• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
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  • 优先权
    • 专利摘要:
    The invention includes a temperature compensated constant current source. Specifically, two PNP transistors are implemented in a circuit wherein one of the transistors is used to provide a DC feedback to the other transistor to stabilize the output current. Temperature independence is effected due to the base emitter saturation voltage occurring when the emitter resistance is equal to the collector resistance at large values for .beta., the common emitter current gain. The circuit provides a stable constant current source independent of temperature variations.
    公开号:CA2298929A1
    申请号:C2298929
    申请日:2000-02-17
    公开日:2000-08-18
    发明作者:Normand T. Lemay, Jr.
    申请人:Itron Inc；
    IPC主号:G05F1-567
    专利说明:
    Attorney Docket No.: 1725.76US01 TEMPERATURE COMPENSATED CONSTANT CURRENT SOURCE Related A~nli This application claims the benefit of U.S. Provisional Application No.60 / 120,641, filed February 18, 1999. Backeround of the Invention Field of the Invention The present invention relates to radio frequency data handling. More specifically, the present invention relates to a constant current source circuit. Description of Prior Art Prior art systems generally implement a bandgap reference to establish a voltage or current reference which is then mirrored to a current sourcing or sinking transistor with a high output impedance. Bandgap refers to the energy gap which results from conduction or empty band located at a higher energy level. The width of the bandgap is equal to the amount of energy a valence electron must gain to break its covalent bond and become a conduction band electron. Moreover, in integrated circuits (IC) containing a number of amplifier stages, a constant direct current (DC) is typically generated at one location and reproduced at many other locations for biasing the different transistors in the circuit. Attorney Docket No.: 1725.76US01 Referring to Fig. 1, a prior art circuit configuration 10 for a current sink is shown. Specifically, the output current in the collector lead, I°, is being pulled from a load connected to a positive voltage. The circuit 10 is equivalent to the classical biasing circuit 11. For circuit 10 to provide the load with a constant current, transistor 12 should be maintained in the active mode at all times. This will occur if the collector is kept at a voltage equal to or higher than that of the base. The output current of the circuit is given by: V,~R~/(R,+R,)~-VBE RE+(R, //R~)/((3+1) Thus, in order for I to be stable and predictable, its dependence on (3, common emitter current gain, and VBE should be minimized. The prior art methods use expected ranges of resistance and voltage using the equations defined hereinbelow to arrive at optimal values at which a constant current may be generated. -(R, //R,) ARE(3 + 1 Rz Vi Rt+Rz ~VBe Attorney Docket No.: 1725.76US01 Accordingly the ohmic emitter resistance RE and the base emitter saturation voltage VBE must lie within a set range of values to provide a constant current source or sink. Referring now to Fig. 2, a prior art temperature -compensated current source circuit 16 is shown. Specifically, circuit 16 is configured to produce an output current Io independent of VBE and therefore independent of temperature. The circuit diagram shows transistors 18 and 20 connected as diodes by shorting the collector to the base. This connection results in a two-terminal device whose i-v characteristic is identical to the ie-vBE characteristics of an identical transistor. The reason for the identical characteristics is that the diode-connected transistor will behave internally as a transistor operating in the active mode. Therefore, the collector and base currents of a diode connected transistor have a ratio equal to (3. Still referring to prior art circuit 16, the three transistors 18, 20 and 22 are preferably identical devices fabricated on the same IC silicon chip. Thus, the three transistors 18, 20, 22 have substantially matched characteristics. Therefore, for equal emitter currents, the three devices will have equal base-emitter voltages.Further, circuit 16 is designed such that the current in transistors 18 and 20 is equal to the current in transistor 22, which is the output current. If ~i is assumed to be much larger than unity, the base current of transistor 22 can be neglected. It can be Attorney Docket No.: 1725.76US01 shown that the condition for equal currents in all transistors for the output current Ito be independent of VBE is given by the following equation: R -R,-RE 1- 2Vsa V, Which is approximately R,.R, -RE Under this condition the input current Io will be given by I~-a2R~ ' g O1 Since a is a relatively constant parameter, the circuit_(current sink) will draw a stable .and predictable current. The prior art circuits discussed hereinabove are current sinks. As is clearly shown, in ,current sink circuits the output terminal pulls a constant current from a load connected to a more positive supply voltage. Thus, the output terminal needs to be kept at a potential higher than that of the base. Generally various applications require that the dual circuit that pushes a constant current be connected Attorney Docket No.: 1725.76US01 to a substantially negative voltage. Such a circuit is called a current source and is usually implemented using a PNP type transistor. Fig. 3 represents such a circuit.Circuit 30 includes transistor 32 and pushes a constant current I into a load Rconnected to a negative voltage V-. The output current is defined by the following mathematical relationship: V, ~R, / (R, +R~) ~+ VBE I° a RE+(R,//R,)/(~i+1) This circuit will operate as long as the collector voltage remains lower or equal to that of the base. Similar to the current sink circuits discussed above, the prior art circuit 30 depicted in Fig. 3 can be made unaffected by the value of (3 by making (R1//RZ)/((3+1) much smaller than RE. Also the technique of using two additional diode-connected transistors to eliminate the VBE in the above output current equation can be implemented in the