Thermal power sensor

• US 5,291,073 A
• Filed: 10/07/1992
• Issued: 03/01/1994
• Est. Priority Date: 10/07/1992
• Status: Expired due to Fees

• Current Assignee: John Fluke Mfg. Co., Inc.
• Original Assignee: John Fluke Mfg. Co., Inc.
• Inventors: Lewandowski, Robert J.
• Primary Examiner(s): Wambach, Margaret R.
• Application Number:
  US07/957,935
• Time in Patent Office: 510 Days
• Field of Search: 307/310, 307/355, 324/95, 324/98, 324/99 R, 324/106
• US Class Current: 327/512
• CPC Class Codes: G01R 21/02 by thermal methods , e.g. calorimetric

Abstract

A thermal power sensor for providing an output signal that is linearly related to the amount of power derived from a power source over a first range and that is square law related to the amount of power derived from a power source over a second range which may overlap the first range. The apparatus comprises a pair of matched transistors connected in a common emitter configuration. Each of the matched transistors is thermally coupled to a matched resistor so that heat generated in the resistor is transmitted to its respective transistor to cause temperature variations therein. The current through each transistor is a function of the temperature of the transistor. The matched transistors are connected in series with a pair of cascode transistors. The cascode transistors are connected to a first differential amplifier which senses and amplifies the differential voltage which results from any difference in current flow through the matched transistors. The output of the first differential amplifier is connected to the bases of the cascode transistors to vary the bias voltage applied to the matched transistors. A second differential amplifier is connected to the differential output of the first differential amplifier and has its output connected to the second matched resistor. The effect of the feedback loop which includes the first differential amplifier, the cascode transistor and the matched transistors is to maintain the current flow through the matched transistors constant, whereby their collector-emitter voltage is linearly proportional to the power being sensed. The effect of the second loop, including the second differential amplifier and the second matched resistor, is to maintain thermal equilibrium and equality between the matched transistors and their associated resistors to provide a square law indication of the power being sensed. A combined indicator is provided for providing an indication of sensed power over the first and second ranges.

Claims

1. An apparatus for deriving an output signal that is related to the magnitude of a signal derived from a source, comprising:

first and second matched circuit devices thermally isolated from each other and substantially isolated from ambient thermal energy, said circuit devices each having a variable characteristic controlled by an amount of thermal energy incident thereon;

means for thermally coupling the first circuit device to said source so that thermal energy from said source is incident on the first device;

means responsive to a control signal for applying additional thermal energy differentially to said first and second circuit devices and for raising the thermal energy of said first and second circuit devices to a quiescent value of thermal energy greater than that due to ambient thermal energy;

means for sensing said variable characteristics of said first and second circuit devices;

means responsive to said variable characteristics for generating said control signal to control said means for applying additional thermal energy so that the first and second circuit devices have a tendency to be thermally balanced; and

means responsive to said control signal for determining the magnitude of said applied additional thermal energy, said magnitude of said applied additional thermal energy corresponding to the magnitude of the signal from said source.

2. The apparatus of claim 1 wherein said means for applying additional thermal energy comprises a first source of thermal energy coupled to said first circuit device and a second source of thermal energy coupled to said second circuit device, and said means for generating said control signal includes means for generating differential control signals and means for applying said differential control signals respectively to said first and second sources of thermal energy, and further including a third source of thermal energy responsive to said differential control signals for applying further additional thermal energy to said second circuit device.

3. The apparatus of claim 2 wherein said first and second matched circuit devices are mounted respectively on first and second substrates which are substantially thermally isolated from each other and from ambient energy, and further wherein said first source of thermal energy is on said first substrate, and said second and third sources of thermal energy are on said second substrate.

4. The apparatus of claim 1 wherein said first and second matched circuit devices are biased, bipolar semiconductor junction devices.

5. The apparatus of claim 4 wherein said first and second bipolar semiconductor junction devices are bipolar transistors, respectively.

6. The apparatus of claim 1 wherein said output signal is linearly related to the amount of power derived from said source.

