Abstract:
The invention relates to a temperature sensor having a first FET transistor circuit and a second FET transistor circuit and also to a method for operating a temperature sensor. Both FET transistor circuits are operated at an operating point that lies outside the temperature-independent operating point. The difference between the voltages at the first and second FET transistor circuits is evaluated as a measure of the temperature at one of the FET transistor circuits. The invention enables the temperature sensor to provide a relatively large output signal even in the case of only small changes in temperature.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Field of the Invention  
           [0002]    The invention relates to a temperature sensor including two FET&#39;s (Field Effect Transistors) that each have an operating point at which the gate voltage is essentially temperature independent for a constant drain-source current. The invention also relates to method for operating the temperature sensor.  
           [0003]    U.S. Pat. No. 5,226,942 describes a temperature sensor that has two field-effect transistors. One of these transistors is operated in the region below the pinch-off voltage (subthreshold region) and the other is operated at an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. By comparing the voltages at the two field-effect transistors, a signal is generated that is dependent on the temperature of the transistor that is operated below the pinch-off voltage.  
           [0004]    A temperature sensor of this type is also disclosed in U.S. Pat. No. 5,796,290. A first field-effect transistor is operated at an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. A second field-effect transistor is operated below the temperature-independent operating point and thus in a region in which, for a constant drain-source current, the gate voltage is temperature-dependent. The difference between the voltages at the first and second field-effect transistors is evaluated as a measure of the temperature.  
           [0005]    One disadvantage of the known temperature sensors is that the change in voltage obtained because of a change in temperature is small because of the small currents and voltages in the region below the pinch-off voltage or the temperature-independent operating point. This change in voltage, therefore, can only be processed further with difficulty.  
         SUMMARY OF THE INVENTION  
         [0006]    It is accordingly an object of the invention to provide a temperature sensor and a method for operating the temperature sensor which overcomes the above-mentioned disadvantages of the prior art apparatus and methods of this general type.  
           [0007]    In particular, it is an object of the invention to provide a temperature sensor and a method for operating the temperature sensor, which for the compensation of temperature dependences in analog or digital circuits, provide an output signal that is dependent on the temperature. In this case, small changes in temperature are intended to bring about an output signal that is as large as possible.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a temperature sensor that includes: a first FET transistor circuit and a second FET transistor circuit that each have a gate voltage and a temperature-independent operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. There is also a circuit for operating the first FET transistor circuit at a first operating point that is different from the temperature-independent operating point. The circuit is also provided for operating the second FET transistor circuit at a second operating point that is different from the temperature-independent operating point. The first FET transistor circuit provides a first voltage, and the second FET transistor circuit provides a first voltage. A difference between the first voltage and the second voltage is evaluated as a measure of a temperature of the first FET transistor circuit or the second FET transistor circuit.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a temperature sensor. The method includes steps of: providing the temperature sensor with a first FET transistor circuit, a second FET transistor circuit, and a circuit for operating the first FET transistor circuit and the second FET transistor circuit; evaluating a difference between a voltage at the first FET transistor circuit and a voltage at the second FET transistor circuit as a measure of a temperature at the first FET transistor circuit or the second FET transistor circuit; providing the first FET transistor circuit and the second FET transistor circuit with a temperature-independent operating point at which, for a constant drain-source current, a gate voltage is essentially temperature-independent; and operating the first FET transistor circuit and the second FET transistor circuit at a respective operating point that is different from the temperature-independent operating point.  
           [0010]    Accordingly, the temperature sensor is distinguished by circuitry for operating the first FET transistor circuit at an operating point that lies above or below the temperature-independent operating point. The circuitry is also for operating the second FET transistor circuit at an operating point that likewise lies above or below the temperature-independent operating point. The difference between the voltages at the first and second FET transistor circuits is evaluated as a measure of the temperature at one of the FET transistor circuits.  
           [0011]    Consequently, unlike in the case of the temperature sensors known in the prior art, in the invention the operating points of both FET transistor circuits are put outside of the temperature-independent operating point of the respective transistor. In particular, the operating point of one FET transistor circuit may be put above and the operating point of the other FET transistor circuit may be put below the temperature-independent operating point. This increases the temperature response since the two temperature responses of the two FET transistor circuits are added together. Accordingly, the temperature sensor provides a relatively large output signal even in the case of small changes in temperature.  
           [0012]    A further advantage of the inventive solution is that the tolerance of the circuit required for setting the current is increased since the temperature-independent operating point no longer has to be found exactly in one of the FET transistor circuits.  
           [0013]    It is pointed out that each field-effect transistor has an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent (zero temperature coefficient point). The drain-source current has a positive temperature coefficient below this operating point, and a negative temperature coefficient above this operating point. This is described in detail in the literature.  
