JP2001134332A - Constant current circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a constant current circuit capable of sufficiently correcting temperature even when a set current is comparatively large. SOLUTION: The constant current circuit is provided with a 1st constant current circuit having a negative temperature characteristic and a 2nd constant current circuit having a positive temperature characteristic and corrects temperature by adding the mutually opposite temperature characteristics of both the 1st and 2nd constant current circuits. Concretely the 1st constant current circuit impresses only VREF to a resistor R1 and the 2nd constant current circuit impresses VREF-VBE to a resistor R2, so that the former is provided with the negative temperature characteristic, the latter is provided with the positive temperature characteristic and temperature correction can be attained by both the constant current circuits.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a constant current circuit, and more particularly to a constant current circuit capable of canceling out temperature characteristics at a relatively large current.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional general constant current circuit. A reference voltage VREF is applied to the base of the transistor Q1, and a resistor R1 is connected between the emitter and the ground GND. The operation of such a circuit is such that the resistance R1 is VREF-VBE.
There is a voltage drop of Here, VBE is the transistor Q1
Is the base-emitter voltage. Therefore, the collector current I0 of the transistor Q1 becomes I0 = (VREF−
VBE) / R1.

The collector current I0 is such that VREF is constant at a band gap 1.2V reference voltage, VBE has a negative temperature characteristic, and a resistor R1 has a positive temperature characteristic. It is necessary to design circuit constants so as to cancel the characteristics. In particular, when the temperature characteristic is to be completely canceled, the resistance R is adjusted to match the temperature characteristic of VBE.
It is necessary to design the resistance value of 1 to be considerably large. At this time, the collector current I0 is about several μ 数.

[0004]

However, when it is desired to set the constant current to several tens of μA, the resistance value of the resistor R1 cannot be increased, and the temperature characteristic of VBE is strongly affected.
As shown in FIG. 4, a problem has arisen that the collector current I0 has a positive temperature characteristic. An object of the present invention is to realize a constant current circuit that can always improve the temperature characteristics regardless of the magnitude of the constant current.

[0005]

SUMMARY OF THE INVENTION The present invention comprises a first constant current circuit having a negative temperature characteristic and a second constant current circuit having a positive temperature characteristic, and adding the opposite temperature characteristics. To achieve temperature compensation.

[0006] More specifically, the first constant current circuit is V
REF alone is applied to the resistor R1, and the second constant current circuit
By applying REF VBE to the resistor R2, the former has a negative temperature characteristic, and the latter has a positive temperature characteristic, thereby realizing temperature correction by both.

[0007]

FIG. 1 shows one embodiment of the present invention.

[0008] The constant current circuit of the present invention comprises a first constant current circuit and a second constant current circuit connected in parallel.

In the first constant current circuit, a resistor R1 is connected between the emitter of the first NPN transistor Q1 and the ground GND, and a band gap 1.2V reference voltage is applied to the resistor R1. The collector current at this time is defined as I1.

A third PN is connected to the base of the transistor Q1.
The emitter of the P-type transistor Q3 is connected, and the collector of the transistor Q3 is grounded. Further, the reference voltage VREF is applied to the base of the transistor Q3, and the reference voltage VREF +
VBE of the transistor Q3 is added and applied. Therefore, the VBE of the transistor Q3 and the VBE of the transistor Q1 cancel each other, and as a result, the resistance R1
, Only the reference voltage VREF is applied.

In the second constant current circuit, a resistor R2 is connected between the emitter of the second NPN transistor Q2 and the ground GND, and a reference voltage VREF is applied to the base. As a result, the transistor R2 is applied to the resistor R2 from the reference voltage VREF. Will be applied. The collector current of the transistor Q2 at this time is represented by I2
And

Therefore, the total current I of the constant current circuit of the present invention is
0 can be represented by the sum of the current I1 of the first constant current circuit and the current I2 of the second constant current circuit. That is, I0
= I1 + I2.

Next, the operation principle of the present invention will be described.

In the first constant current circuit, since only the reference voltage VREF is applied to both ends of the resistor R1, the collector current I1 of the transistor Q1 is represented by I1 = VREF / R1. Since VBE having a negative temperature characteristic does not exist in the equation, and the resistor R1 has a positive temperature characteristic, I1 indicates a negative temperature characteristic.

Since the second constant current circuit has the same configuration as the conventional constant current circuit, the reference voltage VR is applied across the resistor R2.
A voltage obtained by subtracting VBE of the transistor Q2 from EF is applied. Therefore, the collector current I2 of the transistor Q2 is represented by I2 = (VREF-VBE) / R2. Compared to VBE with strong negative temperature characteristics,
Since the resistor R2 wants to have a relatively large collector current I2, the resistance value cannot be set so large that a sufficiently positive temperature characteristic cannot be obtained. As a result, the collector current I2
Would shows the positive temperature characteristic.

FIG. 4 is a characteristic diagram for explaining the temperature correction of the constant current circuit of the present invention. The collector current I1 of the transistor Q1 of the first constant current circuit shows a negative temperature characteristic that decreases as the temperature rises. Conversely, the collector current I2 of the transistor Q2 of the second constant current circuit exhibits a positive temperature characteristic that increases with an increase in temperature due to the effect of the negative temperature characteristic of VBE. However, in the constant current circuit of the present invention in which the two are connected in parallel, the total current I0 is offset by the opposite temperature characteristics of I1 and I2, and the temperature correction is performed even in a region where the set current is as large as several tens of μΑ. Can be realized.

[0017]

As described above, according to the present invention, it is possible to realize a constant current circuit capable of performing sufficient temperature correction without depending on the positive temperature characteristic of the resistor connected to the emitter. This greatly expands the freedom of current circuit design.

[Brief description of the drawings]

FIG. 1 is a circuit diagram illustrating a constant current circuit according to the present invention.

FIG. 2 is a characteristic diagram illustrating the operation principle of the constant current circuit of the present invention.

FIG. 3 is a circuit diagram illustrating a conventional constant current circuit.

FIG. 4 is a characteristic diagram illustrating an operation principle of a conventional constant current circuit.

Claims (3)

[Claims] 1. A first constant current circuit comprising a first transistor and a first resistor connected to an emitter of the first transistor,
A second constant current circuit including a second transistor and a second resistor connected to an emitter of the second transistor, applying a reference voltage to a first resistor, and applying a reference voltage to the second resistor from the reference voltage; The base-emitter voltage VBE of the second transistor
The first constant current circuit has a negative temperature characteristic, the second constant current circuit has a positive temperature characteristic, and the two are connected in parallel. A constant current circuit characterized by performing correction. 2. The method according to claim 1, wherein a base of the first transistor is provided at a base thereof.
2. The method according to claim 1, wherein a voltage between the base and the emitter of the third transistor is added to the reference voltage and applied, and the reference voltage is applied to a resistor connected to the emitter of the second transistor. Constant current circuit. 3. A first constant current circuit comprising a first transistor, a first resistor having one end connected to its emitter and the other end grounded, and a first constant current circuit having a base applied with a reference voltage. A second constant current circuit comprising two transistors, a second resistor having one end connected to the emitter and the other end grounded, and an emitter connected to the base of the first transistor, and a base connected to the base. A third transistor to which the reference voltage is applied, wherein the first constant current circuit has a negative temperature characteristic, and the second constant current circuit has a positive temperature characteristic, and the two are connected in parallel. A constant current circuit characterized by connecting and performing temperature correction.