JPS592347B2 - variable current source - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable current source, and particularly to a constant current circuit or a voltage-to-current conversion circuit that obtains an output current directly proportional to a control voltage.

Conventionally, various constant current power supply circuits have been proposed, but the output current is not accurately proportional to the control voltage, and the configurations are large, complex, and expensive because they require a large number of high-precision resistors. It had various drawbacks as described below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a conventional constant current circuit will be explained based on FIGS. 1 to 4, and then embodiments of the present invention will be explained based on FIGS. 5 to 11.

For simplicity, corresponding parts in each figure are given the same reference numerals. In FIG. 1, voltage V1 is applied to the emitter of transistor 10 via terminal 14 and resistor 12 (resistance value R1). On the other hand, a voltage V2 is applied to the base via terminal 16, and a constant current 1 is applied from the collector. ut flows to terminal 18. Here I. ut+(1-2)/R1, V1
If -V2 is large enough, the output current 10ut will be V, -V2
and varies over a wide range in direct proportion to R1 and exactly inversely proportional to R1. However, in reality, the base-emitter voltage VBO changes due to changes in emitter current and temperature, resulting in an output current of 1. ut has the disadvantage that it is not necessarily exactly proportional to V1-V2. Furthermore, when the emitter current is several MA, it operates well, but when the emitter current is very small, the base current of the transistor 10 cannot be ignored, and the I value increases depending on the temperature. There is a disadvantage that ut changes. In the conventional example shown in FIG. 2, the operational amplifier 20 is used as a so-called voltage follower, and the emitter voltage is set to V.
2, thereby compensating for the drawbacks of the conventional example shown in FIG.

As shown in the figure, a plurality of new emitter resistors 12a, 12
b, 12c, ..., 12x are each connected in parallel via a switch (electronic switch or mechanical switch),
Furthermore, it is connected in parallel to the resistor 12. Here, if the combined resistance value of all emitter resistors connected in parallel is R1', the output current is 1. ut is 10ut=(,-V2)/R,
'. Therefore, the emitter resistors 12a, 12b, 1
If 2c, . . . , 12x are selectively inserted into the circuit using switches connected in series, the output current is 1. For example, ut is 1mA, 2mA, 5mA, 10mA, 20mA,
. . . can be changed to a plurality of values in calibrated (i.e., known) predetermined steps. However, the conventional example shown in FIG. 2 requires a large number of highly accurate (with an error of about 0.1%) resistors and switches, which has the disadvantage of making the structure complicated and expensive. Furthermore, in order to obtain the current between these steps continuously, it is necessary to change the voltage V or V2, but the output current 1. Since ut is proportional to the voltage V1-V2, it has the disadvantage that it is not possible to obtain an output that is improved by only one of V1 and V2. FIG. 3 shows a conventional example of a current source circuit that obtains an arbitrary output current approximately proportional to the control voltage.

In this circuit, operational amplifiers 20 and 22 are used as voltage followers, and another operational amplifier 24 is connected to resistor 3.
The resistance values of 2, 33, and 34 are made equal and used as an inverting amplifier with a gain of -1. First, when the output voltage (control voltage) V2 of the potentiometer 28 is O, the voltage -
The output voltage of the potentiometer 30 connected to the V3 power supply (not shown) via the terminal 36 is adjusted to -V1. Therefore, when control voltage 2 is O,
The output voltage of the operational amplifier 24 is V1, and the emitter voltage of the transistor 10 is also V1. Here, when the control voltage V2 is increased, a voltage of V1-V2 is generated at the output terminal of the operational amplifier 24. Therefore, a potential difference of V2 is substantially generated across the emitter resistor 12. In other words, the output current is I. If ut, IO
It becomes Ut+V2/R1. In other words, the output current is 1. ut
is a value approximately proportional to the control voltage V2. However, the conventional example shown in FIG. 3 has the following drawbacks. That is, (1) when the control voltage 2 is increased to 1, the base of the transistor 10
Since the collector voltage VBC is O, it is limited to applications where no load voltage occurs.

Therefore, in general, it is necessary to set V2<V1, so
Output current 1. The linear variable range (that is, variable ratio) of ut is limited. (2) When the control voltage V2 approaches O, the emitter voltage increases to V, IIC. As a result, the voltage drop at the emitter resistor 12 and the output current also approach O, so the operational amplifier 2
Due to the current flowing to the 0 inverting input terminal (approximately 0.5 μA or less, although it varies depending on the device), the output current change becomes nonlinear.