           [0014]    In one refinement of the invention, the operating point of both FET transistor circuits lies above or below the temperature-independent operating point. A relatively large tolerance of the current feed-in may be present in this variant, too.  
           [0015]    The circuitry for operating the first FET transistor circuit and for operating the second FET transistor circuit preferably have a current generator that feeds the two transistor circuits with constant currents. In this case, the current generator advantageously increases two coupled current sources that feed the FET transistor circuits. By using a current generator that operates the two FET transistor circuits with different currents, the desired operating points of the transistor circuits can be set simply and reliably. For setting the operating points, the current generator in this case contains, for example, a current mirror that defines the operating points of the FET transistor circuits.  
           [0016]    As an alternative, the circuit for operating the first and second FET transistor circuits in each case have a resistor connected in series with the respective FET transistor circuit. In this embodiment of variant, the FET transistor circuits are fed by the resistors.  
           [0017]    In a development of the temperature sensor, an amplifier circuit detects the difference between the voltages at the first and second FET transistor circuits and converts the difference voltage into a control voltage, for example, for an amplifier circuit connected downstream. The amplifier circuit amplifies the difference voltage and matches it to the operating point.  
           [0018]    The FET transistor circuits preferably in each case have at least one MOS transistor that is operated as a diode circuit, i.e. the gate is connected to the drain terminal.  
           [0019]    In order to enable more variable generation of the difference voltage between the two FET transistor circuits, in a further refinement of the invention, cascaded MOS transistors are used for the first and/or the second FET transistor circuit, i.e. FET transistors are connected in series. In this case, the difference voltage can be set over a very wide voltage range in a simple manner by varying the supply currents and the transistor parameters.  
           [0020]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0021]    Although the invention is illustrated and described herein as embodied in a Temperature sensor and method for operating a temperature sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0022]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 shows the basic components of a temperature sensor that provides an output signal for controlling an amplifier circuit, for example;  
         [0024]    [0024]FIG. 2 a  diagrammatically shows the transistor characteristic curve of a MOS transistor for two temperatures;  
         [0025]    [0025]FIG. 2 b  shows the circuit of a MOS transistor when plotting the transistor characteristic curve in accordance with FIG. 2 a ; and FIG. 3 shows a circuit diagram of an exemplary embodiment of the temperature sensor.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a temperature sensor  1  that is realized by two MOS transistors T 1  and T 2  that are each arranged as a diode, i.e. the gate terminal and the drain terminal of the transistors are connected to one another. The transistors T 1  and T 2  are operated with different currents  11  and  12  by using a current generator  11  with two coupled current sources. In this case, the current generator  11  is connected to the gate terminal of each transistor T 1 , T 2 . The source terminal of each transistor T 1 , T 2  is grounded.  
         [0027]    The amplifier circuit  2  has two inputs. One input of the amplifier circuit  2  is connected to the drain terminal of the transistor T 1  and the other input is connected to the drain terminal of the transistor T 2 . The control circuit  2  thus detects the difference between the voltages at the two transistors T 1  and T 2  and converts this difference voltage into a control voltage Ug.  
         [0028]    In the exemplary embodiment shown in FIG. 1, the control voltage Ug is supplied to a plurality of transistors T 6 , which form current sources for differential amplifier stages  3 ,  3 ′,  3 ″. The amplifier stages  3 ,  3 ′,  3 ″, each increase MOS transistors T 7 , T 8  and drain resistors Rd. The current through the individual amplifier stages  3 ,  3 ′,  3 ″ is in each case set by the gate voltage at the transistor T 6 . In this case, the gate voltage can be set such that the current Iv through the amplifier stages is reduced as the temperature decreases.  
         [0029]    However, the control voltage Ug is not restricted to being used to control an amplifier circuit connected downstream. In principle, the control voltage can be used for the compensation of temperature dependences in any desired analog or digital circuit.  
         [0030]    Depending on the current density, a different temperature dependence of the gate voltage results, which is utilized for the temperature measurement. FIG. 2 a  diagrammatically shows the transistor characteristic curve of a MOS transistor (for instance transistor T 1  or the transistor T 2  shown in FIG. 1) for two temperatures TP 1  and TP 2 , where TP 2  is less than TP 1 . The associated circuit for which the transistor characteristic curve is plotted is illustrated in FIG. 2 b . The transistor is operated in a diode circuit during the plotting of the transistor characteristic curve.  
         [0031]    In accordance with FIG. 2 a , there exists an operating point A at which the temperature coefficient is equal to zero, i.e. for a constant drain-source current Id the gate voltage Ug is temperature-independent (Id=I 0  and Ug=U 0 ). The temperature coefficient of the drain-source current Id is positive below this point A and negative above the point A.  