construction of a prior art circuit 30. Generally biasing IC design is based on the use of constant-current sources. On an IC chip with a number of amplifier stages a constant DC current is generated at one'location and is then reproduced at various other locations for biasing the various amplifier stages. This method has the advantage that the bias Attorney Docket No.: 1725.76US01 currents of the various stages track each other in case of changes in power-supply voltage or in temperature. One of the basic problems encountered in the design of transistor amplifiers is that of establishing and maintaining the proper DC emitter current and collector to emitter voltage. The biasing problem is generally encountered because of the change of transistor parameters with temperature and the variation of the parameters between transistors of the same type. Specifically, parameters such as, for example, hFE' ICO'UBE are susceptible to variation. Thus the factors which must be considered in the design of transistor bias circuits include the specified maximum and minimum values of current gain (hFE) at the operating point for the type of transistor used, the variation of hFE, the variation of base to emitter voltage drop (VBE) with temperature and the tolerance of the resistors used in the bias networks including tolerance of the supply voltages. A common technique for providing current reproduction is the current mirror. Generally it consists of two matched first and second transistors with their bases and emitters connected together. Further the first transistor is connected as a diode by shorting its collector to its base. The circuit fed by the collector of the second transistor ensures active-mode operation at all times by keeping its collector voltage higher than that of the base. The performance of the Attorney Docket No.: 1725.76US01 current mirror is independent of the voltage as long as the second transistor is maintained in the active mode. Further, prior art techniques which utilize biasing arrangements generally involve establishing a stable and predictable DC operating point inside the active region of operation of the transistor. A stable operating point is one that is almost independent of variations in the device voltage or current loss. These parameters vary with temperature. Notably the temperature variance is high for different units of the same device type. Precision constant current supply circuits are well known in the art. In many applications requiring precise measurement of DC parameters, it is more convenient to use a current reference rather than a voltage reference. A precision circuit could be designed to provide a current reference supply similar to a voltage reference supply. A reference amplifier is implemented to act together with a transistor to maintain the voltage constant across a resistor at the reference voltage.Further, transistors may be configured to provide opposite and equal currents such that they can be made to compensate for changes in ambient temperature. This requires careful selection of the transistors and their bias points. The temperature coefficient may be minimized by trimming the circuit values. This could be done by .;adjusting the bias current to the collector of the reference amplifier which requires changing the value of the resistor at the base. Attorney Docket No.: 1725.76US01 Although prior art circuits provide constant current sources, most use complicated circuitry and configurations. Moreover, prior art systems which provide temperature compensation are dependent on the base emitter voltage unless mirrored to higher stability current reference, thus requiring complicated circuitry and various components which typically result in unstable and unrepeatable current and voltage values. Therefore there is a need for a highly efficient and reliable stable current source which is not affected by temperature variations and is independent of base emitter voltage. Further, there is a need for a circuit for use as a current reference and source with minimal components and for a current output which can be mirrored to multiple stages with substantially no marked degradation in performance. Summary of the Invention It is the object of the present invention to provide a temperature compensated constant current source using minimal components. It is yet another object of the present invention to provide a simple circuit preferably for use as an independent current reference and source with a high .;output impedance.~~' , It is yet a further object of the invention to provide a temperature compensated constant current source. The present invention advantageously Attorney Docket No.: 1725.76US01 implements the principle of compensation which involves a method of neutralizing some undesirable characteristic of an electronic circuit. For example a crystal may have a positive temperature coefficient and therefore its frequency might increase with increasing temperature. Thus, in an oscillator that must be stable under varying temperature conditions, this characteristic of the crystal can be neutralized by placing a capacitor whose value increases with temperature and pulls the crystal frequency lower. Yet another object of the present invention is the generation of a constant current source at stable temperatures while eliminating the dependence of the circuit on base emitter saturation voltage. It a further object of the invention to provide a circuit for use as a current reference and source with minimal components and to be mirrored to multiple stages with minimal degradation in performance. Yet a further object of the invention is to provide a simple biasing configuration and scheme to enable efficient and reliable current referencing at stable temperature independence. Another object of the invention is to provide a circuit wherein the accuracy of the reference current and its constancy is exclusively dependent on the .;voltage supply and.the resistor tolerances. Attorney Docket No.: 1725.76US01 Other advantages and inventive features will become apparent after considering the following description thereof in conjunction with the illustration of the preferred