JPH07129262A - Constant voltage circuit - Google Patents

Abstract

PURPOSE:To prevent the constant-voltage output of an output transistor(TR) from varying by providing a feedback loop which includes a capacitor connected to the 1st and 2nd part of a detection resistance in parallel. CONSTITUTION:When a load becomes abnormal to cause a constant voltage, appearing at the collector of the output TR 12, to rise abruptly, the base voltage of a differential TR 7 rises before the base voltage of a differential TR 6 rises, and the negative feedback loop consisting of the differential TR 7, the output TR 12, the detection resistance 15, and a capacitor 18 operates. The collector voltage of the output TR 12 becomes constant and stable. When the load becomes abnormal to cause the constant voltage, appearing at the collector of the output TR 12, to fall abruptly to the contrary, the base voltage of the TR 9 falls, the base voltage of the output TR 12 falls, and the collector voltage rises. The collector voltage of the output TR 12, therefore, becomes constant and stable.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant voltage circuit.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional constant voltage circuit.
In FIG. 2, (1) and (2) are NPN type transistors, which are connected by a current mirror. That is, the collector of the transistor (1) is connected to the power source Vcc via the resistor (3), and the emitters of the transistors (1) and (2) are grounded. (4) and (5) are PNP type transistors, which are connected to a current mirror. That is, the bases of the transistors (4) and (5) are connected to the collector of the transistor (2), and the emitters are connected to the power supply Vcc.
(6) and (7) are NPN type differential transistors, which are differentially connected. That is, the collectors of the transistors (6) and (7) are connected to the collectors of the transistors (4) and (5), respectively, and the emitters are grounded via the resistor (8). (9) is an NPN type transistor, whose base is connected to the collector of the transistor (7), whose collector is connected to the power source Vcc via the resistor (10), and whose emitter is grounded via the resistor (11). There is. (12) is PN
It is a P-type output transistor whose base is connected to the collector of the transistor (9) and whose emitter is connected to the power supply Vcc. Reference numeral (13) is a reference voltage generating circuit, which is connected to the collector of the output transistor (12) and the base of the differential transistor (6) via the resistor (14). That is, the reference voltage generating circuit (13) is biased by the collector voltage of the output transistor (12) and is supplied with the reference voltage V.
generate a ref. (15) and (16) are detection resistors,
It is connected in series between the collector of the output transistor (12) and ground, and the connection point of both resistors is the differential transistor (7).
Connected with the base of.

A PNP type output transistor (12)
Has the advantage that the power supply Vcc can be lowered, but has the drawback that the current amplification factor h _FE is small. Therefore, the collector of the differential transistor (7) and the output transistor (12)
A transistor (9) is inserted between the bases of the two to supplement the gain. Also, a differential transistor (7), a transistor (9), an output transistor (12), and a detection resistor (1
5) forms a negative feedback loop, and the output transistor (1
It operates so that the collector of 2) becomes a constant voltage. Further, since the reference voltage generating circuit (13) may be an existing circuit, the description of the specific circuit and its operation description will be omitted.

From the above, the load (not shown) is driven by the constant voltage appearing at the collector of the output transistor (12).

[0005]

Here, the load becomes abnormal due to some factor, and the output transistor (1
It is assumed that the constant voltage appearing at the collector in 2) has changed. That is, when the collector voltage of the output transistor (12) rises, the reference voltage Vref rises, the base voltage of the transistor (9) rises, and the output transistor (1
The base voltage of 2) drops and the collector voltage rises further. On the contrary, when the collector voltage of the output transistor (12) drops, the reference voltage Vref drops, the base voltage of the transistor (9) drops, the base voltage of the output transistor (12) rises, and the collector voltage further increases. It will fall. Therefore, there is a problem that positive feedback works and the collector voltage of the output transistor (12) oscillates. Therefore, conventionally, a capacitor (17) is connected between the collector of the output transistor (12) and the ground to solve the problem of oscillation. As a result of an oscillation prevention experiment, it has been found that the capacitor (17) needs a capacitance of several μF.

However, when the conventional constant voltage circuit is integrated, it is necessary to connect the capacitor (17) to the outside due to the capacity of the capacitor (17), and the number of terminal pins and external parts increases. There was a problem. Therefore, an object of the present invention is to provide a constant voltage circuit in which a capacitor that prevents fluctuations in the constant voltage of the output transistor can be integrated.