Furthermore, in such a case, normal operation of the operational amplifier 20 becomes difficult. (3) Three operational amplifiers, high precision resistors,
Two potentiometers are required and the positive power supply (voltage +
In addition to 1), a negative power supply (voltage -, ) is required, and furthermore, the circuit configuration and adjustment are not simple and are expensive.

The above are the shortcomings of the conventional circuit shown in Fig. 3, but Fig. 4 shows a conventional example that improves the nonlinearity caused by the current to the inverting input terminal of the operational amplifier, which is a drawback of the conventional circuit shown in Figs. 2 and 3. There is a circuit.

In this circuit, a new transistor 38 is added, the base of which is directly connected to the base of the other transistor 10, and the collector connected to the terminal 14 via a resistor 40. In addition,
The resistance value of resistor 40 is made equal to that of resistor 12. In this conventional example, the emitter of the transistor 38 is not connected to the inverting input terminal of the operational amplifier 20, so the current flowing through the resistor 40 is not shunted to the inverting input terminal, and the second,
One of the drawbacks of the conventional example shown in FIG. 3 is eliminated. However, other drawbacks remain unresolved, and there is another drawback in that a pair of transistors 10 and 38 with matching characteristics are required. The conventional example shown in Fig. 4 is Utility Model Application No. 49-41221.
It is disclosed in the specification of

The present invention provides a novel variable current source that does not have the drawbacks of the conventional examples described above, and embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 shows a first embodiment of the present invention, in which, as explained in the conventional example, the output voltage V2 from the potentiometer 28 is
A voltage equal to is developed at the emitter terminal of transistor 10.

Therefore, current 1 flowing through resistor 12 (resistance value R1)
1 is always a constant value determined by (V1-V2)/R1.
On the other hand, since the output voltage V4 of the potentiometer 46 (used as a control voltage) is applied to the non-inverting input terminal of the operational amplifier 44 used as a voltage follower, the output voltage of the operational amplifier 44 is is also V4, and the current 12 flowing through the resistor (resistance value R2) in the direction of the arrow
becomes (V2-V4)/R2. Therefore, the collector current of transistor 10, ie the output current. Ut is equal to the difference between the two currents 11 and i2, and is expressed as follows. If Rl, R2, Vl, and V2 are selected to satisfy the following conditions, then the output current is directly proportional to the control voltage V4 and inversely proportional to R2.

If a high-precision resistor is used as the resistor 42 with a resistance value R2, the output current 10ut can be changed from O to 4/4 in response to changes in the control voltage 4.
It can be very linearly varied over a wide range up to R2. This situation is shown in FIG. According to this embodiment, most of the drawbacks described in the conventional example shown in FIG. 3 are eliminated. That is, (1) the base voltage of the transistor 10 is approximately 2 - constant, and the transistor 10 is capable of linear operation even when a large amplitude output voltage is generated in a charging capacitor inserted between its collector and ground. be.

Furthermore, since the control voltage V4 can be varied from O to V1, the variable range is extremely wide. (2) The emitter current of the transistor 10 is the control voltage V4 and the output current 1.

A constant value (l-V2)/Rl. regardless of ut. or l/(
R1+R2), so if this value is selected to be sufficiently larger than the control input current of the operational amplifier 20, the influence on the operation can be ignored. Therefore, the output current is 1 even when V4 approaches 0. The linearity of ut is good. (3) The structure is simple and inexpensive because it uses few parts and can operate with a single electric wire (voltage 1). FIG. 6 shows a case of a current sink method using an NPN transistor 41 instead of the PNP transistor 10, and the other configurations are the same as the embodiment shown in FIG. 5, except that the power supply voltage is negative. are the same. Therefore, a description of the operation of the embodiment shown in FIG. 6 will be omitted. 7th
The figure shows a circuit in which the embodiment of FIG. 5 is provided with coarse and fine adjustment means for the control voltage V4.

That is, the coarse adjustment potentiometer 54 and the fine adjustment potentiometer 56 are connected to ground and a negative voltage source (e.g., voltage -
output voltages of potentiometers 54 and 56 are applied to the inverting input terminal of operational amplifier 44 via resistors 50 and 52, respectively. If the ratio of the resistance values of the resistors 50 and 52 is set to, for example, 1:100, the control voltage V4 can be coarsely and finely adjusted. If this control voltage V4 is measured with a digital voltmeter (DMV), I will be obtained from the above three equations. ut can be easily determined. FIG. 9 is another embodiment of the present invention, and shows a current source circuit that combines a current source method and a current sink method.

