereof is used as a reference voltage. The constant current source 28 is composed of an operational amplifier 32, an FET 34 and a resistor 36. The relationship between these elements is the same as the relationship between the operational amplifier 14, the transistor 10 and the resistor 12, and is determined by the reference voltage VR and the value of the resistor 36 supplied to the non-inverting input terminal of the operational amplifier 32. Therefore, the current of the current source 28 is changed by changing the voltage VR, and the voltage drop (reference voltage Vref) of the resistor 30 is also changed by changing the current, and the output current Iout is changed. The other operations of the current source circuit of FIG. 3 are the same as those of FIG.

[0018]

As described above, according to the current source circuit of the present invention, the output current can be changed linearly from zero. In addition, the maximum output voltage at the terminal generating the output current can be increased to improve the power supply voltage utilization rate.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a preferred embodiment of the present invention.

FIG. 2 is an operation characteristic diagram of FIG.

FIG. 3 is a detailed circuit diagram of a preferred embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional current source circuit.

FIG. 5 is a circuit diagram of another conventional current source circuit.

6 is an operating characteristic diagram of FIG.

[Explanation of symbols]

 10 Transistors 12 Resistors 14 First Operational Amplifiers 16 Reference Voltage Means 18 Second Operational Amplifiers

Claims (1)

[Claims] 1. A first operational amplifier, a transistor having a radiation electrode coupled to one end of a resistor and an inverting input terminal of the first operational amplifier, and a control electrode coupled to an output terminal of the first operational amplifier; A reference voltage means connected between the non-inverting input of the first operational amplifier and the other end of the resistor, an output end coupled to the other end of the resistor, and an inverting input end coupled to the radiating electrode of the transistor. And a second operational amplifier that receives a predetermined voltage at the non-inverting input terminal, and obtains an output current from the collecting electrode of the transistor.