Electrical constant current circuit, electrical constant current source, measurement arrangement

Abstract

The invention relates to an electrical constant current circuit (CCC) in particular for supplying electrical power to a sensor, the circuit (CCC) comprising - a power supply input terminal (INT), - a constant current output terminal (OUT), - a first transistor (TRF) - a shunt-type voltage reference (RVS), - wherein the base of the first transistor (TRF) being biased by the shunt-type voltage reference (SVR) in series with the base-emitter junction of the second transistor (TRS), - wherein the emitter of the first transistor (TRF) being connected to the power supply input terminal (INT) via a current set resistor (RST), - wherein the collector of the first transistor (TRF) being connected to said output terminal (OUT) through a Schottky diode (SKD). The improved temperature stability, initial accuracy, drop out voltage and power consumption the invention proposes: - that the circuit (CCC) comprises a second transistor (TRS) in the line between the base of the first transistor (TRF) and the shunt-type voltage reference (SVR), - wherein the second transistor's (TRS) base and collector are connected to the first transistor's (TRF) base.

Description

[0001]202301400 Auslandsfassung 1 Description Electrical constant current circuit, electrical constant cur- rent source, measurement arrangement FIELD OF THE INVENTION The invention relates to an electrical constant current cir- cuit in particular for supplying electrical power to a sen- sor, the circuit comprising - a power supply input terminal, - a constant current output terminal, - a first transistor, - a shunt-type voltage reference, - wherein the base of the first transistor being biased by the shunt-type voltage reference in series with the base-emitter junction of the second transistor, - wherein the emitter of the first transistor being con- nected to the power supply input terminal via a current set resistor, - wherein the collector of the first transistor being con- nected to said output terminal through a Schottky diode. Further, the invention relates to an electrical constant cur- rent source comprising such constant current circuit and a measurement arrangement comprising such source. BACKGROUND OF THE INVENTION From “Linden T. Harrison, Current Sources and Voltage Refer- ences”, 2005, Chapter 4 - Using BJTs to Create Current Sources, Pages 47-124, and Chapter 5 - Using Precision Matched-Pairs, Duals, and Quads, Pages 125-135, 2005 design suggestions about approaches of constant current circuits are known. From US 3784 844 A an electrical constant current circuit of the incipiently mentioned type is known. The arrangement com- prises an individual diode which will compensate the drifts to some degree but will have much higher accuracy uncertain- ties and higher thermal drifts due to the unequal thermal 202301400 Auslandsfassung 2 behavior and thermal condition of the diode with respect to the base-emitter junction of the transistor. Conventional constant current circuits, e.g., the traditional Zener diode-based circuit, suffer from a high drop-out volt- age. The Zener diode-based circuit is limited to the voltage of available precision Zener diodes and increasing thermal drift of the circuit with lower voltage Zener diodes. The drop-out voltage of the traditional circuit is relatively high because of the voltage of the available precision Zener diodes. Lower voltage Zener diodes are available but with high tolerances which makes the overall circuit tolerance not suitable for precision applications. An increased dropout voltage requires higher supply voltage and causes higher dis- sipated power in the circuit. Further, the thermal drift be- havior and the initial accuracy of the Zener diode-based cir- cuit are poor due to the limited tolerances of available Ze- ner diodes and the large tolerances of the junction voltage of the transistor which normally has big variations. In sensor applications where a minimum output current should be guaranteed over the full operation temperature range of the system (e.g. ICP®-acceleration sensors by PCB SYNOTECH GMBH), the nominal output current should be set to higher values which results in higher dissipated power and higher consumed power. Integrated Circuit (IC) current sources are available but their maximum voltage ratings are limited and do not meet the requirements for most of the industrial applications. Currently, an improved solution avoiding these drawbacks is not available. SUMMARY OF THE INVENTION It is one object of the invention to avoid or diminish above explained drawbacks. It is another object of the invention to improve initial accuracy and / or reduce the operating power 202301400 Auslandsfassung 3 consumption and / or reduce the temperature drift and / or to lower the drop out voltage. The object of the invention is achieved by the independent claims. The dependent claims describe advantageous develop- ments and modifications of the invention. The invention proposes a circuit of the incipiently mentioned type being prepared such: - that the circuit comprises a second transistor in the line between the first transistor's base and the shunt- type voltage reference, - wherein the second transistor's base and collector are connected to the first transistor's base. A preferred embodiment of the invention provides that a ca- pacitor is provided parallel to the shunt-type voltage refer- ence in series with the second transistor to reduce noise. This feature enables a more stable constant current. Another preferred embodiment of the invention provides that the circuit comprises a ground reference of the shunt type voltage reference via a grounding bias resistor. In other words and in more detail that means that the circuit com- prises a ground reference of the shunt-type voltage reference connected to the ground of the input voltage source via base- emitter junction of the second transistor in series with a grounding bias resistor said grounding bias resistor belong- ing to the constant current circuit Said grounding bias resistor, which embodies the ground ref- erence of the shunt voltage reference, is an integral part of the constant current circuit. In other words, the shunt-type voltage reference includes a ground reference terminal, wherein said terminal is connected to a ground terminal via the grounding bias resistor. 