Since this circuit is a combination of the circuit shown in FIG. 5 and a modification of the circuit shown in FIG. 6, only the modified portion will be explained. Note that the circuit elements in FIG. 9 that correspond to the elements in FIG. 6 are given dashes.

Now, if the resistance values of resistors 62 and 64 are made equal, operational amplifier 60 operates as an amplifier with a gain of -1. Therefore, the voltage applied to the non-inverting input terminal of the operational amplifier 2' in the next stage is -2, which is the same as in the embodiment shown in FIG. On the other hand, the output voltage V4 of potentiometer 46 is applied via resistor 66 to the inverting input terminal of operational amplifier 44I. If the resistance value of resistor 68 is made equal to that of resistor 66, a voltage of -V4 will be generated at the output terminal of operational amplifier 44'. This voltage is applied to the operational amplifier 4 in FIG.
It is equal to the output terminal voltage of 4. Therefore, in the embodiment shown in FIG. 9, if attention is paid to the current flowing out and flowing into and out of the transistors 10 and 415, it can be seen that the circuit is a combination current source circuit of the current source and current sink type. FIG. 10 shows another embodiment of a current source/current sink combination current source circuit according to the present invention. The main differences from the embodiment shown in FIG. 411 is directly connected, and one end of the load 68 is connected to the connection point, (3)
Operational amplifier 44'', resistors 42'', 66, 68 in Fig. 9
etc. are excluded. Note that the current-voltage characteristics of this example are shown in FIG. Now, since the voltage applied to operational amplifiers 20 and 60 is V1/2, the emitter voltages of transistors 10 and 411 are maintained at V1/2 and -V1/2, respectively. Therefore, the emitter current of the transistor 411 is constant at V, /2R, where R is the resistance value of the resistor 12''.
It can be seen that the condition of the above-mentioned equation (3) is satisfied if both the resistance values are R. Here, if the control voltage 4 is set to 0, all of the current flowing through the resistor 12 flows through the resistor 42, so that the voltage I shown in the figure is applied from the load side. A current 1/2R in the opposite direction to ut flows through the collector and emitter of the transistor 41I and the resistor 12.
On the other hand, the control voltage V4 will flow through V
When the voltage is 1/2, all of the current V1/2R flowing through the collector and emitter of the transistor 10 flows through the collector and emitter of the transistor 41'. ut is O.
Furthermore, when the control voltage V4 is V, the transistor 10
The current V1/R flowing through the collector and emitter of is the load 68
V,/2 since it is divided into two parts: the transistor 41' side and the transistor 41' side.
It becomes R. For convenience of explanation, we have considered the control voltage V4 in terms of the three points mentioned above, but if the control voltage V4 is changed from O to V1, the collector current of the transistor 10 can be made continuously variable from O to V1/R. Since the current 1 flows into or out of the load 68. ut becomes as shown in FIG. Note that it is not an essential requirement that the resistance values of the resistors 12, 12' and 42 be equal, but the resistance value of the resistor 125 is equal to the I of the transistor 10,1' when V4=V1/2. If ut is selected to be equal, the characteristics shown in FIG. 11 can be obtained. Furthermore, it is clear that coarse and fine adjustment means can be added to the embodiments of FIGS. 9 and 10 as in the embodiment of FIG. 6. Also, a resistive voltage divider can be used in place of the operational amplifier 60. According to the experimental results, the output current is 1.

The linear variation of ut is 1 compared to 10 to 30 in the conventional circuit.
00 to 1000 or more (for example, 10 μA to 10 mA)
It was found that the characteristics were extremely good. Therefore, it has various uses such as a sweep generator for an oscilloscope, a voltage-controlled oscillator, and a function generator. Note that the output current is 1. The ut value can be directly read from the dial scale by using a multi-turn potentiometer with a vernier. However, the control voltage V3 can be measured directly with a digital voltmeter (D
LED (Light Emitting Diode) by measuring with VM)
, it can also be displayed digitally on an LCD (liquid crystal display) or the like. In one preferred embodiment, V1: 50 volts, V2: 10 volts, R1: 20KΩ, R2: 5K.
It is Ω.

