JPH0413692Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Technical field This invention relates to the improvement of constant current circuits.
In particular, it is an object of the present invention to provide a constant current circuit that can obtain a stable output current against fluctuations in power supply voltage.

(b) Background of the technology A constant current circuit must be configured to generate a constant current even when the power supply voltage fluctuates, such as when a ripple is applied to the power supply. For example, FIG. 1 shows a bias circuit using a conventional constant current circuit, in which a series circuit of resistor 1 and diodes 2 and 3 is connected between the power supply (+V _cc ) and the ground, and the diode 2 A constant voltage obtained by A constant current is obtained at the collector of the PNP transistor 6, and a constant voltage is generated from one end of a load resistor 7 connected to the collector of the PNP transistor 6. In the circuit of FIG. 1, when the power supply voltage (+V _cc ) changes, the current flowing through the diodes 2 and 3 changes, and the voltage across the diodes 2 and 3 changes, so the NPN transistor 4 and the PNP This has the disadvantage that the collector current of the transistor 6 changes, making it impossible to obtain a constant current.

When power supply voltage fluctuations, especially ripples, occur, the adverse effects can be prevented by inserting a ripple filter into the power supply line. However, since the use of a ripple filter causes a drop in the power supply voltage, it has not been possible to use the ripple filter particularly in equipment that uses a low voltage power supply.

(c) Main points of the invention The present invention was created in view of the above points, and has excellent ripple rejection characteristics and can be used at low power supply voltages.
The present invention aims to provide a constant current circuit suitable for integration into an IC (integrated circuit), which includes first and second transistors of a first polarity and a third transistor of a second polarity opposite to the first polarity. The present invention is characterized in that a constant voltage is applied to the base of the third transistor, and the collector of the second transistor is connected to the emitter of the third transistor to form a negative feedback path.

(iv) Embodiment Figure 2 shows an embodiment of the present invention, in which 8 is a PNP type first transistor whose emitter is connected to the power supply (+V _cc ) via a resistor 9 and which generates a constant current in the collector. The transistor 10 has an emitter connected to the power supply (+
V _cc ), a PNP type second transistor 11 has a base connected to the base of the first transistor 8, and a collector 11 has a collector connected to the common base of the first and second transistors 8 and 10, and an emitter fixed to NPN connected to earth via current source 12
type third transistors, 13 and 14 are diodes that generate a base bias voltage of the third transistor 11, and 15 is a diode whose emitter and base are commonly connected to the emitter and base of the second transistor 10, and whose collector current is connected to the diode. 1
3 and 14, and 16 is a starting resistor.

Next, the operation will be explained. When the power is turned on at a certain point, a current flows through the diodes 13 and 14 via the starting resistor 16, and the base of the third transistor 11 becomes a predetermined voltage, so that the collector current of the third transistor 11 flows. Therefore, base currents of the first, second, and fourth transistors 8, 10, and 15 flow, and correspondingly, respective collector currents flow. Therefore, the fourth transistor 15
The collector current is supplied to the diodes 13 and 14, and the base voltage of the third transistor 11 can be made constant. Further, since the collector of the second transistor 10 is connected to the emitter of the third transistor 11, the collector current of the second transistor 10 flows to the constant current source 12 together with the emitter current of the third transistor 11. At this time, since the collector current of the second transistor 10 is sufficiently larger than the emitter current of the third transistor 11, the collector current of the second transistor 10 is approximately equal to the current flowing through the constant current source 12. Become. Furthermore, the collector current of the first transistor 8 is obtained at the output terminal 17, and if the collector current of the second transistor 10 is constant, the collector current of the first transistor 8 is also constant, and therefore the collector current of the first transistor 8 is obtained at the output terminal 17. The resulting output current is also constant.

Now, if the power supply voltage fluctuates, the collector currents of the first, second, and fourth transistors 8, 10, and 15 change due to the Early effect. Therefore, when the collector current of the second transistor 10 increases due to the Early effect, it is compared with the current flowing through the constant current source 12, and the emitter current of the third transistor 11 decreases. Therefore, the collector current of the third transistor 11 decreases, and the base currents of the first, second, and fourth transistors 8, 10, and 15 decrease accordingly. Therefore, first,
Among the collector currents of the second and fourth transistors 8, 10 and 15, the current increased by the Early effect is reduced, and the collector currents of the first, second and fourth transistors 8, 10 and 15 are reduced. returns to its original stable state. Therefore, the collector current of the first transistor 8 does not change even if the ripple continues and the power supply voltage fluctuates, and an extremely stable constant current can be obtained. Note that the negative feedback operation of increasing the collector current due to the Early effect of the second transistor 10, decreasing the emitter current and collector current of the third transistor 11, decreasing the base current of the second transistor 10, and decreasing the collector current rapidly occurs. Therefore, the constant current is accurately set, and it is also possible that the collector current of the second transistor 10 decreases due to the Early effect in response to fluctuations in the power supply voltage.

Figure 3 shows the voltage change (dotted line) obtained at the output terminal 18 in Figure 1 when the power supply voltage (+ _Vcc ) is changed, and the load resistance at the output terminal 17 in Figure 2 as shown in Figure 1. This shows the change in voltage (solid line) that occurs across the load resistor when the load resistor is connected.
As is clear from the figure, the embodiment of the present invention shown in FIG. 2 can generate an extremely stable output voltage.

Note that the resistor 9 is connected to compensate for variations in the Early effect, and if the first
If the Early effect of the transistor 8 and the Early effect of the second transistor 10 are equal, the resistor 9 can be omitted.

(e) Effects As described above, according to the present invention, the circuit configuration is designed to stabilize the base voltage of the third transistor 11 and compensate for current changes due to the Early effect of each transistor. This has the advantage of providing an extremely stable constant current circuit. Furthermore, unlike when a ripple filter or the like is inserted, voltage loss does not occur, so it can be used with a low power supply voltage, and has the advantage that it can be used, for example, in an amplifier circuit driven by a 3V battery. Furthermore, the constant current circuit according to the present invention uses a capacitor, etc.
There is no need to use elements that are difficult to integrate into ICs, so
It is a practical device that can be easily incorporated into an IC and can also be used as a constant voltage circuit by simply connecting an appropriate load.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a bias circuit using a conventional constant current circuit, Fig. 2 is a circuit diagram showing an embodiment of the present invention, and Fig. 3 is a characteristic diagram for explaining the present invention. be. Explanation of main figure numbers, 8...First transistor, 1
0...Second transistor, 11...Third transistor, 17...Output terminal.

Claims (1)

[Scope of utility model registration request] a first transistor of a first polarity whose emitter is connected to one end of the power supply; a second transistor of a first polarity whose emitter is connected to one end of the power supply and whose base is commonly connected to the first transistor; On the common base of the first and second transistors,
a third transistor of a second polarity, the emitter of which is connected to the other end of the power supply via a constant current source, the base of which is connected to a constant voltage source, and an output terminal connected to the collector of the first transistor; By connecting the collectors of the two transistors to the emitters of the third transistor, the base currents of the first and second transistors are controlled according to the collector current of the second transistor, and the collector current of the first transistor is A constant current circuit characterized in that the constant current is maintained at a constant value and a constant current is obtained at the output terminal.