7. An apparatus for generating an output signal indicative of the magnitude of an input signal, said apparatus comprising:

first and second matched circuit devices thermally isolated from each other and substantially isolated from ambient energy, each of said circuit devices having a variable characteristic controlled by the amount of thermal energy incident thereon;

means for coupling the first circuit device to said input signal so that thermal energy derived from said input signal is incident on said first circuit device, thereby controlling the variable characteristics of said first circuit device;

means for sensing the variable characteristics of said first and second circuit devices and for generating first and second control signals corresponding thereto;

means linearly responsive to said first and second control signals for applying additional thermal energy differentially to said first and second circuit devices over a first range of said input signal to raise the thermal energy of said first and second circuit devices toward a quiescent, thermally balanced condition that is greater than that due to ambient thermal energy, the magnitude of said additional differential thermal energy being linearly related to the magnitude of said input signal over said first range;

a source of thermal energy responsive to at least one of said first and second control signals in accordance with a predetermined relationship for applying further additional thermal energy to said second circuit device over a second range of said input signal, the magnitude of said further additional thermal energy having said predetermined relationship to the magnitude of said input signal over said second range; and

means for generating said output signal as a function of the magnitude of said additional thermal energy and said further additional thermal energy.

8. The apparatus of claim 7 wherein said first and second ranges overlap each other for at least a portion of said first and second ranges of said input signal.

9. The apparatus of claim 7 wherein said first and second matched circuit devices are bipolar transistors, the means for applying additional thermal energy differentially to said first and second circuit devices includes means for adjusting the bias power dissipated in said devices responsive to said control signal, and said source of further additional thermal energy is a resistor.

10. In a voltage and power sensing device including a pair of matched, forward-biased bipolar transistors connected in a differential common emitter configuration, wherein each transistor is associated with one of a pair of matched resistors, heat transfer means thermally coupling each resistor to a respective transistor so that heat generated in the resistors is coupled to their respective transistors to cause temperature variations therein, said transistors exhibiting a collector-emitter current that is a function of the temperature of the transistor, whereby the collector-emitter current through said transistors varies as a result of the power being dissipated by the respective resistor, said transistors being connected to a first amplifier which senses and amplifies a differential voltage resulting from different currents flowing through said transistors and produces an output signal in response thereto, the output signal being connected in a feedback loop to the second of the matched resistors, the first matched resistor being connected to an input terminal for receiving a signal or power source to be sensed, the improvement comprising:

a respective cascode transistor connected in series with each matched transistor, said cascode transistors having their respective bases coupled to differential outputs of a first differential amplifier in an attempt to balance the temperatures of said matched transistors by varying bias power dissipated in said matched transistors as a function of the differential outputs of said first differential amplifier, a voltage drop across said matched transistors varying as a linear function of the power being sensed; and

means connected to the output of said first differential amplifier providing an indication of the said power being sensed, said indication bearing a substantially linear relationship to said differential voltage.

11. The apparatus of claim 10, further including a second amplifier means connected to the output of said first differential amplifier, said second amplifier providing a signal of the second of said matched resistors, thereby causing said matched resistor to dissipate further additional power, said signal bearing a substantially square root relationship to said power being sensed.

12. The apparatus of claim 11, including means for combining said indications of power being sensed and providing an indication of power sensed over an effective range of said power sensing device.

13. A voltage and power sensor for determining the voltage or power from an external source, said sensor comprising:

first and second matched sense transistors thermally isolated from each other and substantially isolated from ambient thermal energy, said sense transistors having their bases coupled to each other and to a fixed voltage and their emitters connected to each other and to a current source;

a first thermal element thermally coupled to said first sense transistor, said first thermal element dissipating power, said first thermal element being coupled to said external source so that the current through said first sense transistor varies as a function of the heat coupled to said first transistor by power dissipated in said first thermal element;

a differential amplifier generating a differential output that is proportional to the difference in currents through said first and second sense transistors;

first and second heating transistors thermally coupled to said first and second sense transistors, respectively, said first and second heating transistors having respective emitters connected to collectors of respective sense transistors and respective bases connected to each other and to a higher fixed voltage so that the collector-emitter voltages of said sense transistors remain substantially constant; and

first and second cascode transistors having respective bases connected to respective outputs of said differential amplifier and respective emitters connected to collectors of respective heating transistors, said cascode transistors varying the collector-emitter voltages of respective heating transistors causing a change in the power dissipated in the heating transistors which results in a temperature change of the heating transistors as a linear function of the output from said differential amplifier, thereby attempting to thermally balance said sense transistors, whereby the output of said differential amplifier is indicative of the power from said external source.

