Constant Current Regulating Circuit

Abstract

Circuitry for providing a constant current to a load includes a source of current and a regulating circuit interposed between the current source and the load. The regulating circuit includes a current shunting transistor coupled to the output of the current source and a feedback transistor also coupled to the output of the current source and to the base of the current shunting transistor. A diode connected between the current shunting transistor and the current source is chosen to have a dynamic admittance substantially equal to the transconductance of the current shunting transistor to provide the latter with unity voltage gain. A variation in current output of the source is accommodated by the current shunting transistor so that a substantially constant voltage at the output thereof is maintained. An output circuit including a transistor, coupled to the output of the current shunting transistor and to the load, and controlled by the output voltage, provides a substantially constant current to the load despite current variations from the current source.

Description

BACKGROUND

This invention relates generally to circuitry for providing a constant current output and more particularly to circuitry for regulating such constant current producing circuitry.

Often when it is required to provide a constant reference or the like current to a load, such as, for example, an operational amplifier, the current from the constant current source varies, sometimes because of a variation in the voltage powering the constant current source. When the latter occurs, the operation of the load can be greatly affected. This is especially true when the current produced is being used for biasing circuitry in the operational amplifier.

Regulating circuits which attempt to maintain the current to the load constant are known in the prior art. These circuits, however, are often sensitive to changes in the constant current source and therefore introduce such things as poor power supply sensitivity which in turn affects the current to the load.

SUMMARY

Accordingly, it is an object of the present invention to provide new and improved circuitry which maintains a constant current output for a load.

It is another object of the present invention to provide new and improved circuitry for regulating the current output from a "constant current source" to maintain the current substantially constant to a load.

It is still another object of the present invention to provide circuitry of the above described type which is easily adaptable to integrated circuit techniques.

Briefly, a preferred embodiment of the constant current regulating circuitry according to the invention comprises a first transistor coupled at the collector thereof to the cathode of a diode and a feedback arrangement including a second transistor, the base of which is coupled to the anode of the diode. The emitter of the second transistor is coupled to the base of the first-mentioned transistor. The dynamic admittance of the diode is substantially equal to the transconductance of the first transistor so that the voltage amplification of the latter is one. The output electrode (collector) of the first-mentioned transistor is coupled to the base of a third transistor which provides the regulated, constant reference current to a load.

A known "constant current source," such as, for example, a field effect transistor (FET) appropriately connected, is coupled to the anode of the diode and base of the second (feedback) transistor. Any variation in the current through the FET, due to variations in the power supply powering the latter or due to fabrication variations which may cause the FET current to vary from unit to unit, are compensated for by the action of the current regulating circuitry. In operation, the first-mentioned transistor shunts varying amounts of current, and maintains a substantially constant voltage at the output thereof, to in turn maintain a substantially constant current flow through the third transistor to the load.

DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a schematic diagram of a preferred embodiment of the constant current regulating circuitry according to the invention.

DETAILED DESCRIPTION

Referring now to the single FIGURE of the drawing in greater detail, there is illustrated therein current regulating circuitry according to the invention, designated generally by the numeral 10 and outlined in dotted lines. The current regulating circuitry 10 is coupled at the input 12 thereof to the output 13 of a commonly used "constant current source" 14, herein taking the form of a field effect transistor (FET) connected to produce a constant current output. The FET is powered from a first voltage source or power supply (not shown) connected at point 16. While a "constant current source" has been illustrated herein and is preferred, the current regulating circuit 10 according to the invention is operable also wherein the FET is replaced by a resistor or like component.

The output 18 of the current regulating circuit 10 is coupled via lead 22 to a load 20. The load is also connected to a voltage source at point 24, normally different than that powering the FET.

The current regulating circuitry 10 includes a diode 26, the anode or input electrode 28 of which is coupled to the constant current source (FET) 14. The cathode 30 of the diode is connected to the collector 32 of a transistor 34. A second transistor 36 forming a feedback stage, is connected at the base 38 thereof to the output 13 of the FET circuit source and to the anode of diode 26. The collector 40 of transistor 36 is connected via lead 39 to the junction 16 and the power supply (not shown) supplying the constant current source 14. The emitter 42 of transistor 36 is connected over lead 44 to the base 46 of transistor 34, as well as to ground through a resistor 48. The emitter 50 of transistor 34 is grounded as well.

An output stage 52 of the current regulating circuit includes a transistor 54, the base 56 of which is connected to the output (collector) 32 of transistor 34. The collector or output electrode 58 of the transistor 54 is connected to the load 20 at junction 18 and the emitter 60 of the transistor 54 is connected to ground through a resistor 62.

