# Patent

## Title
Constant output reference voltage circuit

## Abstract

The voltage reference circuit includes: a first MOS transistor; a second MOS transistor including a gate terminal connected to a gate terminal of the first MOS transistor and having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value and a K value of the first MOS transistor; a current mirror circuit flowing a current based on a difference between the absolute values of the threshold values of the first MOS transistor and the second MOS transistor; a third MOS transistor flowing the current; and a fourth MOS transistor having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value of the third MOS transistor and flowing the current.

## Background
RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-211220 filed on Sep. 27, 2011, the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage reference circuit.
2. Background Art
FIG. 6 is a circuit diagram illustrating a conventional voltage reference circuit.
The conventional voltage reference circuit includes PMOS transistors 101 to 103, NMOS transistors 201 to 204 and 301, an output terminal 401, a power supply terminal 501, an earth terminal 502, and a resistor 601. A threshold voltage (hereinafter referred to as Vtnl) of the NMOS transistor 301 is lower than a threshold voltage (hereinafter referred to as Vtnh) of the NMOS transistors 201 to 204. The PMOS transistors 102 and 103 constitute current mirror circuits with the PMOS transistor 101 to flow a drain terminal current of a desired ratio to a drain terminal current of the PMOS transistor 101. The NMOS transistor 204 constitutes a current mirror circuit with the NMOS transistor 203 to flow a drain terminal current of a desired ratio to a drain terminal current of the NMOS transistor 203.
Source terminals of the PMOS transistors 101 to 103 are connected to the power supply terminal. Gate terminals of the PMOS transistors 102 and 103 are connected to a gate terminal and a drain terminal of the PMOS transistor 101 and a drain terminal of the NMOS transistor 201. Gate terminals of the NMOS transistors 201 and 202 are connected to a drain terminal of the NMOS transistor 201 and a drain terminal of the PMOS transistor 102. A source terminal of the NMOS transistors 202 is connected to the earth terminal. One end of the resistor 601 is connected to a source terminal of the NMOS transistor 201, and the other end thereof is connected to the earth terminal. Gate terminals of the NMOS transistors 203, 204, and 301 are connected to drain terminals of the NMOS transistors 203 and the PMOS transistor 103. Source terminals of the NMOS transistors 203 and 204 are connected to the earth terminal. A drain terminal of the NMOS transistor 301 is connected to the power supply terminal. The output terminal 401 is connected to a drain terminal of the NMOS transistor 204 and a source terminal of the NMOS transistor 301.
Respective K values of the NMOS transistors 201 to 204 and 301 are K201, K202, K203, K204, and K301, and a resistance value of the resistor 601 is R601.
The PMOS transistors 101 and 102, the NMOS transistors 201 and 202, and the resistor 601 constitute a constant current circuit. For example, in a case where each transistor works in a saturation region, if K values of the PMOS transistors 101 and 102 are equal to each other, currents flowing in the PMOS transistors 101 and 102 are equal to each other, and their current value takes 0A or a certain constant current value (hereinafter referred to as IK). When a start circuit is provided so that the current will not be 0A, the PMOS transistors 101 and 102, the NMOS transistors 201 and 202, and the resistor 601 work as a constant current circuit. The constant current IK is represented by the following formula:




  
    
      
        
          
            I
            K
          
          =
          
            
              1
              
                R
                601
                2
              
            
            ·
            
              
                (
                
                  
                    1
                    
                      
                        K
                        201
                      
                    
                  
                  -
                  
                    1
                    
                      
                        K
                        202
                      
                    
                  
                
                )
              
              2
            
          
        
        ⁢
        
          

        
        ⁢
        
          
            where
            ⁢
            
              
            
            ⁢
            K
            ⁢
            
              
            
            ⁢
            201
          
          >
          
            K
            ⁢
            
              
            
            ⁢
            202.
          