embodiment of the invention shown in the accompanying drawings. The present invention is a temperature compensated constant current source. Specifically, two PNP transistors are implemented in a circuit wherein one of the transistors is used to provide a DC feedback to the other transistor to stabilize the output current. Temperature independence is effected due to the base emitter saturation voltage occurring when the emitter resistance is equal to the collector resistance at large values for (3, the common emitter current gain. The circuit provides a stable constant current source independent of temperature variations. Brief Description of the Drawings Fig. 1 is a schematic diagram of a circuit representing prior art configurations for a constant current sink; Fig. 2 is a schematic diagram of a _circuit representing prior art configurations for a temperature compensated current sink; Fig. 3 is a schematic diagram of a circuit representing prior art configurations for .a current source;Fig. 4 is'a schematic diagram of a circuit representing the circuit of the present invention; and Attorney Docket No.: 1725.76US01 Fig. 5 is a schematic diagram of a circuit representing the implementation of the present invention mirrored to multiple stages. Description of the Preferred Embodiment Referring to the circuit 40 depicted in Fig. 4, a temperature compensated constant current source is shown in accordance with the present invention. Specifically the invention implements PNP transistors 41 and 42 with the current output lout. Transistor 42 is implemented to provide DC feedback to transistor 41 which stabilizes the output current to be set by V~~/ (2 RX ). The circuit of the present invention enables eliminating, substantially, the effect of temperature. More specifically, temperature independence is achieved when RE=R~ when (3 is sufficiently large. The output current is dependent only on Vcc 44, and resistors RE, R X and Rc if (3 is large. Unlike the prior art, circuit 40 provides a current reference and a current source with a minimum number of components. , For example when compared to the prior art circuit 16, circuit 40 of the present invention uses only two transistors, namely transistors.41 and 42. Thus, the invention provides a simple circuit which can be implemented as an independent current reference andvsource with a high output impedance. Attorney Docket No.: 1725.76US01 One of the principles governing the present invention includes a circuit where the collector current is defined by the following mathematical relationship:RE V cc 2V aERs V aE IcE= ~Rc+R$) Rx _ (Rc+RE)Rx + Rx R E V~~ - 2 VBERE + VBE (R~+RE) (Rc + RE) Rx Temperature compensation for VBE occurs at R~ = RE Thus, V~ IcE- 2Rx As can be seen from the above equation, the collector current is independent of VBE when RE = R~, and since VBE varies with temperature, and is no longer in the expression defined above, this circuit is no longer temperature dependent. The bias is made temperature independent for VBE by using direct current feedback with the output buffer stage. The circuit of the present invention may be used in one exemplary application as a current source for a diode detector.In such use, the circuit has demonstrated very repeatable performance of the diode detector over a wide range of temperature variance and from unit to unit. Attorney Docket No.: 1725.76US01 One of the advantages of the present invention includes its unique circuit structure and topology which requires that only the two biasing circuits Rc and RE be equal and that ~i (common emitter current gain) be significantly large to ensure temperature stability by eliminating the parametric dependence on VBE. The circuit can be mirrored to multiple stages. Specifically, referring to Fig. 5, circuit 50 shows a mirrored version of the present invention. Particularly, transistors 52, 54 and voltage 60 including the resistors RE, R~, RX form the main elements of the present invention. The circuit produces output current 61. In an integrated circuit having a number of amplifier stages, for example, a constant DC current is usually generated at one location and reproduced at many other locations for biasing the different transistors in the circuit. This approach in the prior art is called current mirror. Current mirror consists of two matched transistors with their bases and emitters connected together. The output circuit 61 of Fig. 5 of the present invention is mirrored and reproduced at current outputs of transistors 56 and 58.Further, for prior art current mirrors with multiple outputs, the errors due to the base currents accumulate and the output currents are not uniformly reproducible. The present invention overcomes these limitations because of the DCfeedback provided by Q2. Attorney Docket No.: 1725.76US01 While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes, variations and modifications can be made therein without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true scope and spirit of the invention.
    权利要求:
    Claims (22)
    [1] 1. A temperature compensated current source having an output current I out wherein a first and second transistors Q1 and Q2 are structured in a circuit to provide DC feedback and to stabilize the output current I out comprising:first and second terminals;said first and second transistors each having a base, an emitter and a collector;a first resistor coupled in series to the collector and a second resistor coupled in series to the emitter of said second transistor;a third resistor coupled to the emitter of said first transistor;a DC voltage (V cc) supply at said first terminal connected in series to said second resistor and said third resistor;said base of said first transistor being connected to the collector of said second transistor; and said base of said second transistor being connected to the emitter of said first transistor, said first and second transistors, said first, second and third resistors including said voltage supply forming the circuit wherein the emitter of said first transistor forms the second terminal for the output I out which is set by V cc/2R, where R is equivalent to said third resistor.