The dynamic admittance of the diode 26 is substantially equal to the transconductance of transistor 34. It should be noted that the gain of a transistor is equal to the transconductance thereof multiplied by the load resistance or the transconductance divided by the admittance of the load. The transistor thus exhibits a voltage amplification of one or unity gain when the admittance of the diode is equal to the transconductance of the transistor. In operation, ragardless of the current flowing from the current source FET 14, the voltage supplied to the base of transistor 54 is maintained substantially constant.

Transistor 36, in the feedback path, adjusts the current shunted by transistor 34 thereby maintaining the voltage at junction 64 substantially constant regardless of changes in the supply voltage or variations in the FET. The voltage at point 64, however, may vary with temperature and/or resistor variations. This variation in voltage at junction 64 due to temperature, however, compensates for changes in the base-emitter voltage of transistor 54, thus producing a constant current at collector 58 thereof.

For explanatory purposes, it will be assumed that load 20 comprises circuitry in an operational amplifier, requiring biasing with a constant reference current. With the regulating circuitry 10 of the invention, the latter is insured. Any variation in the current derived from the constant current source 14 is compensated for by the regulating circuitry, as described, to maintain the current at lead 22 substantially constant.

For example, assuming the current emanating from source 14 increases, the latter flows through diode 26 as well as through base 38 of transistor 36. The increased current in base 38 causes an increase in the current flow through emitter 42 of transistor 36. The last-mentioned current increase also increases the current flow through resistor 48 and base 46 of transistor 34. This increased base current causes an increase in the collector current of transistor 34, thereby shunting excess current through diode 26 to ground and thus maintaining the voltage to the base of transistor 54 substantially constant.

Looking at the above from a voltage consideration, an increase in current from the FET 14 causes a corresponding voltage increase at junction 66. The latter however, due to the feedback transistor 36, also causes the voltage at the base of transistor 34 to increase by a like amount. Since transistor 34 has unity gain, the voltage at the collector thereof will be reduced by the increase in voltage at the base to maintain the voltage at junction 64 constant, thus not affecting the current flow to the load 20. In addition, the regulating circuitry 10 which includes only three transistors and a diode, can be easily fabricated as an integrated circuit.

While a particular embodiment of the invention has been shown and described, it should be understood that the invention is not limited thereto since many modifications may be made. It is therefore contemplated to cover by the present application any and all such modifications as fall within the true spirit and scope of the appended claims.

Claims

We claim:

1. Circuitry for regulating the output current of a current source to provide a substantially constant current to a load, the current source being connected to a reference potential, the improvement comprising:

a. a junction point;

b. a P-N semiconductive junction device, connected to the current source, and oriented to pass current to the junction point;

c. a current shunting semiconductive device having a first main electrode connected to the junction point, a second main electrode connected to a common reference point, and a control electrode, the dynamic electrical characteristics of the current shunting semiconductor device and the P-N junction semiconductive device being equal for providing the former with unity gain;

d. feedback circuit means connected to the connection between the current source and the P-N junction semiconductive device and the control electrode of the current shunting semiconductive device for providing the latter with signals representative of changes in current supplied by the current source for maintaining the junction point at a constant voltage; and

e. output circuit means connected to the load and to the junction point, responsive to the voltage at the junction point, for providing constant current to the load.

2. The circuit of claim 1 wherein the feedback circuit means further comprise a semiconductive device having a first main electrode connected to the reference potential, a control electrode connected to the connection between the current source and the semiconductive junction device, and a second main electrode connected to the control electrode of the current shunting semiconductive device.

3. The circuit of claim 2 wherein the output circuit means further comprises a semiconductive device having a first main electrode connected to the load, a second main electrode electrically coupled to the common reference point and a control electrode electrically connected to the junction point.

4. The regulating circuitry of claim 1 wherein the current shunting semiconductive device is a transistor, the collector and emitter thereof being the first and second main electrodes and the base being the control electrode, and wherein the P-N junction semiconductive device is a diode whose dynamic admittance is substantially equal to the transconductance of the current shunting transistor.

5. The regulating circuitry of claim 2 wherein the feedback semiconductive device is a transistor whose collector and emitter are the first and second main electrodes and whose base is the control electrode.

6. The regulating circuitry of claim 3 wherein the output circuit semiconductive device is a transistor whose collector and emitter are the first and second main electrodes and whose base is the control electrode.

7. The regulating circuitry of claim 4 wherein the feedback semiconductive device is a transistor whose collector and emitter are the first and second main electrodes and whose base is the control electrode.

8. The regulating circuitry of claim 5 wherein the output circuit semiconductive device is a transistor whose collector and emitter are the first and second main electrodes and whose base is the control electrode.

9. Circuitry of the type claimed in claim 8 wherein said constant current source comprises a field effect transistor connected to provide a relatively constant current output.