        
      
    
    
      
        (
        1
        )
      
    
  




The constant current IK is mirrored to the PMOS transistor 103, and a drain terminal current of the PMOS transistor 103 is mirrored to the NMOS transistor 204. For example, when all the transistors of FIG. 6 work in a saturation region, K values of the PMOS transistors 101 and 103 are equal to each other, and when K values of the NMOS transistors 203 and 204 are equal to each other, the constant current IK flows in the NMOS transistors 204 and 301. When respective gate-source voltages necessary for the NMOS transistors 204 and 301 to flow the constant current IK are assumed VGS204K and VGS301K, a voltage (hereinafter referred to as VrefK) of the output terminal 401 is represented by the following formula with the use of the formula (1):




  
    
      
        
          
            
              
                V
                refK
              
              =
              
              ⁢
              
                
                  V
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    204
                    ⁢
                    K
                  
                
                -
                
                  V
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    301
                    ⁢
                    K
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  
                    
                      I
                      K
                    
                  
                  ⁢
                  
                    (
                    
                      
                        1
                        
                          
                            
                              K
                              204
                            
                          
                          ⁢
                          
                            
                          
                        
                      
                      -
                      
                        1
                        
                          
                            K
                            301
                          
                        
                      
                    
                    )
                  
                
                +
                
                  (
                  
                    
                      V
                      tnh
                    
                    -
                    
                      V
                      tnl
                    
                  
                  )
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  
                    
                      1
                      
                        R
                        601
                      
                    
                    ·
                    
                      (
                      
                        
                          1
                          
                            
                              K
                              201
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              202
                            
                          
                        
                      
                      )
                    
                  
                  ⁢
                  
                    (
                    
                      
                        1
                        
                          
                            K
                            204
                          
                        
                      
                      -
                      
                        1
                        
                          
                            K
                            301
                          
                        
                      
                    
                    )
                  
                
                +
                
                  (
                  
                    
                      V
                      tnh
                    
                    -
                    
                      V
                      tnl
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          where
          ⁢
          
            
          
          ⁢
          K
          ⁢
          
            
          
          ⁢
          201
        
        >
        
          K
          ⁢
          
            
          
          ⁢
          202.
        
      
    
    
      
        
      
    
  




As mentioned earlier, the voltage reference circuit of FIG. 6 is a circuit to output a reference voltage VrefK determined by Vtnl, Vtnh, K201, K202, K204, K301, and R601.

