JPS59191627A - Constant current circuit - Google Patents

Abstract

PURPOSE:To obtain a constant current source which has no variation of the set current despite the variation of the threshold voltage of a constant current generating transistor TR, by using a voltage source which varies in response to the variation of the threshold voltage of said TR. CONSTITUTION:A voltage dividing circuit consisting of transistor FET12-15 is provided between a power supply VDD10 and a power supply VSS11. The output voltage 21 of the voltage dividing circuit is supplied to a constant current generating FET20. Then the voltage 21 varies in response to the variation of the threshold voltage of the FET20. Thus it is possible to obtain a constant current source which has no variation of the set current despite the variation of the threshold voltage of the TR.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an integrated constant current circuit, and particularly to a constant current circuit that allows fluctuations in the threshold voltage of a transistor.

[Background of the invention]

As shown in Fig. 1, a conventional constant current circuit uses diode-connected complementary conductivity type PMO8 and NMO8 transistors (transistors with gates and drains connected) 12 and 13 as resistors, and connects the power supply DD10 and the power supply VBI.
Construct a voltage divider circuit for +11, and output 2 of this voltage divider circuit.
1 is applied to a constant current generating transistor 20 to generate a constant current at its drain, which is the output 22 of the constant current circuit.

Furthermore, as another example of a conventional constant current circuit, FIG. 2 shows a constant current circuit suitable for generating a negative current in an active load type differential amplifier circuit. On the other hand, ■Transistor 12 on the DD10 side and voltage divider circuit output 2
A diode-connected transistor with a channel of the same polarity as the transistor 13 on the V 1111 side is inserted between the transistors 1 and 1. This circuit is a constant current circuit in which the constant current value increases or decreases in response to variations in element characteristics due to variations in chip manufacturing so that the operating point of the differential amplifier circuit becomes stable. Therefore, this circuit is not suitable for applications where fluctuations in the set current must be suppressed even if the element characteristics change.

In the case of the constant current circuit shown in FIG. 1 as well, since such applications are not particularly taken into consideration, there is a drawback that the constant current value fluctuates due to fluctuations in element characteristics, particularly fluctuations in threshold voltage. In the circuit shown in Figure 1, the threshold voltage of the NMO8) transistor is VTR, and its fluctuation is ΔVTN.

PMO8) The transistor threshold voltage is VTP, its fluctuation is ΔVTP%, and the power supply voltage is Van (Vs s = 0
), then the output current I. The variation in ut is ΔI out, and when the power supply voltage is low, the rate of variation in the output current is large.

[Purpose of the invention]

An object of the present invention is to provide a constant current source whose set current does not change even if the threshold voltage of a transistor changes.

[Summary of the invention]

In order to achieve the above object, the present invention improves a voltage divider circuit so that the output voltage of the voltage divider circuit changes in response to changes in the threshold voltage of a constant current generating transistor.

[Embodiments of the invention]

Hereinafter, the present invention will be explained in detail with reference to Examples. Third
The figure is a circuit diagram of an embodiment of a constant current circuit according to the present invention.

Transistors 12, 13, 14, and 15 are a voltage divider circuit, and a diode-connected PMO8) transistor 15 is inserted between transistors 13 and 14 of the voltage divider circuit in FIG.
The connection point between transistors 13 and 15 is a divided voltage V. ut3
The voltage dividing circuit output 21 outputs the voltage. The transistor 20 is an element for generating a set current from this divided voltage. Transistor 23 is transistor 2
This element is used to bias the terminal 22, which has a drain of 0, to a sufficiently high voltage so that the set current generated by the transistor 200 does not depend on the voltage at the terminal 22. Terminal 2 is connected to terminal 2 by a current mirror circuit formed by transistors 23 and 24.
5 to obtain a constant current supply output equal to the set current. In addition, if a constant current sink source is required, the transistor 23.26
A constant current sinking output equal to the set current can be obtained at the terminal 29 by the current mirror circuit according to 27.28. Note that when the voltage of the load is sufficiently high, the terminal 22 can be used as a direct current sink output.

The feature of the present invention lies in the configuration of the voltage dividing circuit. With this configuration, it is possible to design the output voltage of the voltage dividing circuit to vary in accordance with variations in the threshold voltage of the NMO 8 and not to vary in response to variations in the threshold voltage of the PMOS.