[0007]

The present invention has been made to solve the above problems, and is characterized in that an output transistor and an output transistor connected in series are provided. A detection resistor for converting a current flowing in the voltage into a voltage, a reference voltage generator for generating a reference voltage in accordance with the voltage of the first portion of the detection resistor, and the reference voltage and the voltage of the second portion of the detection resistor. An error amplifier that includes first and second differential transistors to be compared, and that connects the output voltage of the output transistor to a constant voltage according to the error voltage of both voltages, and the first and second portions of the detection resistor are connected in parallel. And a capacitor that absorbs a variation in the first portion of the detection resistor.

[0008]

According to the present invention, the fluctuation of the constant voltage output of the output transistor can be prevented by using the negative feedback loop including the capacitors connected in parallel to the first and second portions of the detection resistor.

[0009]

The details of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a constant voltage circuit of the present invention. 1 and 2 are denoted by the same reference numerals, and the description thereof will be omitted. In FIG. 1, (18) is a capacitor,
It is connected in parallel to both ends of the detection resistor (15). The impedances of the detection resistor (15) and the capacitor (18) are the resistance (14) existing between the collector of the output transistor (12) and the base of the differential transistor (6).
Also, the impedance is set smaller than the impedance including the wiring capacitance. Theoretically, the impedance of the capacitor (18) is
It is 1 / j2πfC and has a characteristic inversely proportional to the frequency. According to experiments, it has been found that when the load becomes abnormal due to some factor, noise of several hundred kHz to several MHz is superimposed on the constant voltage appearing in the collector of the output transistor (12). . Therefore, the capacitance of the capacitor (18) may be about several tens pF, which is suitable for integration. Moreover, it has been confirmed that the circuit operates properly when the resistance values of the resistors (14), (15) and (16) are set to 12 KΩ, 24 KΩ and 13 KΩ, respectively, and the capacitance of the capacitor (18) is set to 10 pF. .

The operation of FIG. 1 will be described below. First, if the load receives an abnormality due to some factor, and the constant voltage appearing at the collector of the output transistor (12) rises sharply, the differential transistor (6) before the base voltage of the differential transistor (6) rises. The base voltage of 7) rises, and a negative feedback loop composed of a differential transistor (7), a transistor (9), an output transistor (12), a detection resistor (15), and a capacitor (18) works. That is, the base voltage of the transistor (9) rises, the base voltage of the output transistor (12) rises, and the collector voltage falls. Therefore, the collector voltage of the output transistor (12) stabilizes at a constant voltage. On the other hand, when the load receives an abnormality due to some factor and the constant voltage appearing at the collector of the output transistor (12) sharply drops, the differential transistor (6) before the base voltage drops. The base voltage of (7) drops, and the differential transistor (7), the transistor (9), and the output transistor (1
A negative feedback loop composed of 2), the detection resistor (15), and the capacitor (18) works. That is, the base voltage of the transistor (9) drops, the base voltage of the output transistor (12) drops, and the collector voltage rises. Therefore, the collector voltage of the output transistor (12) stabilizes at a constant voltage.

From the above, the capacitor (1) for preventing the fluctuation of the constant voltage appearing at the collector of the output transistor (12)
8) can be integrated, and the number of terminal pins and external parts can be reduced.

[0012]

According to the present invention, it is possible to integrate a capacitor for preventing the fluctuation of the constant voltage output of the output transistor and to reduce the number of terminal pins and external parts.

[Brief description of drawings]

FIG. 1 is a diagram showing a constant voltage circuit of the present invention.

FIG. 2 is a diagram showing a conventional constant voltage circuit.

[Explanation of symbols]

 (6) (7) Differential transistor (12) Output transistor (13) Reference voltage generation circuit (15) (16) Detection resistor (18) Capacitor

Claims (2)

[Claims] 1. A reference voltage is generated according to a voltage of an output transistor, a detection resistor connected in series with the output transistor and converting a current flowing through the output transistor into a voltage, and a voltage of a first portion of the detection resistor. A reference voltage generator and first and second differential transistors for comparing the reference voltage and the voltage of the second portion of the detection resistor are included, and the output voltage of the output transistor is determined according to the error voltage of both voltages. A constant voltage circuit comprising: an error amplifier that outputs a voltage; and a capacitor that is connected in parallel to the first and second portions of the detection resistor and that absorbs fluctuations in the first portion of the detection resistor. 2. The impedance of the detection resistor and the capacitor connected in parallel is smaller than the impedance from the voltage of the first portion of the detection resistor until the reference voltage is generated. Constant voltage circuit.