         [0032]    In the case of the circuit shown in FIG. 1, the transistor T 2  is now operated at a current intensity  12  that lies below the temperature-independent operating point A. In this region, the temperature coefficient of Id is positive, i.e. as the temperature increases, a smaller voltage is required for realizing a predetermined current intensity  12 . Thus, in FIG. 2 a , for realizing the current intensity  12 , the voltage U 2 TP 2  is required at the lower temperature TP 2  and the lower voltage U 2 TP 1  is required at the higher temperature TP 1 .  
         [0033]    By contrast, the transistor T 1  is operated at an operating point above the temperature-independent operating point A. In this region, the temperature coefficient of Id is negative i.e. as the temperature increases, a higher voltage is required for realizing a predetermined current intensity I 2 . Thus, in FIG. 2 a , for realizing the current intensity I 1 , the voltage U 1 TP 2  is required at the lower temperature TP 2  and the higher voltage U 1 TP 1  is required at the higher temperature TP 1 .  
         [0034]    Since the transistor T 2  is operated at the operating point  12  and the transistor T 1  is operated at the operating point I 1 , the voltage varies at both operating points I 1  in dependence on the temperature, to be precise with temperature coefficients of different signs. This means that the two temperature responses of the transistors T 1 , T 2  are added together. Consequently, a small change in temperature (from TP 2  to TP 1 ) already leads to a relatively large change in the voltage difference at the two transistors T 1 , T 2  (from U 1 TP 2 -U 2 TP 2  to U 1 TP 1 -U 2 TP 1 ). In this case, the difference voltage (U 1 -U 2 ) increases as the temperature TP increases.  
         [0035]    It can be discerned, in particular, that the difference voltage U 1 Tx-U 2 Tx is greater than for the case where a transistor is operated at the temperature-independent operating point A.  
         [0036]    The difference voltage (U 1 -U 2 ) is converted into the control voltage Ug by the amplifier circuit  2 . The amplifier circuit  2  matches the operating point in addition to performing amplification. In the exemplary embodiment shown in FIG. 1, the control voltage Ug sets the gate voltage of the transistors T 6  in accordance with the desired current Iv.  
         [0037]    When the temperature decreases, the voltage U 1  at the transistor T 1  falls and the voltage U 2  of the transistor T 1  of the temperature sensor  1  rises. Accordingly, the difference voltage (U 1 -U 2 ) also falls. As a result, the control voltage Ug, which represents the gate voltage of the transistors Q 1 , is also reduced. At decreasing temperatures, this leads to a reduced current Iv through the amplifier stages. Consequently, the circuit illustrated ensures that, at low temperatures, only a reduced current flows through the amplifier stages. This leads to a reduced gain which compensates for an increase in the gain that is usually effected at low temperatures.  
         [0038]    [0038]FIG. 3 shows a detailed illustration of an exemplary embodiment of a temperature sensor  1 ′. A current source  11 ′ is formed by three field-effect transistors T 3 , T 4 , T 5 , which constitute a current mirror. The reference current Iref impressed on the transistor T 5  determines the gate voltage at the transistors T 3 , T 4 . The currents I 1 , I 2 , through the transistors T 3 , T 4 , are set by the reference current Iref and the dimensions of the transistors T 3 , T 4 , T 5 .  
         [0039]    The currents I 1 , I 2  through the transistors T 3 , T 4  define the operating points of the further field-effect transistors T 1 , T 2 ′, T 2 ″. The field-effect transistors T 1 , T 2 ′, T 2 ″ serve for the actual temperature measurement. In contrast to FIG. 1, the transistor T 2  is in this case replaced by two series-connected transistors T 2 ′, T 2 ″. As an alternative, it is also possible for more than two transistors to be connected in series.  
         [0040]    The amplifier circuit  2 ′ further processes the temperature-dependent voltage U 1 -U 2 , i.e. the difference between the voltages at the transistor T 1  and the transistors T 2 ′ and T 2 ″. The amplifier circuit  2 ′ is formed by a subtraction circuit—known per se—for the two input voltages U 2  and U 1 . The subtraction circuit has an inverting operational amplifier  21 , to which the input voltage U 1  is fed to the noninverting input via a voltage divider. Since the ratio of the resistors R 1 , R 2  at the inverting input and at the noninverting input is identical, only the difference between the input voltages (U 1 -U 2 ) is amplified and output as the control voltage Ug.  
         [0041]    In this case, the input currents into the amplifier (through R 1 ) are intended to be negligible such that the operating point currents I 1 , I 2  are not corrupted.  
         [0042]    Through varying the currents I 1  and I 2  and the parameters of the transistors T 1 , T 2 ′, T 2 ″, the difference voltage (U 1 -U 2 ) can be set virtually arbitrarily in the circuit shown in FIG. 3. As a result, this temperature-dependent voltage can be converted to the required voltage Ug by using the operational amplifier  21 .