For this purpose, the output voltage V of the voltage divider circuit is required. The transistor size may be determined so that ut3 is equal to the sum of 1/2 of the power supply voltage vDD and the threshold voltage ■τN of NMO8, as shown in equation (2). (For simplicity, calculations are made with V fill as Ov - NMO8) If the drain terminal 22 of transistor 20 is biased to a sufficiently high voltage, the output current of transistor 20 . ut is determined by the gate-source voltage Van and the threshold voltage vTN. If the mobility of the transistor 20 is β20% and the size is (W/L)20, then the output current is: ut is......(3). In the configuration of this embodiment, the gate-source voltage is
. Since it is equal to ut3, the output current is. That is, the output current does not depend on the threshold voltages of NMO8 and PMO8.

Therefore, even if the threshold voltage value fluctuates, the output current does not change.

Now, it will be explained that if the transistor size of the voltage dividing circuit is designed to satisfy the relationship shown below, the output voltage can be made to satisfy the expression (2).

If the drain current of a diode-connected MOS transistor is If, the gate-source voltage Voε1 of the transistor is expressed by equation (5).

Here, VT is the threshold voltage, β is the mobility, and (W/L) is the size. Therefore, the output voltage of the voltage divider circuit V out
3 and the power supply voltage DD are represented by the current Il flowing through the voltage dividing circuit as follows. (6) (7) Here, the subscript corresponds to the transistor number. For simplicity's sake, if we eliminate the current I+ from equations (6) and (7), we get
・・・・・・・・・・・・・・・(9) Therefore, if the transistor size is designed so that K 12 = 13 + 14 + 15, the sum of the net gate-source voltages excluding the threshold voltage (Van
If the transistor size of the voltage dividing circuit is designed so as to divide 2VTN 2P) into two equal parts, the output voltage will be as follows, and equation (2) can be satisfied.

Note that the PM of the lower active resistance in this voltage dividing circuit is
Although the order of connection of O8 and NMO8 may be arbitrary, it is preferable to connect PMO8 at the top as in this embodiment. P
This is because the influence of the substrate effect of MO8 can be reduced. Further, the influence of the substrate effect of the NMO 8 can be eliminated by connecting the well and the source as shown in the transistor 14 (9).

〔Effect of the invention〕

According to the simulation, in the circuit shown in Fig. 3, using a 5-inch power supply and setting the transistor threshold voltage VTR to IV, VTH
'c If you change 0.2 V, the fluctuation of the output current value will be 0.
．． It was 3 chi. The variation in the circuit of FIG. 1 is 26%, which is a significant improvement.

Note that the present invention is to invert the polarity of a current mirror circuit by using the drain of NMO8) transistor 13 on the Vgall side as a voltage dividing circuit output, and to create a complementary MO using an N-well.
It is clear that variations such as using the S process are included. Further, another diode-connected transistor (it may be plural) is added to the transistors 14 and 15, and another diode-connected transistor (it may be plural) is added to the transistor 12. is also possible.

As explained in detail above, according to the present invention, a constant current source that does not depend on threshold voltage fluctuations can be realized (10).

[Brief explanation of drawings]

1 and 2 are examples of conventional constant current circuits, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. 20...Transistor for constant current generation, 12, 13゜1
4.15...Transistor constituting a voltage dividing circuit, 21
... Output terminal of voltage divider circuit, 23.24 ... Current mirror circuit constituting constant current supply source, 23°26.27
．． 28...Current mirror circuit that constitutes a constant current sinking source. Agent Patent attorney Akio Takahashi (11) ■ 1 Figure Atsushi 2 Kyōγ 3 Figure Iθ ss

Claims (1)

[Scope of Claims] 1. In a constant current circuit consisting of a constant current generating transistor and a voltage source for driving the transistor, a voltage source whose generated voltage fluctuates in response to threshold voltage fluctuations of the constant current generating transistor is provided. A constant current circuit characterized in that it is used. 2. A combination in which a first conductivity type transistor is used as a constant current generating transistor, and two diode-connected first conductivity type transistors and one diode-connected second conductivity type transistor are connected in series as a voltage source for driving the transistor. A constant current circuit using a voltage divider circuit with an active resistor and one active resistor of a second conductivity type transistor connected to a diode. 3. The constant current generating circuit according to item 2, wherein the output voltage of the voltage dividing circuit is approximately equal to the sum of half the power supply voltage and the threshold voltage of the first conductivity type transistor. 4. A first conductivity type transistor is used as a constant current generating transistor, and two or more diode-connected first conductivity type transistors and one or more diode-connected second conductivity type transistors are used as a voltage source for driving the transistor. This is a voltage divider circuit using an active resistor in which a composite active resistor is connected in series with a second conductivity type transistor connected in series with one or more diodes, and the output voltage of the voltage divider circuit is almost a component obtained by dividing the voltage of the power supply. A constant current circuit characterized by using a voltage divider circuit consisting only of a threshold value component of a first conductivity type transistor.