Patent Publication Number: US-2018052477-A1

Title: Low voltage bandgap reference generator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application Ser. No. 62/376,933, filed Aug. 19, 2016, titled “Low Voltage Band-Gap,” which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The techniques described herein relate generally to bandgap reference voltage generators. 
     2. Discussion of the Related Art 
     Bandgap reference voltage generators are widely used in integrated circuits as a way to provide a constant voltage reference. Bandgap reference voltage generators are designed to produce a fixed voltage despite power supply variations, temperature fluctuations, fabrication tolerances, and variable loading conditions. 
     SUMMARY 
     Some embodiments relate to reference voltage generator circuit. The circuit has a first branch, having a first current and a first voltage, a second branch, having a second current and a second voltage, and a third branch, having a third current and a third voltage. The circuit has an amplifier that couples the first voltage to the second voltage. The circuit also has a feedback circuit that couples the third voltage to at least one of the first or second voltages. 
     Some embodiments relate to a reference voltage generator circuit including: a first branch having a first transistor, a first impedance in series with the first transistor and a first terminal between the first transistor and the first impedance; a second branch having a second transistor, a second impedance in series with the second transistor and a second terminal between the second transistor and the second impedance; a third branch having a third transistor, a third impedance in series with the third transistor, a fourth transistor coupled between the third transistor and the third impedance and a third terminal coupled between the third transistor and the fourth transistor; a first amplifier having: a first input coupled to the first terminal; a second input coupled to the second terminal; and a first output coupled to respective control terminals of the first, second and third transistors; a second amplifier having: a first input coupled to the third terminal; and a second input coupled to the first terminal or the second terminal; and a second output coupled to a control terminal of the fourth transistor. 
     The foregoing summary is provided by way of illustration and is not intended to be limiting. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like reference character. For purposes of clarity, not every component may be labeled in every drawing. The drawings are not necessarily drawn to scale, with emphasis instead being placed on illustrating various aspects of the techniques and devices described herein. 
         FIG. 1  shows a bandgap reference voltage generator. 
         FIG. 2  shows an embodiment of a bandgap reference voltage generator with an additional transistor and amplifier. 
         FIG. 3  shows another embodiment of the circuit of  FIG. 2  with alternate electrical connections. 
         FIG. 4  shows another embodiment of the circuit of  FIG. 2  with additional impedance components. 
         FIG. 5  shows another embodiment of the circuit of  FIG. 2  with an additional output stage. 
         FIG. 6  shows another embodiment of the circuit of  FIG. 5  with a third output stage. 
     
    
    
     DETAILED DESCRIPTION 
     A bandgap voltage reference may be an important part of many integrated circuit solutions. Bandgap voltage references are ideally independent of power supply voltage, fabrication tolerances, and temperature. However, with modern transistors being scaled-down in size, power supply voltages have been reduced and become more sensitive to temperature fluctuations and fabrication tolerances. As the power supply voltage is reduced it becomes more challenging to produce a stable bandgap voltage reference. Described herein is a bandgap voltage reference suitable for low power supply voltages. 
       FIG. 1  shows a bandgap voltage reference generator  100 . The bandgap voltage reference generator  100  comprises three transistors,  102 ,  104 , and  106 , three impedance components,  108 ,  110 , and  112 , and an amplifier  114 . Transistors  102  and  104  may be of substantially similar sizes. Transistor  106  may be of substantially similar size as transistors  102  and  104  or of a different size proportional to a desired output current. The transistors  102 ,  104 , and  106  may be field-effect transistors (FETs), bipolar junction transistors (BJTs) or any other suitable types of transistors. The impedance components  108 ,  110 , and  112  may include any of transistors, diodes, resistors or any combination thereof. The amplifier  114  may receive the drain voltages  102 D and  104 D of transistors  102  and  104 , respectively, as its two inputs (non-inverting + and inverting −), coupling the drain voltages  102 D and  104 D to each other at a virtual short. The amplifier  114  may provide its output to the control terminals (e.g., gates) of transistors  102 ,  104 , and  106  to control transistors  102 ,  104 , and  106  based on its output. The bandgap voltage reference generator  100  includes three branches B 1 , B 2  and B 3 . Branch B 1  includes transistor  102  and impedance component  112  connected in series. Branch B 2  includes transistor  104  and impedance component  110  connected in series. Branch B 3  includes transistor  106  and impedance component  108  connected in series. Each transistor  102 ,  104 , and  106  may allow a current to flow through the series-connected components of its respective branch B 1 , B 2 , and B 3 . In some embodiments, it may be advantageous for the currents in branches B 1 , B 2 , and B 3  to be approximately equal. Since the gates of the transistors  102 ,  104 , and  106  may be connected to the output of the amplifier  114 , the source voltages  102 S,  104 S, and  106 S, of the transistors may be connected to the supply voltage VDD, the drain voltages  102 D and  104 D of transistors  102  and  104  are coupled to one another through the inputs of the amplifier  114 , the currents through transistors  102  and  104 , and consequently branches B 1  and B 2 , may be approximately the same. Transistor  106  does not have a drain voltage  106 D coupled to amplifier  114 . Due to fluctuations in temperature, fabrication tolerances, supply voltage, and/or load, the current through branch B 3  may vary relative to the currents in the branches B 1  and B 3 . As a result, though the impedance of impedance component  108  may be constant, fluctuations in the current through transistor  106  on branch B 3  may vary the output voltage VOUT. 
