US3659121A

US3659121A - Constant current source

Info

Publication number: US3659121A
Application number: US89538A
Authority: US
Inventors: Thomas M Frederiksen
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1970-11-16
Filing date: 1970-11-16
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25

Abstract

An improved constant current source, which may provide very small current without requiring the construction of a large resistor, is disclosed. This constant current source includes a transistor having a bias source comprising a second transistor that is biased to low current conduction, the biasing resistor for the biased transistor being advantageously small.

Description

United States Patent Frederiksen 51 Apr. 25, 1972 [541 CONSTANT CURRENT SOURCE [72] Inventor: Thomas M. Frederiksen, Scottsdale, Ariz.

[73] Assignee: Motorola, Inc., Franklin Park, Ill.

[22] Filed: Nov. 16, 1970 [21] App]. No.: 89,538

[52] US. Cl ..307/297, 307/296, 330/22 [51] Int. Cl. 1 t i ..H03k 1/14 [58] Field of Search ..307/296, 297, 230; 330/22 [56] References Cited UNITED STATES PATENTS 3,534,245 10/1970 Limberg ..307/297 2,822,434 2/1958 Ehret ..330/22 3,364,434 1/1968 Widlar.. 3,536,986 10/1970 Perlman ..307/297 Primary Examiner-Donald D. Forrer Assistant ExaminerHarold A. Dixon Attorney-Mueller and Aichele [5 7] ABSTRACT An improved constant current source, which may provide very small current without requiring the construction of a large resistor, is disclosed. This constant current source includes a transistor having a bias source comprising a second transistor that is biased to low current conduction, the biasing resistor for the biased transistor being advantageously small.

I 3 Claims, 3 Drawing Figures PATENTEDAPRZS I872 3,659,121

INVENTOR.

Thomas M. Frederf/rsen CONSTANT CURRENT SOURCE BACKGROUND Constant current sources are known in which a transistorized constant current source draws a small current from a supply source and which includes an emitter resistor. However, the emitter resistor is so large where current is quite small as to take up a large portion of the surface area of a chip on which the constant current source is applied that would otherwise be used for other circuit elements. Attempts have been made to provide the high emitter resistor by pinch resistor methods, however it is difficult to make a high resistor by pinch resistor methods with the degree of accuracynecessary to cause production of the desired small value of constant current.

It is an object of this invention to provide an improved constant current source which provides or draws a small current.

It is a further object of this invention to provide a'constant,

current source which does not require ahigh or largevalued resistor.

SUMMARY In accordance with this invention, the constant current source comprises a transistor whose base iskept at a bias value such that the transistor draws current through a load circuitin series with the collector and emitter circuit of the transistors that is constant and quite small. The bias means includes another transistor and a resistor, however the resistor, since the current flowing therethrough is relatively large, canbe small compared to the emitter resistor used in known constant current circuits, and therefore the required resistor may be' easy to put on a chip and require less roomthan'the required high value resistor of known constant .current. sources. Ac.- cording to another feature of this invention, a second constant current source can be added to the low current constant cur.- rent source, the second constant current source however drawing higher current than the low constant current source.

DESCRIPTION The invention will be better understood upon reading'the following description in connection with the accompanying drawings in which:

FIG. 1 illustrates a known constant current source;

FIG. 2 illustrates a circuit embodying the constant-current source of this invention; and

FIG. 3 illustrates two constant current sources, one drawing, greater current than the other.

Turning to FIG. 1, the base and collector of an NPN transistor are connected directly together and, through aresistor 14, to the terminal 12 to which the positive terminal of a voltage regulated source of supply current (not shown) is to be connected. The transistor 10 therefore becomes a'diode whose anode is connected not only to the terminal 1 2:by way of the resistor 14, but also directly to the base of an NPN transistor 16. The cathode of the diode 10 is connected to a point of reference potential such as ground 18. The collector of the transistor 16 is connected by way of a load 20.to the supply terminal 12 and the emitter of the transistor 16 is connected by way of emitter resistor 22 to ground 18. In this known circuit, the voltage between the base and ground of the transistor 16 is fixed at the forward voltage drop of the diode 10. The voltage across the base to emitter of the transistor 16 is reduced by the voltage drop across the resistor 22 which is equal to the value of the resistor 22 multiplied by the current flow therethrough. Therefore, the current flow into the collector of the transistor 16 and therefore through the load 20 is constant and smaller in magnitude than the reference current flow through resistor 14. If this constant current flow through the resistor 22 is to be low, its resistance value must be high. In

fact, for very small constant current flow, the resistor 22 must be so high that it takes up a large part of a chip on which the constant current source is applied, decreasing the amount of circuit that can be put on that chip. If the resistor 22 is made by the known pinch resistor method, the area of the chip taken up by the resistor is small but since the accuracy of resistors made by the pinch method is low, the current flow through the load 20 may be quite different from the value thereof which is required. The circuit of FIG. 2, which embodies a constant current source of this invention does not require a large resistor.

