JPS6323567B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a constant current circuit.

2. Description of the Related Art Conventionally, known constant current circuits include those that obtain a constant current by using a constant forward voltage of a diode and those that obtain a constant current by using a constant voltage of a Zener diode.

The former constant current circuit using a diode has a drawback of high temperature dependence due to the temperature coefficient of the diode.

The latter, a constant voltage circuit using a Zener diode, combines a Zener diode with a positive temperature coefficient and a diode with a negative temperature coefficient.
Although the respective temperature coefficients can be canceled out,
The Zener diode has a high on-voltage and a lower operating limit voltage of about 5V, which has the disadvantage of poor voltage reduction characteristics and the generation of Zener noise.

This invention was made in order to provide a constant current circuit with improved voltage reduction characteristics and temperature characteristics.

Further, the present invention has been made to provide a constant current circuit in which the temperature coefficient can be arbitrarily set within a certain range.

This invention provides a constant voltage circuit composed of a plurality of series diodes (or diode-connected transistors, the same applies hereinafter) and a load means, and a voltage dividing resistor is provided in the diodes connected to the load side to provide different constant voltages. This is applied to the bases of differential pair transistors each having a common emitter with a resistor, and the output is obtained from the collectors of the transistors biased to a relatively low degree of conductivity.

Hereinafter, the present invention will be specifically explained with reference to Examples.

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

Voltage dividing resistors R ₃ and _{R 4} are provided in parallel with the diode Q ₁ provided on the load side of a constant voltage circuit configured with a resistor R 1 as a load and series forward diodes Q ₁ to _{Q 3 ,} This forward constant voltage V _F is 1/
A voltage V _F /n divided by n is formed.

In addition, differential transistors Q ₄ and Q ₅ are provided with a resistor R ₂ on the common emitter, and a constant voltage formed by the above constant voltage circuit is connected to the base of transistor Q ₄ .
2V _F is applied to the base of transistor Q ₅ , and a constant voltage 2V _F +V _F / formed by the above voltage dividing resistors R ₃ and R ₄ is applied to the base of transistor Q 5.
Apply n.

Among these differential transistors Q ₄ and Q ₅ , the transistor Q ₄ biased to a relatively low degree of conductivity
The collector current of the other transistor Q5 is used as the output constant current, and the collector of the other transistor _Q5 is connected to the power supply terminal _Vcc .

In this circuit, a constant suction current is obtained, and in order to obtain a constant extrusion current, a diode as a constant voltage element is provided on the power supply voltage V _cc side, as shown in Fig. 2, and a dynamic pair transistor
Q ₄ and Q ₅ may be configured with pnp transistors.

Furthermore, by adding a resistor R5, the constant voltage applied to the bases of the differential pair transistors _Q4 and _Q5 is determined by the ratio of the voltage dividing resistors _R3 to _{R5 ,} mV _F , mV _F +V _F /n may be applied to the bases of transistors Q ₄ and Q ₅ , respectively.

The circuit configured as described above can achieve the purpose for the following reasons.

The current ₀ flowing through the resistor R ₂ provided at the common emitter of the differential transistors Q ₄ and Q ₅ can be determined by the following equation (1).

₀ = (m-1) V _F /R ₂ ... (1) Here, V _F is the forward voltage of the diodes Q ₁ to _{Q 3} and the voltage between the base and emitter of the transistors Q ₄ and Q ₅ , Both can be treated as equal. In addition, in the circuit of FIG. 1, m=
2.

The bases of the differential transistors Q ₄ and Q ₅ are
Since an offset of V _F /n is provided, the ratio / ₀ of the collector current of the transistor _Q4 , which is relatively biased to the off side, and the current ₀ flowing through the resistor _R2 is the same as that of the bipolar differential pair. It can be determined from the following equation (2) from the transfer characteristics of the transistor.

Here, e is the natural logarithm, k is Boltzmann's constant,
T is the absolute temperature and q is the electron charge.

From the above equations (1) and (2), the output current can be calculated using the following equation (3).

It will be understood from equation (3) that the output current is a stable constant current that is unrelated to fluctuations in the power supply voltage _Vcc . Since the circuit of this embodiment uses the forward voltage of the diode, the lower limit voltage for operation can be reduced to about 2.5V, and the voltage reduction characteristics can be improved.

Then, the temperature dependence is determined by the above voltage dividing resistors R ₃ and R ₄
With the ratio n, the temperature characteristics of the species can be obtained as shown in FIG.

Figure 3 shows the ratio n due to the voltage dividing resistor as 20, 30,
This is a plot of equation (3) with 50 and ∞.

In Figure 3, the horizontal axis indicates temperature (°C),
The vertical axis represents the current at 25° C. as 1.

In equation (3), at room temperature around 27℃, q/kT=1/
Since 26mV and V _F = 0.7V, for example, if n = 30, q・V _F /kT・n=0.7/0.026×30=
Since 0.7/0.78≒1, the temperature characteristics can be extremely small in the range of room temperature from 25℃ to 100℃, and if the value of n is increased from this value, it will have a negative temperature coefficient, and conversely, if it is decreased, it will have a positive temperature coefficient. Become something.

In addition to obtaining a constant current with a small temperature coefficient by setting n to approximately 30 as described above, the ability to obtain positive or negative temperature characteristics is useful in the following cases.

For example, when forming a voltage signal by flowing the above-mentioned constant current, if the temperature coefficient of the resistor is positive, a current with a negative temperature coefficient that cancels this is used, and conversely, the temperature coefficient of the resistor is When is negative, a stable voltage signal can be obtained by using a current with a positive temperature coefficient that offsets this.

This invention can be widely used as a constant current circuit.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing one embodiment of the present invention, and FIG. 3 is a temperature characteristic diagram thereof.

Claims (1)

[Claims] 1. A constant voltage circuit constituted by a plurality of series diodes having one end connected to one power supply voltage terminal and the other end connected to a load means, and a diode provided at the other end. It is equipped with a voltage dividing resistor provided in parallel and a differential pair transistor having a common emitter resistor on one of the power supply voltage sides. A voltage is applied, an output constant current is obtained from the collector of the transistor biased to a relatively low degree of conductivity among the differential pair transistors, and the voltage is divided by the voltage dividing ratio of the voltage difference between the different voltages by the voltage dividing resistor. A constant current circuit characterized by set temperature characteristics. 2. The constant current circuit according to claim 1, wherein the voltage dividing ratio of the differential voltage between the different voltages by the voltage dividing resistor is set to approximately 30.