Temperature Stable Constant Current Source

Abstract

An ultrastable high-speed constant DC current source for generating a precise reference voltage in other apparatus such as a high-speed analogue to digital converter. A continuous constant load current is selectively switched between two current paths, one of which comprises an output load across which said reference voltage is developed. A high-speed digitally controlled driver circuit including a differential amplifier configuration controls the flow of the constant current selectively through one of two hot carrier diodes. The diodes serve as electronic switches from the constant current source which comprises an operational amplifier connected in a feedback loop including a Darlington transistor configuration and controlled by an externally applied input reference voltage and an error signal developed by the flow of said load current across a temperature compensated resistor.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a constant or otherwise regulated electric current from a voltage source to a load. The magnitude of the current is unaffected by changes in load or device parameters as a result of temperature change or operating point.

2. Description of the Prior Art

Circuits for supplying a controlled magnitude of electrical current flow from a voltage source to a load are well known to those skilled in the art. One example of such a circuit is disclosed in U.S. Pat. No. 3,114,872, issued to G. A. Allard. This reference discloses a constant current source in which the magnitude of current supplied by the source is responsive to temperature and changes inversely with variations in temperature in the relative amount required by one or more magnetic cores. While the output current of such a circuit is not constant in a strict sense, the combination of the regulating circuit and the voltage source may be considered a constant current source in that the magnitude of the current supplied is determined fully by the parameters of the regulating circuit and is independent of the mode and the magnitude of the voltage of the source. Such a constant current source finds utility in supplying read and write half currents to a coincident current magnetic core memory in which the hysteresis characteristics of the magnetic cores are quite temperature dependent.

Another type of circuit that supplies a constant current to a load is a switching regulator such as disclosed in U.S. Pat. No. 3,350,628, issued to L. E. Gallaher, et al. According to that invention, constant current is supplied to a load from a regulated power supply of the type employing a transistor switch connected between the source and the load. The switch is driven by a Schmitt trigger which is responsive not only to a first feedback signal derived from deviations from a desired value of a current supply to the output filter, but also to a second feedback signal. The second feedback signal is an alternating current signal derived from the switching action of the transistor and applied in a sense to reduce the hysteresis of the trigger circuit.

A regulated power supply circuit which discloses a feedback circuit wherein the error signal is derived solely from deviations of the output from a desired value is furthermore disclosed in U.S. Pat. No. 3,233,915, issued to H. R. Ryerson, et al. The frequency of switching varies in response to the direct current feedback signal to facilitate rapid correction of the output current.

SUMMARY

While the above noted prior art operates in the manner intended, the present invention is directed to a temperature stable constant current source which constantly delivers a predetermined value of current selectively to one of two signal paths by means of a pair of hot carrier diodes which are operated in accordance with a differential amplifier driver circuit controlled by means of a digital input trigger signal. An output load is connected to one of the switching diodes. The current source is connected in series between a supply potential and the pair of switching diodes and comprises a feedback amplifier in which a Darlington transistor circuit is connected in a DC feedback loop to one DC amplifier input. The DC feedback loop supplies a second input which is applied to the DC amplifier and which is a function of the desired constant current supplied to the pair of switching diodes. The second input is developed across a temperature stabilized resistor and any parameter variations in the circuit are effectively nulled out by the Darlington feedback loop.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is an electrical schematic diagram of the preferred embodiment of the subject invention.

Description of the Preferred Embodiment

Referring now to the drawing, the circuit diagram shown is comprised of four circuit portions, namely a current source 10, a load current switch circuit 12, a load current switch driver circuit 14 and a voltage reference source 16. An output load 18 which may be, for example an input device for a binary ladder network of an analogue to digital converter for a digital signal processor, is coupled to the switch circuit 12 and more particularly to one semiconductor diode 20 of a pair of diodes 20 and 22 having their respective anode electrodes directly connected at the circuit junction 24. The load 18 is connected from the opposite or anode electrode of the diode 20 to a point of reference potential illustrated as ground.