202301400 Auslandsfassung 4 Preferably the voltage reference is a micropower precision shunt-type voltage reference. This may be e.g., an AD1580 (available at Analog Devices, Inc.) 2-terminal preci- sion band gap shunt-type voltage reference. The AD1580 pro- vides a 1.225 V output for input currents between 50 μA and 10 mA. The initial voltage accuracy is ±0.1% and the tempera- ture drift is ±50 ppm / °C maximum. The operating range lies between 50 µA to 10 mA. The term “micropower” is a common term in electronics meaning that the consumed power is in the range of microwatts. One preferred embodiment provides a Schottky diode at the output terminal to avoid reverse current for output protec- tion. Employing the Schottky diode reduces the drop-out volt- age of the circuit by about 0.3 V compared to using a normal rectifier diode. Preferably said first transistor and said second transistor are identical. Most preferably said first transistor and said second transistor are both part of a matched-pair dual tran- sistor. This matched-pair dual transistor may e.g., be a DMMT5401-7-F bipolar transistor BJT matched PNP SM signal trans of Diodes Incorporated. Another preferred embodiment provides an electrical constant current source, the source comprising a constant current cir- cuit as described herein wherein the source comprises a power supply, suppling power to said constant current circuit. One preferred field of application is a measurement arrange- ment comprising a constant current source according to one of the herein described embodiments. According to the invention a circuit with two identical tran- sistors, which are parts of a matched-pair PNP transistor is employed, resulting in significant improvement in thermal drift and accuracy of the circuit due to the perfectly 202301400 Auslandsfassung 5 matched junction voltage, thermal behavior, and also ther- mally coupled structure of the matched-pair transistor. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is now described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 shows a schematic diagram of an electrical constant current source comprising a constant current cir- cuit, Figure 2 shows a measured temperature stability of the cir- cuit in the temperature chamber. The illustration in the drawings is in schematic form. It is noted that in different figures, similar or identical elements may be provided with the same reference signs. DESCRIPTION OF THE DRAWINGS Figure 1 shows a schematic diagram of an electrical constant current source CCS comprising a constant current circuit CCC. Said constant current circuit CCC for supplying electrical power to a sensor SNR receives the operation power from a (preferably regulated) DC power supply POS. The circuit CCC comprises a power supply input terminal INT and a constant current output terminal OUT. A first transis- tor TRF and an identical second transistor TRS are provided as a matched-pair dual transistor MDT. The transistors are configured such that - said first transistor’s TRF emitter being connected to the power supply input terminal INT via a -current set resistor RST; - said first transistor’s TRF base being biased by the shunt-type voltage reference SVR via the second transis- tors emitter-base-line (base-emitter junction); 202301400 Auslandsfassung 6 - the second transistor’s TRS base is connected to its collector and to the first transistor's TRF base; - the first transistor’s TRF collector being connected to said output terminal OUT via a Schottky diode SKD, - an optional noise reduction capacitor CNR connected in parallel to the shunt-type voltage reference SVR in se- ries with the base-emitter junction of the second tran- sistor TRS. In the circuit CCC, the transistor TRF is the main pass ele- ment and equates the voltage across the reference voltage SVR plus the base-emitter voltage of the second transistor TRS to the voltage across the current set resistor RST and its base- emitter voltage VTRFsuch that: VSVR+ ^^TRS= ^^RST+VTRFEmploying said matched-pair dual transistor MDT perfectly cancels out ^^TRSand VTRFand their drifts due to the ambient temperature variations and self-heating of the transistor. This mechanism results in significant improvement on tempera- ture drift and the accuracy of the circuit. Due to the can- cellation of the ^^TRSby VTRF, and neglecting the base cur- rents inequality of transistors (<50µA), the output [Iout] current will be The drop-out voltage [VDO] of the circuit will be: ^^^^ ^^= ^^SVR+ VTRF ^^ ^^ ^^ ^^ ^^+ ^^SKDwhere the VTRF ^^ ^^ ^^ ^^ ^^is the collector-emitter saturation voltage of the TRF and VF is the forward voltage of the diode SKD. The optional capacitor CNRis used for noise reduction and power supply rejection ratio [PSRR] improvement in high fre- quencies. 202301400 Auslandsfassung 7 Figure 1 symbolically illustrates the schematic diagram of the invention. Figure 2 shows the measured temperature stability in the tem- perature chamber. Over a wide range of -40°C to 100°C the temperature drift is about 3,96mA – 4,09mA which is approxi- mately ±1.5% (this result is achieved with a ±50 ppm / °C set resistor (Rset), further improvement possible by choosing a lower thermal coefficient resistor). Although the present invention has been described in detail with reference to the preferred embodiment, it is to be un- derstood that the present invention is not limited by the disclosed examples, and that numerous additional modifica- tions and variations could be made thereto by a person skilled in the art without departing from the scope of the invention. It should be noted that the use of "a" or "an" throughout this application does not exclude a plurality, and "compris- ing" does not exclude other steps or elements. Also, elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims. Independent of the grammatical term usage, individuals with male, female or other gender identities are included within the term.