14. The voltage and power sense of claim 13, further including a second thermal element thermally coupled to said second sense transistor, said second thermal element dissipating power as a function of the square of a voltage across said second thermal element, said second thermal element being coupled to the output of said differential amplifier so that the current through said second sense transistor varies as a function of heat coupled to said second transistor by power dissipated in said second thermal element, whereby the power from said external source is determined as a function of the sum of the square of the output of said differential amplifier and the output of said differential amplifier.

15. The voltage and power sensor of claim 14 wherein said second thermal element is coupled to the output of said differential amplifier through a nonlinear element so that said second thermal element does not couple significant thermal energy to said second sense transistor until the power from said external source exceeds a predetermined value.

16. The voltage and power sensor of claim 13 wherein said first thermal element is a resistor.

17. A voltage and power sensor for determining the power from an external source, said sensor comprising:

first and second semiconductor means thermally isolated from each other and substantially isolated from ambient thermal energy, each of said semiconductor means generating an electrical output that is a function of the temperature of said semiconductor means, said first and second semiconductor means dissipating power as a linear function of respective first and second control signals applied to said first and second semiconductor means, respectively;

a first thermal element thermally coupled to said first semiconductor means, said thermal element dissipating power as a function of the square of a voltage across said thermal element, said first thermal element being coupled to said external source so that the current through said first semiconductor means varies as a function of the heat coupled to said first semiconductor means by power dissipated in said first thermal element; and

a differential amplifier generating said first and second control signals as a function of the difference in electrical outputs from said first and second semiconductor means, thereby varying the power dissipated in said first and second semiconductor means as a linear function of the output from said differential amplifier in an attempt to thermally balance the temperatures of said first and second semiconductor means, whereby the output of said differential amplifier is indicative of the power from said external source.

18. The voltage and power sensor of claim 17 wherein said first and second semiconductor means include respective first and second transistors thermally isolated from each other and substantially isolated from ambient thermal energy, the current through said transistors varying as a function of the temperature of said transistors.

19. The voltage and power sensor of claim 17 wherein said first and second semiconductor means include respective first and second transistors, and wherein said first and second control signals vary the collector-emitter voltages of said first and second transistors, respectively, thereby varying the power dissipated by said first and second transistors as a linear function of said first and second control signals.

20. The voltage and power sensor of claim 17 wherein said first and second semiconductor means include respective first and second sense transistors thermally isolated from each other and substantially isolated from ambient thermal energy, the current through said sense transistors varying as a function of the temperature of said sense transistors, said first sense transistor being thermally coupled to said first thermal element, said first and second semiconductor means further including respective first and second heating transistors connected in series with, and thermally coupled to, respective first and second sense transistors, said first and second control signals varying respective collector-emitter voltages of said first and second heating transistors, respectively, thereby varying the power dissipated in said first and second heating transistors as a linear function of said first and second control signals.

21. The voltage and power sensor of claim 17, further including a second thermal element thermally coupled to said second semiconductor means, said second thermal element dissipating power as a function of the square of a voltage across said second thermal element, said second thermal element being coupled to the output of said differential amplifier so that the current through said second semiconductor means varies as a function of the heat coupled to said semiconductor means by power dissipated in said second thermal element, whereby the power from said external source is a function of the sum of the square of the output of said differential amplifier and the output of said differential amplifier.

22. The voltage and power sensor of claim 21 wherein said second thermal element is coupled to the output of said differential amplifier through a nonlinear element so that said second thermal element does not couple significant thermal energy to said second semiconductor means until the power from said external source exceeds a predetermined value.

23. The voltage and power sensor of claim 17 wherein said first thermal element is a resistor.