The semiconductor diodes, moreover, 20 and 22 are comprised of high-speed switching diodes and are preferably hot carrier diodes generically referred to as Schottky barrier diodes. These devices are extremely fast acting due to their inherent low internal electrode capacitance. Additionally these devices exhibit an extremely low back bias leakage current characteristic, being in the order of 2.times.10.sup.-.sup.9 amperes.

The anode electrode of the second switching diode 22 is connected to the driver circuit 14 and more particularly to the collector electrode of transistor 26 which is additionally coupled to a +12 volt power supply potential through a zener diode 28 and resistor 30. Transistor 26 forms one-half of a differential amplifier circuit additionally including transistors 32, 34, 36 and transistor 38. The collector of transistor 32 is returned to ground. However, the input or base electrode of both transistors 26 and 32 are adapted to have a digital input trigger signal and its complement respectively applied thereto by means of transistors 36 and 38. Zener diode 40 and resistor 41 couple the emitter of transistor 36 to the base of transistor 26 while zener diode 42 and resistor 43 couple the emitter of transistor 38 to the base of transistor 32. The emitter electrodes of transistors 26 and 32 are connected at a common junction 44 which is also connected to the collector of transistor 34. With the exception of transistor 26 all of the transistors included in the driver circuit 14 are powered by means of a -18 volt supply potential coupled to their respective emitter electrodes. Mutually opposite polarity trigger input signals are simultaneously applied to the transistors 36 and 38 to drive them mutually "on" and "off" and vice versa, causing transistors 26 and 32 to also be rendered mutually "on" and "off." Thus if transistor 36 is driven "on" by means of a signal applied to its base, transistor 26 will also be driven "on;" however, transistors 38 and 32 will be driven "off."

With respect to transistor 34, however, resistors 46, 48, and 50 as well as the semiconductor diode 52 maintain transistor 34 in a constantly conductive state. Thus when transistor 26 is turned "on," the relatively low DC impedance path through transistors 26 and 34 provides a current path through resistor 30 and a negative potential (.apprxeq. -8 volts) appears at the anode electrode of diode 22 which back biases it into its nonconductive state. When this condition occurs, a load current I.sub.L in the order of 20 milliamperes (10.sup.-.sup.3 amp.) as controlled by the current source 10 flows through the diode 20 and into the load 18. On the other hand, when transistor 26 is driven "off" by means of a negative going trigger signal applied to transistor 36, a positive potential appears at the anode electrode of the switching diode 22 forward biasing it into conduction whereupon the current I.sub.L as determined by the current source 10 flows through the diode 22, the resistors 30 and the zener diode 28. The zener diode 28 acts to limit the magnitude of the voltage appearing at the anode of diode 22 to for example approximately (+6)-(I.sub.L R.sub.30) volts. This volt potential appears at the cathode electrode of the switching diode 20, thereby back biasing it into its nonconductive state.

It is immediately evident, therefore, that the driver circuit 14 under the control of two complementary trigger signals respectively applied to the transistors 36 and 38 to act to cause a continuous DC current I.sub.L to either flow through the load 18 or to divert it through switching diode 22, the zener diode 28 and resistor 30. Since the current I.sub.L flows continuously either through hot carrier diode 20 or hot carrier diode 22, load current from the current source 10 is not interrupted, L does not vary between zero and the desired value. The problem of switching transients is immediately eliminated thereby. Secondly, electronic switching of the current to and from the load 18 occurs in the order 10.times. 10.sup.-.sup.9 seconds due to the extremely fast acting capability of the hot carrier diodes 20 and 22. It should also be pointed out that the back bias leakage current of the hot carrier diode is in the order of 2.times. 10.sup.-.sup.9 amperes, so that substantially 100 percent of the 20 ma. current I.sub.L is flowing at any one time through only one of the diodes 20 or 22, depending upon which of the two is driven into conductivity by means of the driver circuit 14.