Claims

202301400 Auslandsfassung 8 Patent Claims 1. Electrical constant current circuit (CCC) in particular for supplying electrical power to a sensor (SNR), the cir- cuit (CCC) comprising - a power supply input terminal (INT), - a constant current output terminal (OUT), - a first transistor (TRF) - a shunt-type voltage reference (SVR), - wherein the first transistor (TRF) base being biased by the shunt-type voltage reference (SVR) in series with second transistor (TRS) base-emitter junction, - wherein the first transistor (TRF) emitter being con- nected to the power supply input terminal (INT) via a collector resistor (RST), - wherein the first transistor (TRF) collector being con- nected to said output terminal (OUT), characterized in - that the circuit (CCC) comprises a second transis- tor (TRS) in the line between the first transis- tor's (TRF) base and the shunt-type voltage refer- ence (SVR), - wherein the second transistor's (TRS) base and collec- tor are connected to the first transistor's (TRF) base, - wherein said first transistor (TRF) and said second transistor (TRS) are identical, - wherein said first transistor (TRF) and said second transistor (TRS) are both part of a matched-pair dual transistor (MDT).

2. Circuit (CCC) according to claim 1, wherein a capacitor (CNR) is provided parallel to the shunt-type voltage reference (SVR) to reduce noise.

3. Circuit (CCC) according to at least one of the preceding claims 1-2, wherein the shunt-type voltage reference (SVR) is a mi- cropower precision shunt-type voltage reference with a power consumption in the range of microwatts.202301400 Auslandsfassung 9 4. Circuit (CCC) according to at least one of the preceding claims 1-3, wherein a Schottky diode (SKD) is provided at the output terminal to avoid reverse current.

5. Circuit (CCC) according to at least one of the preceding claims 1-4, the circuit (CCC) comprising: - a ground reference of the shunt-type voltage refer- ence (SVR) connected to the ground of the power supply (POS) via base-emitter junction of the second transistor in series with a grounding bias resistor (RBS).

6. Electrical constant current source (CCS), the source (CCS) comprising a constant current circuit (CCC) according to at least one of the preceding claims 1-5, wherein the source comprising a power supply (POS), sup- pling power to said constant current circuit (CCC).

7. Measurement arrangement comprising a constant current source (CCS) according to the preceding claim 6.

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