As shown in FIG. 2, the base and collector of an NPN transistor 24are connected together by way of a resistor 26, the baseof the transistor 24 also being connected to the supplytenninal 28-by way of a resistor 30. The emitter of the transistor. 24 is connected to the reference point 32, which may be ground. The collector of the transistor 24 is connected to the base of an NPN transistor 34 whose emitter is connectedto the ground 32 and whose collector is connected to the terminal 28 by way of a load 36. In the circuit of FIG. 2, the resistor 30 is of a size to establish the current flow therethrough,,the voltage applied at the terminal 28 again being regulated. Some of the current through the resistor 30 flows into the base of the transistor 24 but most thereof flows through the resistor 26 and the collector to emitter path of the transistor 24. By choice of the size of the resistor 26, the voltage applied to the base of thetransistor 34 is adjusted to the value whereby the collector current drawn by the transistor 34 may be very small and yet the resistor 26 may be of a size that is easily made accurately and which takes up little chip area. Therefore the current through the load 36 may be very small without constructing a large resistor.

FIG. 3 closely. resembles FIG. 2 wherein similar elements, similarly connected, are given the same reference characters in these two Figures. FIG. 3 differs from FIG. 2 in that a resistor 38 is included between the resistor 30 and the base of the transistor 24, the junction of the

resistors

30 and 38 being connected to the base of an NPN transistor 40. The collector of the transistor 40 is connected to the terminal 28 by way of a load 42. The emitter of the transistor 40 is connected to ground 32 by way of a resistor 44. In operation, the current through the load 36 is kept low in the manner described in connection'withFIG. 2. The current through the load 42 is constant and depends on the ratio of resistor 38 to the resistor 44. By changing the value ofthis ratio, the current flowing through the load 42 and into the collector of the transistor 40 may be larger or smallerthan the current flowing into the collector. of the transistor 24. This is due to the fact that the voltage drop across the resistor 44 must be essentially the same as the voltage drop across the resistor 38, whereby relative increase or decrease in the size of the resistor 44 causes the current flow in the load 42 to go down or up. Therefore, in FIG. 3, two constant current sources, one of which supplies a small current and the other ofwhich supplies a relatively large current, are shown.

While all the transistors shown are of the NPN type, PNP transistors may be used, using appropriate connection and polarity of the source. While the resistor 30 is shown by its ordinary symbol, it may actually take the form of any means for providing a reference current.

What is claimed is:

1. A source for a load requiring a low constant current comprising:

a first transistor having base, emitter and collector electrodes, the collector and emitter of the transistor being connected in series with the load across a supply source;

biasing source means connected to the base electrode of the first transistor for maintaining said transistor in a constant low current conduction stage, said biasing source means comprising a second transistor having base, emitter and collector electrodes; and

means for maintaining said second transistor in a high current conduction state comprising first and second resistors connected in series with the collector and emitter of the second transistor across the supply source, the base of the second transistor being connected directly to the collector thereof through said second resistor.

2. A current source as recited in claim 1 wherein the emitter of the first transistor and the emitter of the second transistor are connected to a common terminal of the supply source.

3. A constant current source comprising: a first and a second resistor; first and second transistors each having main electrodes and a control electrode;

means for connecting said resistors in series between the main electrode of said first transistor and a first supply terminal;

means for connecting a point between said resistors to the control electrode of said first transistor;

means for connecting the other main electrode of said first transistor to another supply terminal;

means for connecting a load which requires constant current between said first supply terminal and a first main electrode of said second transistor;

means for connecting a control electrode of said second transistor to the first main electrode of said first transistor;

means for connecting the other main electrode of said second transistor to the other supply terminal;

a third transistor having main electrodes and a control electrode;

a third and a fourth resistor; means for connecting a second load requiring constant current flow between said first terminal and the first main

Claims

1. A source for a load requiring a low constant current comprising: a first transistor having base, emitter and collector electrodes, the collector and emitter of the transistor being connected in series with the load across a supply source; biasing source means connected to the base electrode of the first transistor for maintaining said transistor in a constant low current conduction stage, said biasing source means comprising a second transistor having base, emitter and collector electrodes; and means for maintaining said second transistor in a high current conduction state comprising first and second resistors connected in series with the collector and emitter of the second transistor across the supply source, the base of the second transistor being connected directly to the collector thereof through said second resistor.

3. A constant current source comprising: a first and a second resistor; first and second transistors each having main electrodes and a control electrode; means for connecting said resistors in series between the main electrode of said first transistor and a first supply terminal; means for connecting a point between said resistors to the control electrode of said first transistor; means for connecting the other main electrode of said first transistor to another supply terminal; means for connecting a load which requires constant current between said first supply terminal and a first main electrode of said second transistor; means for connecting a control electrode of said second transistor to the first main electrode of said first transistor; means for connecting the other main electrode of said second transistor to the other supply terminal; a third transistor having main electrodes and a control electrode; a third and a fourth resistor; means for connecting a second load requiring constant current flow between said first terminal and the first main electrode of said third transistor; means for connecting the control electrode of said third transistor through said first resistor to said first supply terminal and through said third and second resistors to the first main electrode of said first transistor; and means to connect the second main electrode of said third transistor by way of said fourth resistor to said second supply terminal.