The current I.sub.L is, as has been said before, a continuous DC current and is typically in the order of 20 ma. This is provided by the current source 10 which is comprised of a high gain DC operational amplifier 54 coupled to a +12 volt and a -12 volt supply potential and operated as a differential amplifier. Amplifier 54 includes a pair of input terminals 56 and 58 as well as an output terminal 60. An adjustable reference voltage is applied thereto which is generated by the circuit 16 which is comprised of a constant voltage source including an operational amplifier 62 coupled into an emitter follower circuit including transistor 64. The output or reference voltage is applied to terminal 56 of the operational amplifier 54 by means of the resistor 66 which is connected to the emitter of transistor 64. The reference voltage magnitude is determined by the potentiometer 68 coupled between resistor 70 and 72. One end of resistor 72 is coupled to a zener diode 74 which establishes a fixed voltage thereacross depending upon its threshold voltage while one end of resistor 70 is coupled back to the emitter of transistor 64 and thereby provides a DC feedback path for the amplifier 62. The voltage reference circuit 16 then is simply a voltage regulator circuit and is shown for sake of illustration only, it being understood that any other constant voltage source may be employed when desired.

Returning now to a consideration of the constant current source 10, which is the principal circuit portion of the subject invention, input terminal 56 has an adjustable reference voltage applied thereto as determined by the potentiometer 68 of the voltage reference source 16. The other input terminal 58 of the operational amplifier 54 is connected to circuit junction 76 to which is connected to temperature stabilized resistor 78 and which has its other end connected to circuit junction 80. A -12 volt supply potential is connected to circuit junction 80. Additionally, a resistor 82 is connected between circuit junction 80 and the first input terminal 56 of the operational amplifier 54. Capacitor means including the electrical capacitors 84 and 86 are coupled between circuit junction 80 and ground to provide a filtering action for the -12 volt supply potential.

A feedback loop comprising a Darlington transistor configuration including transistors 88 and 90 is coupled between the amplifier output terminal 60 and the second input terminal 58 which is also common to the resistor 78 at circuit junction 76. The output of the operational amplifier comprises the base input current I.sub.B for the transistors 88 and 90 while the collector current (I.sub.C) is the desired constant load current I.sub.L so that I.sub.L =I.sub.C. This is accomplished by means of commonly connecting the collector electrodes of transistors 88 and 90 to circuit junctions 24 of the hot carrier diode switch circuit 12. The emitter current I.sub.E flowing from transistor 90 is substantially equal to the collector current I.sub.C plus the base current I.sub.B into the transistor 88. The voltage drop across resistor 78 caused by the emitter current I.sub.E appears as an input signal at terminal 58 of the operational amplifier 54. The operational amplifier 54 is internally connected so that the output signal current I.sub.B changes until the voltage at terminal 58 equals the reference voltage applied to the other input terminal 56. The DArlington configuration of transistors 88 and 90, moreover, provides an overall current gain (I.sub.C =h.sub.FE I.sub.B) which is equal to the product of the h.sub.FE of each transistor. Thus for example where the DC h.sub.FE of both transistors is in the order of 50, a current I.sub.B in the order of 8 microamperes will produce the required collector current I.sub.C of 20 milliamps. Any changes occurring in the gain characteristic of the transistors 88 and 90 is reflected as a voltage change across resistor 78; however, the output I.sub.B of the operational amplifier 54, however, will be driven in the proper direction to force the voltages at input terminals 58 and 56 to be equal, thus bringing the value of the load current I.sub.L to the required value.

The following Table I represents a tabulation of typical operating results obtained for a temperature range between -50.degree. C. to +100.degree. C. The output current was measured by monitoring the voltage across a load resistor 18 having a resistance value of 170 ohms. A current change of less than 0.0013 percent was observed.

Temperatures in .degree. C. Voltage across load resistor 18 __________________________________________________________________________ -50 -2.702 -40 -2.701 -30 -2.701 -20 -2.701 -10 -2.701 0 -2.701 +10 -2.700 +20 -2.700 +30 -2.699 +40 -2.700 +50 -2.699 +60 -2.699 +70 -2.699 +80 -2.699 +90 -2.699 +100 -2.699 --------------------------------------------------------------------------- TABLE I

in summation, therefore, the collector current flowing through the Darlington configuration of transistors 88 and 90 powered by the -12 volt supply potential applied to circuit junction 80 is maintained at a constant value by means of the feedback amplifier configuration including the operational amplifier 54. Moreover, the collector current of the feedback transistors 88 and 90 is a continuously flowing current which is switched by means of the driver circuit 14 so that it either flows through the diode 20 and into the load resistor 18 or is diverted into the diode 22 which then flows through resistor 30 and the zener diode 28.