    [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-148530


SUMMARY OF THE INVENTION
However, in the conventional voltage reference circuit illustrated in FIG. 6, the resistance value as well as the K values and the threshold values of the transistors determine a reference voltage level according to the formula (2), which causes a problem that an influence on process variation and an influence of temperature characteristics are large. Further, there is also such a problem that variability factors due to process variation increase when correction is performed to reduce a temperature characteristic of the reference voltage level. Further, in order to perform the correction, it is necessary to include a logic circuit for a temperature sensor and correction, which disadvantageously increases a circuit scale.
The present invention has been achieved in view of the above problems, so as to provide a voltage reference circuit which can reduce variability factors due to process variation and easily correct a reference voltage level and a temperature characteristic of the reference voltage level within desired ranges, without increasing a circuit scale.
In order to solve the above problems, a voltage reference circuit according to the present invention includes: a first MOS transistor; a second MOS transistor including a gate terminal connected to a gate terminal of the first MOS transistor and having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value and a K value of the first MOS transistor; a current mirror circuit flowing a current based on a difference between the absolute values of the threshold values of the first MOS transistor and the second MOS transistor; a third MOS transistor flowing the current of the current mirror circuit; and a fourth MOS transistor having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value and a K value of the third MOS transistor and flowing the current of the current mirror circuit, and the voltage reference circuit is configured to output, as a reference voltage, a constant voltage based on the absolute values of the threshold values and the K values of the third MOS transistor and the fourth MOS transistor.
With the use of the voltage reference circuit of the present invention, it is possible to reduce variability in a reference voltage level due to process variation by resistance and variability of corrected values of a reference voltage level and temperature characteristics without increasing a circuit scale.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a circuit diagram illustrating a voltage reference circuit according to the first embodiment.
FIG. 2 is a graph illustrating curves of a gate-source voltage relative to a drain terminal current of two NMOS transistors having different threshold values and K values.
FIG. 3 is a circuit diagram illustrating a voltage reference circuit according to the second embodiment.
FIG. 4 is a circuit diagram illustrating a voltage reference circuit according to the third embodiment.
FIG. 5 is a circuit diagram illustrating a voltage reference circuit according to the fourth embodiment.
FIG. 6 is a circuit diagram illustrating a conventional voltage reference circuit.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following will describe embodiments of the present invention with reference to drawings.
Embodiment 1
FIG. 1 is a circuit diagram illustrating a voltage reference circuit according to the first embodiment.
The voltage reference circuit according to the first embodiment includes PMOS transistors 101 and 102 and NMOS transistors 201, 301, and 302, an output terminal 401, a power supply terminal 501, and an earth terminal 502. A threshold voltage (hereinafter referred to as Vtnl) of the NMOS transistors 301 and 302 is lower than a threshold voltage (hereinafter referred to as Vtnh) of the NMOS transistor 201. Respective K values of the NMOS transistors 201, 301, and 302 are K201, K301, and K302. The PMOS transistor 101 and the PMOS transistor 102 constitute current mirror circuits.
Next will be explained connections in the voltage reference circuit according to the first embodiment.
Source terminals of the PMOS transistors 101 and 102 are connected to the power supply terminal 501. A gate terminal of the PMOS transistors 102 is connected to a gate terminal and a drain terminal of the PMOS transistor 101 and a drain terminal of the NMOS transistor 301. Gate terminals of the NMOS transistors 201 and 301 are connected to a drain terminal and a gate terminal of the NMOS transistor 302 and a drain terminal of the PMOS transistor 102, and source terminals thereof are connected to the earth terminal 502. The output terminal 401 is connected to a drain terminal of the NMOS transistor 201 and a source terminal of the NMOS transistor 302.
Next will be explained an operation of the voltage reference circuit according to the first embodiment.
Respective drain terminal currents of the PMOS transistors 101 and 102 are assumed I101 and I102. A voltage of the output terminal 401 is assumed Vref. The PMOS transistors 101 and 102 constitute a current mirror circuit, and therefore, if their respective K values are equal to each other, equal currents flow as the current I101 and the current I102. FIG. 2 illustrates characteristics of a gate-source voltage (hereinafter referred to as VGS) relative to a drain terminal voltage (hereinafter referred to ID) when the NMOS transistor 201 and the NMOS transistor 301 work in a saturation region. A rise position and a tilt of each curve are each determined by a threshold voltage and a K value. Since the current I101 is equal to the current I102 and the gate terminals of the NMOS transistor 201 and the NMOS transistor 301 are connected to each other, when these two transistors work in the saturation region, voltages VGS reach a point A. When a start circuit is provided, the current I101 (=I102) reaches a current value (hereinafter referred to as IA) at the point A, and this value is represented by the following formula with Vtnl, Vtnh, K201, and K301:




  
    
      
        
          
            I
            A
          
          =
          
            
              
                (
                
                  
                    V
                    tnh
                  
                  -
                  
                    V
                    tnl
                  
                
                )
              
              2
            
            
              
                (
                
                  
                    1
                    
                      
                        K
                        301
                      
                    
                  
                  -
                  
                    1
                    
                      
                        K
                        
                          201
                          ⁢
                          
                            
                          
                        
                      
                    
                  
                
                )
              
              2
            
          
        
        ⁢
        
          

        
        ⁢
        
          
            where
            ⁢
            
              
            
            ⁢
            
              K
              201
            
          
          >
          
            
              K
              301
            
            .
          
        
      
    
    
      
        (
        3
        )
      
    
  




When respective voltages VGS necessary for the NMOS transistors 201 and 302 to flow the current IA are assumed VGS201A and VGS302A, and an earth terminal voltage is assumed VSS, a reference voltage Vref of the output terminal 401 is such that Vref=VSS+VGS201A−VGS302A. Values of the voltage VGS201A and the voltage VGS302A are determined by the values of IA, Vtnl, Vtnh, K201, and K302. The current IA is determined by the values of Vtnl, Vtnh, K201, and K301 according to the formula (3), and thus, the value of the reference voltage Vref of the output terminal 401 is determined only by the values of Vtnh, Vtnl, K201, K301, and K302.
When the NMOS transistor 201 and the NMOS transistor 302 work in the saturation region, the reference voltage Vref is represented by the following formula:




  
    
      
        
          
            
              
                V
                ref
              
              =
              
              ⁢
              
                VSS
                +
                
                  V
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    201
                    ⁢
                    A
                  
                
                -
                
                  V
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    302
                    ⁢
                    A
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                VSS
                +
                
                  
                    
                      I
                      A
                    
                  
                  ⁢
                  
                    (
                    
                      
                        1
                        
                          
                            K
                            201
                          
                        
                      