In this case, of course, the condition of equation (2) above is satisfied, and the output current is 1. ut varies from O to 100. In addition, DMV
When using R1, normally R1 is 1KΩ, 10KΩ (n=1
, 2, 3, ...) is convenient. Further, the condition of the above equation (2) is I as shown in FIG. ut O → 3/R
2, but even if formula (2) is not satisfied, the straight line in Figure 8 only moves in the vertical direction (however, IOut》0), so it is not suitable for such applications. (2
) does not have to be satisfied. As explained above, according to the variable current source according to the present invention, a fixed voltage is applied to one end of two resistors whose one end is connected to the emitter of a transistor, and a control voltage is applied to the other end of the resistor. With this circuit configuration, it is possible to obtain an output current with an extremely wide variable range that is precisely proportional to the control voltage.

[Brief explanation of the drawing]

1 to 4 each show a conventional current source circuit, FIGS. 5 to 7 each show an embodiment according to the present invention, FIG. 8 is an explanatory diagram of the operation of the embodiment of FIG. The figures each show other embodiments of the present invention, and FIG. 11 is an explanatory diagram of the operation of FIG. 10. 10, 1', 41, 41'...transistor, 12,
12″, 33, 34, 40, 42, 42″, 48, 6
4, 66...Resistor, 20, 22, 24, 44, 44
1,60... operational amplifier, 28,30,46,56.
...potentio meter.

Claims (1)

[Claims] 1. A transistor to which a constant voltage is applied to its emitter, first and second resistors each having one end connected to the emitter of the transistor, and the other ends of the first and second resistors respectively connected to the emitter of the transistor. A variable current source comprising a controlled voltage source and a controlled voltage source, the variable current source obtaining from the collector of the transistor an output current that changes according to changes in the voltage of the controlled voltage source. 2. A patent claim characterized in that the emitter voltage of the transistor is equal to the product of the resistance of the second resistor and the voltage source divided by the sum of the resistance values of the first and second resistors. The variable current source according to item 1. 3. The variable current source according to claim 1 or 2, wherein the control voltage source is configured to vary from 0 to the voltage of the voltage source connected to one end of the first resistor. . 4. The variable current source according to claim 1, wherein the control voltage source is provided with coarse and fine adjustment means. 5 a first transistor with a constant voltage applied to its emitter;
first and second resistors having one end connected to the emitter of the first transistor, a first voltage source and a first control voltage source respectively connected to the other ends of the first and second resistors; a second transistor to which a constant voltage is applied; third and fourth resistors each having one end connected to the emitter of the second transistor; and a voltage source having a polarity opposite to the first voltage source connected to the other end of the third resistor. and a second control voltage source connected to the other end of the fourth resistor, the input/output current changing according to changes in the voltages of the first and second control voltage sources. from the collectors of the first and second transistors. 6 The emitter voltage of the first transistor is equal to the product of the resistance value of the second resistor and the first voltage source divided by the sum of the resistance values of the first and second resistors, and The emitter voltage of the second transistor is selected to be equal to the product of the resistance of the fourth resistor and the second voltage source divided by the sum of the resistances of the third and fourth resistors. A variable current source according to claim 5, characterized in: 7 The first and second control voltage sources are respectively set from 0 to the first control voltage source.
7. The variable current source according to claim 5 or 6, characterized in that the voltage of the second voltage source is changed. 8. The variable current source according to claim 5, wherein each of the first and second control voltage sources is provided with coarse and fine adjustment means. 9. The variable current source according to claim 5, wherein the first and second control voltage sources are controlled by a single control means. 10 A first transistor to which a constant voltage is applied to the emitter, first and second resistors each having one end connected to the emitter of the first transistor, and a second resistor connected to the other end of the first and second resistor, respectively. a second transistor having a collector connected to the collector of the first transistor and applying a constant voltage to the emitter; a third resistor having one end connected to the emitter of the second transistor; a second voltage source having a polarity opposite to that of the first voltage source connected to the other end of the three resistors; and a load having one end connected to the connection point of the collectors of the first and second transistors, and the voltage of the control voltage. A variable current source that obtains an input current to or an output current from the load that changes in response to changes in the load. 11 The emitter voltage of the first transistor is changed to the emitter voltage of the first transistor.
The variable according to claim 9, characterized in that the product of the resistance value of the resistor and the first voltage source is equal to the value obtained by dividing the sum of the resistance values of the first and second resistors. current source. 12. The variable current source according to claim 9 or 10, wherein the first control voltage source is configured to vary from 0 to the voltage of the first voltage source. 13. The variable current source according to claim 9, characterized in that the control voltage source is provided with coarse and fine adjustment means. 14. A claim characterized in that the resistance values of the first and second resistors are equal, and the emitter voltages of the first and second transistors are selected to be half of the voltage of the first and second voltage sources, respectively. The variable current source according to item 9.