Claims

Having thus described the present invention with respect to its preferred embodiment, I claim as my invention:

1. Apparatus for delivering a constant current to a load from a source of supply voltage over a relatively wide temperature range of operation, comprising in combination:

electronic switch means having a first and a second current switch including circuit means coupling said first current switch to one side of said load, said load having its opposite side coupled to said source of supply voltage;

driver circuit means operated in accordance with at least one control signal applied thereto and including an electronic circuit coupled to said second current switch, said electronic circuit being operated by said control signal to cause said second current switch to become selectively nonconductive and conductive while causing said first current switch to become conductive and nonconductive and thereby switch substantially all of said constant current respectively to said first or second current switch;

a reference voltage source providing an output of reference voltage; and

a constant current source coupled in series between said electronic switch means and said source of supply voltage and additionally including circuit means coupled to said reference voltage source and being responsive to said reference voltage to generate said constant current of a predetermined magnitude in response to the magnitude of said reference voltage, said constant current thereby being adapted to flow continuously either through said first or said second current switch.

2. The invention as defined by claim 1 wherein said first and second current switch comprises a first and a second semiconductive diode.

3. The invention as defined by claim 2 wherein said first and second semiconductor diodes are comprised of Schottky barrier diodes.

4. The invention as defined by claim 1 wherein said constant current source comprises:

Dc amplifier means including a feedback control loop and having first and second input terminals and an output terminal, being operable to provide an output signal which is a function of the signal difference between signals respectively applied to said first and second input terminal;

first circuit means coupling said first input terminal to said reference voltage;

second circuit means coupling said source of supply voltage to said first input terminal;

first feedback means comprising at least one transistor having an input electrode and a first and second output electrode, including circuit means coupling said output terminal of said DC amplifier means to said input electrode, said first output electrode to said electronic switch means, and said second output terminal to said second input terminal; and

second feedback means coupling said second output electrode of said at least one transistor and said second input terminal of said DC amplifier means to said source of supply voltage, whereby said first and second feedback means defines said feedback control loop whereby a voltage is produced across said second feedback means and is applied to said second input terminal for causing a substantially constant current flow between said switch means and said second output electrode of said at least one transistor.

5. The invention as defined by claim 4 wherein said first feedback means additionally comprises a second transistor coupled to said at least one transistor in a Darlington circuit configuration.

6. The invention as defined by claim 5 wherein said first and second current switch comprises a first and a second Schottky barrier diode.

7. The invention as defined by claim 4 wherein said first feedback means comprises a first and a second transistor each having an input electrode and a first and second output electrode and including circuit means commonly coupling said first output electrodes together to said switch means, said input electrode of said first transistor to said output terminal of said DC amplifier means, said second output electrode of said first transistor to said input electrode of said second transistor, and said second output electrode of said second transistor to said third circuit means providing thereby a Darlington circuit configuration.

8. The invention as defined by claim 7 wherein said switch means comprises a pair of hot carrier diodes and wherein one of said pair of diodes is directly connected between said load and said common coupling of said first output electrodes of said first and second transistor.

9. The invention as defined by claim 8 wherein said first and second circuit means and said second feedback means each comprises resistor means.

10. The invention as defined by claim 1 wherein said reference voltage source comprises a DC voltage regulator circuit including means for selectively adjusting the magnitude of said reference voltage thereby controlling said substantially constant current supplied to said load.

11. The invention as defined by claim 1 wherein said driver means comprises a differential amplifier having a first and a second input circuit adapted to be coupled respectively to and operated by a binary input signal and the complement thereof and an output circuit coupled to said second current switch of said switch means, said output circuit being rendered selectively operative to control current flow in said second current switch.