     The output impedance at VOUT may be provided primarily by the drain resistance of transistor  106 . However, the scaling down of semiconductor fabrication technology has reduced the size of modern transistors. As a result, if transistor  106  is small, the output impedance at VOUT may be small. The branch B 3  on which transistor  106  is connected then may become a weak link for the power supply rejection performance of the bandgap voltage reference generator  100 , as fluctuations in the supply voltage VDD may pass easily the output voltage VOUT due to the low impedance of transistor  106 . 
     The techniques and circuits described herein enable improving the performance of the bandgap voltage reference generator.  FIG. 2  shows an example of a bandgap voltage reference generator  200 , according to some embodiments. As shown in  FIG. 2 , the bandgap voltage reference generator  200  includes a feedback circuit  206 . Feedback circuit  206  couples the drain  106 D of transistor  106  to the drain  104 D of transistor  104  at a virtual short. As a result, the drain voltage of transistor  106  is held approximately equal to the drain voltages of transistors  102  and  104 . Further, the feedback circuit may include at least one component between the transistor  106  and VOUT, which can increase the output impedance of the bandgap voltage reference generator  200 . 
     As shown in  FIG. 2 , the feedback circuit  206  may comprise a transistor  202  connected between transistor  106  and impedance component  108 , and an amplifier  204 . The gate of transistor  202  may be driven by the output of amplifier  204 . The amplifier  204  may have an inverting input connected to the junction between transistors  106  and  202 , at  106 D. The amplifier  204  may have a non-inverting input connected to the drain of transistor  104 , at  104 D. In other embodiments, the non-inverting and inverting inputs of amplifier  204  may be reversed; the non-inverting input of amplifier  204  may be connected to the drain of transistor  102 , at  102 D. Amplifier  204  may force the drain voltage of transistor  106 , at  106 D, to be approximately equal to the drain voltage of transistor  104 , at  104 D, which is equal to the drain voltage of transistor  102 , at  102 D, due to amplifier  114 . Any excess voltage caused by fluctuations in temperature or in the value of impedance component  108  may be dropped across transistor  202 . The introduction of feedback circuit  206  may improve the accuracy of the output voltage VOUT by more accurately matching the current in transistor  106  with the currents in transistors  102  and  104 , as drain voltages  102 D,  104 D, and  106 D are approximately the same. The introduction of transistor  202  may also improve the power supply rejection performance of the bandgap voltage reference generator  200 , since transistor  202  is in series with transistor  106 , which increases the impedance seen at the output VOUT. 
       FIG. 3  shows another embodiment of the bandgap voltage reference generator in which the non-inverting input of amplifier  104  is connected to the drain of transistor  102 , at  102 D, rather than the drain of transistor  104 . Since amplifier  114  holds the drains of transistors  102  and  104  at the same voltage, the embodiment of  FIG. 3  operates similarly to the embodiment of  FIG. 2 . 
       FIG. 4  shows another embodiment of a bandgap voltage reference generator  400 , with the addition of impedance components  402  and  404 . Impedance components  402  and  404  may be DC level shifting components. Impedance component  402  may be connected between the drain of transistor  104 , at  104 D, and impedance component  110 . Impedance component  404  may be connected between the drain of transistor  102 , at  102 D, and impedance component  112 . The rest of the circuit may be substantially similar to the circuit of  FIG. 3 . In other embodiments, one input of the amplifier  204  may be tied to the drain of transistor  104 , at  104 D, instead of transistor  102 , at  102 D, as shown in the embodiment of  FIG. 2 . The impedance components  402  and  404  may include any of transistors, diodes, resistors or any combination thereof. While impedance components  402  and  404  are shown as separate from impedance components  110  and  112 , in some embodiments the same effect may be obtained by increasing the impedance of impedance components  110  and  112 . The DC level shifting impedance components  402  and  404  may increase the drain voltages of transistors  102  and  104 , at  102 D and  104 D respectively. Since the drain voltage of transistor  106 , at  106 D, may be tied to the drain voltages of transistors  102  and  104 , at  102 D and  104 D, through amplifiers  204  and  114 , the drain voltage of transistor  106 , at  106 D, may increase as well, allowing a higher value of the output reference voltage VOUT to be implemented. The impedance components  404  and  402  may be chosen so that, regardless of the value of the output voltage VOUT to be implemented, there is a sufficient voltage drop across transistor  202 . 