    [2] 2. The temperature compensated current source of claim 1 wherein said first and second transistors are of the same conductivity PNP type.
    [3] 3. The temperature compensated current source of claim 1 wherein said first and second resistors are of equal resistance value.
    [4] 4. A temperature compensated current source having first and second terminals wherein the first terminal is adapted to be connected to a do source voltage and the second terminal is adapted to be connected to an electrical load, the current source comprising:means for making a bias output current, I out, temperature independent of V BE; and means for providing DC feedback and for stabilizing said output current.
    [5] 5. The temperature compensated current source of claim 4 wherein said means for making the bias temperature independent includes means for equalizing emitter and collector resistance between a first and second transistors wherein the first transistor is implemented in a circuit to provide said DC feedback to the second transistor and to stabilize the output current.
    [6] 6. The temperature compensated current source of claim 5 wherein total temperature independence is acquired by a circuit structure which maintains the emitter and collector resistance of the first transistor equal.
    [7] 7. A method of making a bias temperature independent, for a known base emitter voltage (V BE), the circuit-implemented method comprising the steps of:implementing a do voltage feedback with an output stage in the circuit;structuring resistors R C and R E to be equal to generate temperature compensation; and structuring a first and second transistor to be equal to allow tracking.
    [8] 8. The method according to claim 7 wherein said step of implementing a dc voltage feedback includes providing a dc collector voltage supply at a terminal connected in series to a plurality of resistors and a transistor.
    [9] 9. A circuit system which implements principles of compensation to neutralize temperature effects and provide a constant current reference and source wherein the accuracy of the current output is exclusively dependent on the voltage supply and the resistor tolerances, the circuit system comprising:a first and second transistors each having a base, an emitter and a collector;a first resistor coupled in series to the collector and a second resistor coupled in series to the emitter of said second transistor;a third resistor coupled to the emitter of said first transistor;a DC voltage supply connected in series to said second resistor and said third resistor;said base of said first resistor being connected to the collector of said second transistor; and said base of said second transistor being connected to the emitter of said first transistor;said first and second transistors, said first, second and third resistors including said voltage supply forming said circuit system wherein an output current at the emitter of said first transistor is constant and set by the value of V CC/2R, where R is equivalent to said third resistor, and is independent of temperature.
    [10] 10. A circuit for providing a constant current source, comprising: substantial temperature independence, the independence being achievable by setting the resistance values of a first emitter biasing resistor and a second collector biasing resistor to be equal.
    [11] 11. The circuit of claim 10 providing a current reference and a current source and having no more than two transistors.
    [12] 12. The circuit of claim 11 being mirrorable in multiple stages with minimal degradation in performance.
    [13] 13. The circuit of claim 11 being substantially independent of a base emitter voltage.
    [14] 14. The circuit of claim 11 having a high output impedance.
    [15] 15. The circuit of claim 11 providing for current referencing, the accuracy of the current referencing being dependent only on a voltage supply and tolerances of the first emitter biasing resistor and the second collector biasing resistor.
    [16] 16. The circuit of claim 10 wherein a bias is made substantially temperature independent for V BE by using direct current feedback with an output buffer stage.
    [17] 17. A circuit being operably coupled to a diode detector for providing a constant current source to the diode detector, comprising:substantial temperature independence, the independence being achievable by setting the resistance values of a first emitter biasing resistor and a second collector biasing resistor to be equal.
    [18] 18. The circuit of claim 17 providing a current reference and a current source and having no more than two transistors.
    [19] 19. The circuit of claim 18 being mirrorable in multiple stages with minimal degradation in performance.
    [20] 20. The circuit of claim 18 being substantially independent of a base emitter voltage.
    [21] 21. The circuit of claim 18 having a high output impedance.
    [22] 22. The circuit of claim 18 providing for current referencing, the accuracy of the current referencing being dependent only on a voltage supply and tolerances of the first emitter biasing resistor and the second collector biasing resistor.
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    同族专利:
    公开号 | 公开日
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    法律状态:
    2003-02-17| FZDE| Dead|
    优先权:
    申请号 | 申请日 | 专利标题
    US12064199P| true| 1999-02-18|1999-02-18||
    US60/120,641||1999-02-18||
    US32505099A| true| 1999-06-03|1999-06-03||
    US09/325,050||1999-06-03||
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