JPS624737B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a constant current circuit which attempts to obtain a small current with little fluctuation without using a large resistance.

FIGS. 1 and 2 show conventional constant current circuits for obtaining minute currents.

In FIG. 1, 1 and 2 are NPN transistors, and 3 is a resistor _R1 . In this circuit, when the bias current applied to transistor 1 is I _B and the collector current of transistor 2, which is a small output current, is I _p , the following equation holds true.

R ₁ I _p =kT/qlnI _B /I _p (1) where k: Boltzmann's constant T: absolute temperature q: electronic charge In the circuit shown in Figure 1, let's obtain a small collector current I _p If so, a large resistance value is required for resistor _R1 . For example, bias current I _B =100μA
If we try to obtain a collector current I _p of 1 μA, the resistor R ₁ 3 needs to be 120 kΩ from equation (1).
Generally, in semiconductor integrated circuits, it is easy to obtain a resistance of several kilohms, and if a resistance of as much as 120 kilohms is to be obtained, the chip area will increase.

In FIG. 2, 4 and 5 are NPN transistors, and 6 is a resistor R ₂ connected to the collector of transistor 4. In the circuit of FIG. 2, the following equation holds between the bias current I _B applied to the collector of the transistor 4 and the collector current I _p of the transistor 5, which is a very small current.

I _p =I _B exp (-qR ₂ /kTI _B ) (2) In this circuit, the collector current I _p changes exponentially with respect to the change in the bias current I _B. Therefore, under the condition for obtaining a small collector current I _p , that is, when qR ₁ /kTI _B ≫1, the collector current I _p changes greatly in response to a change in the bias current I _B. For example, bias current I
When _B = 100 μA, in order to obtain a collector current I _p of 1 μA, the resistance R ₂ =1.2KΩ, but under this condition, the bias current I _B increases by 20%, that is, I _B =
When the collector current is 120 μA, the collector current I _p is 0.47 μA, which is a decrease of about 50%, and the collector current has poor stability against changes in the bias current.

This invention was made to eliminate these conventional drawbacks, and aims to provide a highly stable minute constant current circuit without using a large resistor.

The present invention will be explained below using an embodiment shown in FIG. In the figure, 11 is a first transistor, 12 is a second transistor, 13 is a third transistor, 14 is a fourth transistor, 15
16 is a transistor constituting the first constant current source section; 17 is a transistor constituting the second constant current source section; 18 is a transistor constituting the first constant current source section;
The resistor R ₁ , 19 represents the second resistor R ₂ .

In the circuit of FIG. 3, the current applied to the collectors of transistor 11 and transistor 13 is I
_B , the emitter current of transistor 12 is I ₂ , the collector current of transistor 13 is I ₃ , the emitter current of transistor 14 is I ₄ , the collector current of transistor 5 which is a minute output current is I _p , and the base current of each transistor is If we ignore , the following formula holds true.

V _BE1 +V _BE2 =R ₁ I ₃ +V _BE4 +V _BE5 ......(3) V _BE1 =kT/qlnI _B -I ₃ /I _S ......(4) V _BE5 =kT/qlnI _p /I _S ... …(5) R ₂ I ₃ =kT/qlnI _B −I ₃ /I ₃ ………(6) Here, V _BE1 , V _BE2 , V _BE4 , and V _BE5 are the bases of transistors 11, 12, 14, and 15, respectively. ,
Represents the emitter voltage.

Furthermore, if the emitter areas of transistors 16 and 17 are equal, their respective emitter currents I ₂ and I ₄
are equal, so V _BE2 = V _BE4 holds true. Therefore, equation (3) becomes as follows.

V _BE1 = R ₁ I ₃ + V _BE5 (3') From equations (3'), (4), and (5), the following equation (7) holds true.

As in the case of FIGS. 1 and 2 shown as conventional examples, the bias current I _B =100 in this embodiment as well.
μA, and find the conditions for collector current I _p = 1 μA, now the collector current of transistor 13 is
If I ₃ = 10 μA, the resistance R ₂ = 5.7K from equations (3) and (5).
Ω, R ₁ = 11.7KΩ. Therefore, the resistance R ₁ ,
R ₂ can form a constant current circuit without requiring a large resistance value, and can also obtain a minute current as an output. Under these conditions, if the bias current I _B increases by 20%, that is, I _B =120 μA, the collector current I _P =0.93 μA, and the collector current decreases by less than 10%, resulting in good stability.

FIG. 4 shows another embodiment of the present invention, in which the same reference numerals as in FIG. 3 indicate the same parts. In FIG. 4, 20a, 20b, 20c, . . . are output transistors, and minute currents I _p1 , I _p2 , I _p3 .
is obtained. 21 is the third resistor R ₃ , 22 is the fourth resistor
If R ₃ =R ₄ , the emitter current I ₂ of the transistor 12 and the collector current I ₃ of _the transistor 13 become approximately I ₂ =I ₃ . The operation is almost the same as the case shown in Fig. 3, and the minute current I _p1 ,
I _p2 and I _p3 are obtained. In the circuit shown in FIG. 4, since the impedance of the emitter of the transistor 14 is low, a large number of output transistors can be connected, and there is no possibility that one of the output transistors 20a, 20b, or 20c will become saturated and its base current will increase. Even if this occurs, it has the effect of having little effect on the collector currents of other output transistors.

As explained above, the constant current circuit of the present invention can obtain a stable minute current without using a large resistance due to its structure, so it can be widely applied to bias current circuits of high impedance circuits, leakage current prevention circuits, etc. It is.

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing conventional constant current circuits, and FIGS. 3 and 4 are circuit diagrams showing embodiments of the present invention. In the figure, 11 is the first transistor, 12
13 is a third transistor, 14 is a fourth transistor, 15 is a fifth transistor, 16 is a first constant current source section, 17 is a second constant current source section, and 18 is a third transistor. 1 resistor, 19 is the second
, 21 is a third resistor, and 22 is a fourth resistor. In the figures, the same reference numerals indicate the same or equivalent parts.

Claims (1)

[Claims] 1 Based on the collector of the first transistor,
a second transistor having the base of the first transistor connected to the emitter; the base being connected to the base of the first transistor; a first resistor being connected between the collector and the collector of the first transistor;
A third transistor with a second resistor connected between the emitters of the transistor, a fourth transistor with its base connected to the collector of the third transistor, and a base connected to the emitter of the fourth transistor and the collector connected to an output terminal. A fifth transistor, a first constant current source section connected to the base of the first transistor, and a second constant current source section connected to the emitter of the fourth transistor. Constant current circuit. 2. The constant current circuit according to claim 1, wherein a third resistor is connected to the first constant current source section, and a fourth resistor is connected to the second constant current source section.