       FIG. 5  shows another embodiment of a bandgap voltage reference generator  500  with two output stages. The first output stage may comprise transistors  106  and  202  and impedance component  108 , as shown on branch B 3  in previous embodiments. The second output stage may comprise two transistors,  502  and  504 , and an impedance component  506  in branch B 4 . The source of transistor  502 , at  502 S may be connected to the supply voltage VDD, and the gate of transistor  502  may be connected to the gates of transistors  102 ,  104 , and  106  through the output of amplifier  114 . The source of transistor  504  may be tied to the drain of transistor  502 , at  502 D. The gate of transistor  504  may be tied to the gate of transistor  202  through the output of amplifier  204 . One input of amplifier  204  is shown as connecting to the drain of transistor  104 , at  104 D, but in other embodiments may be connected to the drain of transistor  102 , at  102 D, as shown in  FIG. 3 . Impedance component  506  is connected between the drain of transistor  504  and a lower reference node, shown in this embodiment as ground. The second output stage may pass a current substantially similar to the current in the first output stage, as the drain and gates of transistors  502  and  106  may be tied, and the gates of transistors  504  and  202  may be tied. As a result, the second output reference voltage VOUT 2  may be substantially similar to the first output reference voltage VOUT 1 . While only two output stages are shown in  FIG. 5 , in other embodiments any number of additional output stages are possible, with the additional output stages configured and connected in a substantially similar fashion to the second output stage on branch B 4 , as shown in  FIG. 6 . 
       FIG. 6  shows another embodiment of a bandgap voltage reference generator  600 . The bandgap voltage reference generator  600  may be the same as or substantially similar to the one of  FIG. 5 , with an additional output stage shown as branch B 5 . Branch B 5  may comprise transistors,  602  and  604 , and an impedance component  606 . The source of transistor  602 , at  602 S, may be connected to the supply voltage VDD, and the gate of transistor  602  may be connected to the gates of transistors  102 ,  104 ,  106 , and  502  and the output of amplifier  114 . The source of transistor  604  may be tied to the drain of transistor  602 , at  602 D. The gate of transistor  604  may be tied to the gate of transistors  202  and  504  and the output of amplifier  204 . One input of amplifier  204  is shown as connecting to the drain of transistor  104 , at  104 D, but in other embodiments may be connected to the drain of transistor  102 , at  102 D, as shown in  FIG. 3 . Impedance component  606  is connected between the drain of transistor  604  and a lower reference node, shown in this embodiment as ground. The third output stage may pass a current substantially similar to the currents in the first output stage and the second output stage, as the drain and gates of transistors  602 ,  502 , and  106  may be tied, and the gates of transistors  604 ,  504 , and  202  may be tied. As a result, the third output reference voltage VOUT 3  may be substantially similar to the first output reference voltage VOUT 1  and the second output reference voltage VOUT 2 . While only three output stages are shown in  FIG. 6 , in other embodiments any number of additional output stages are possible, with the additional output stages configured and connected in a substantially similar fashion to the second output stage on branch B 4  and/or the third output stage on branch B 5 . 
     In the examples above, the use of the term “substantially similar current” is used on the assumption that the transistors passing the current are of substantially similar sizes. In some embodiments, a transistor of the one or more output stages may be sized relative to transistors  102  and  104  to pass current in the output stage with approximately the same magnitude relative to the currents passed by transistors  102  and  104 . For example, if the transistor  106  is approximately twice the size of transistors  102  and  104 , transistor  106  may pass a current approximately twice the size of the currents passed by transistors  102  and  104 . In embodiments with multiple output stages, such as the one shown in  FIG. 5 , the transistors of the output stages need not be of the same size. For example, in  FIG. 5 , transistors  106  and  502  may be of substantially similar sizes to pass substantially similar currents, or of different sizes to pass currents of proportionally different sizes. Furthermore, while the figures herein show PMOS type transistors, NMOS type transistors could also be used by altering the source connections from VDD to Ground. Similarly, other types of transistors could be used, such as bipolar junction type transistors. 
     Various aspects of the apparatus and techniques described herein may be used alone, in combination, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing description and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.