                      -
                      
                        1
                        
                          
                            K
                            202
                          
                        
                      
                    
                    )
                  
                
                +
                
                  (
                  
                    
                      V
                      tnh
                    
                    -
                    
                      V
                      tnl
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        4
        )
      
    
  




Here, if all the transistors work in the saturation region, the reference voltage Vref is obtained by substituting the formula (3) for the current IA in the formula (4), as represented by the following formula:




  
    
      
        
          
            V
            ref
          
          =
          
            VSS
            +
            
              
                (
                
                  
                    
                      1
                      
                        
                          K
                          301
                        
                      
                    
                    -
                    
                      1
                      
                        
                          K
                          302
                        
                      
                    
                  
                  
                    
                      1
                      
                        
                          K
                          301
                        
                      
                    
                    -
                    
                      1
                      
                        
                          K
                          
                            201
                            ⁢
                            
                              
                            
                          
                        
                      
                    
                  
                
                )
              
              ⁢
              
                (
                
                  
                    V
                    tnh
                  
                  -
                  
                    V
                    tnl
                  
                
                )
              
            
          
        
        ⁢
        
          

        
        ⁢
        
          
            where
            ⁢
            
              
            
            ⁢
            
              K
              201
            
          
          >
          
            
              K
              301
            
            .
          
        
      
    
    
      
        (
        5
        )
      
    
  




From the formula (5), it is found that the value of the reference voltage Vref is a voltage determined by Vtnh, Vtnl, K201, K301, and K302. Thus, a reference voltage which does not vary depending on process variation of resistance can be obtained. Further, it is possible to easily correct a temperature characteristic by adjusting only the values of K201, K301, and K302.
Here, the above description takes, as an example, a case where the NMOS transistors 201, 301, and 302 work in the saturation region, but even in a case where any of or all of the transistors work in a weak inversion region, if K201 and K301 are set so that VGS−ID curves of both transistors cross each other, it is possible to create the current IA determined by the values of Vtnl, Vtnh, K201, and K301 as described above. Further, the reference voltage Vref can be also determined by the values of Vtnl, Vtnh, K201, K301, and K302. Therefore, the temperature characteristic can be corrected by adjusting only K values of respective transistors.
Note that the above description takes, as an example, a method in which on the premise that K values in a current mirror circuit are equal to each other, a reference voltage is corrected by adjusting the K values of respective transistors. However, it is also possible to correct a reference voltage level by adjusting a ratio of drain terminal currents of the respective transistors by changing K values of a mirrored pair of the current mirror circuit.
As such, it is possible to obtain a reference voltage which can be easily corrected by adjusting only the values of K201, K301, and K302 so as to correct the temperature characteristic with no variability depending on process variation of resistance.
Embodiment 2
FIG. 3 is a circuit diagram illustrating a voltage reference circuit according to the second embodiment.
The voltage reference circuit according to the second embodiment includes PMOS transistors 101 to 106, NMOS transistors 201 to 204 and 301 to 303, an output terminal 401, a power supply terminal 501, an earth terminal 502, and resistors 601 to 602. A threshold voltage (hereinafter referred to as Vtnl) of the NMOS transistors 301 to 302 is lower than a threshold voltage (hereinafter referred to as Vtnh) of the NMOS transistors 201 to 202. Respective K values of the NMOS transistors 201, 202, 301, and 302 are K201, K202, K301, and K302. Respective resistance values of the resistors 601 and 602 are R601 and R602. The NMOS transistors 203 and 204 constitute a current mirror circuit. The PMOS transistor 101 and the PMOS transistors 102, 103, and 104 constitute current mirror circuits.
Next will be explained connections of the voltage reference circuit according to the second embodiment.
Source terminals of the PMOS transistors 101 to 106 are connected to the power supply terminal 501. Gate terminals of the PMOS transistors 102 to 104 are connected to a gate terminal and a drain terminal of the PMOS transistor 101 and a drain terminal of the NMOS transistor 301. Gate terminals of the NMOS transistors 201 and 301 are connected to a drain terminal of the NMOS transistor 201 and a drain terminal of the PMOS transistor 102, and source terminals thereof are connected to the earth terminal 502. One end of the resistor 601 is connected to a gate terminal of the NMOS transistor 202 and a source terminal of the NMOS transistor 303, and the other end thereof is connected to a drain terminal of the NMOS transistor 204 and a gate terminal of the NMOS transistor 302. A drain terminal of the NMOS transistors 202 is connected to the drain terminal of the PMOS transistor 103 and to the gate terminal of the NMOS transistor 303, and a source terminal of the NMOS transistors 202 is connected to the earth terminal. A drain terminal of the NMOS transistor 303 is connected to the power supply terminal 501. A drain terminal of the NMOS transistor 302 is connected to a drain terminal of the PMOS transistor 104 and gate terminals of the PMOS transistors 105 and 106, and a source terminal thereof is connected to the earth terminal 502. Gate terminals of the NMOS transistors 203 and 204 are connected to a drain terminal of the NMOS transistor 203 and a drain terminal of the PMOS transistor 105, and source terminals thereof are connected to the earth terminal 502. One end of the resistor 602 is connected to a drain terminal of the PMOS transistor 106, and the other end thereof is connected to the earth terminal 502.
Next will be explained an operation of the voltage reference circuit according to the second embodiment. A voltage of the output terminal 401 is assumed a reference voltage Vref. If K values are equal to each other, currents flowing in the PMOS transistors 101 and 102 are a current IA determined by the values of Vtnl, Vtnh, K201, and K301 as described with reference to the formula (3) in the first embodiment.
As for the currents flowing in the PMOS transistors 103 and 104, since the PMOS transistors 103 and 104 constitute current mirror circuits with the PMOS transistor 101, if their respective K values are the same, the current IA flows therein.
The NMOS transistor 303 controls a gate terminal voltage of the NMOS transistor 202 so that a gate-source voltage of the NMOS transistor 202 reaches a voltage necessary to flow the current IA. The PMOS transistor 104, the NMOS transistor 203, and the NMOS transistor 204 control a gate terminal voltage of the NMOS transistor 302 so that a gate-source voltage of the NMOS transistor 302 reaches a voltage necessary to flow the current IA.
When respective gate-source voltages necessary for the NMOS transistors 202 and 302 to flow the current IA are assumed a voltage VGS202A and a voltage VGs302A, a voltage Vref2 of VGS202A−VGS302A appears at both ends of the resistor 601. This voltage Vref2 is determined by values of IA, Vtnl, Vtnh, K202, and K302. Since the current IA is determined by the values of Vtnl, Vtnh, K201, and K301, the voltage Vref2 is accordingly determined by the values of Vtnl, Vtnh, K201, K202, K301, and K302. Thus, a reference voltage which does not vary depending on process variation of resistance can be obtained. Further, a temperature characteristic of the voltage Vref2 can be corrected to be flat with respect to temperature characteristics of IA, VGS202A, and VGS302A by adjusting the values of K202 and K302.
When each transistor works in a saturation region, the value of the voltage Vref2 is represented by the following formula:




  
    
      
        
          
            
              
                V
                
                  ref
                  ⁢
                  
                    
                  
                  ⁢
                  2
                
              
              =
              
              ⁢
              
                
                  V
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    202
                    ⁢
                    A
                  
                
                -
                
                  G
                  
                    GS
                    ⁢
                    
                      
                    
                    ⁢
                    302
                    ⁢
                    A
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  (
                  
                    
                      
                        
                          1
                          
                            
                              K
                              
                                202
                                ⁢
                                
                                  
                                
                              
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              302
                            
                          
                        
                      
                      
                        
                          1
                          
                            
                              K
                              301
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              201
                            
                          
                        
                      
                    
                    +
                    1
                  
                  )
                
                ⁢
                
                  (
                  
                    
                      V
                      tnh
                    
                    -
                    
                      V
                      tnl
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        6
        )
      
    
  
  
    
      
        
          where
          ⁢
          
            
          
          ⁢
          
            K
            201
          
        
        ⁢
        
          
        
        >
        
          
            K
            301
          
          .
        
      
    
    
      
        
      
    
  




From the formula (6), it is found that the value of the reference voltage Vref2 is a reference voltage determined by Vtnh, Vtnl, K201, K202, K301, and K302. Further, in order to correct the temperature characteristic, only the values of K201, K202, K301, and K302 may be adjusted.
Since the NMOS transistors 203 and 204 constitute a current mirror circuit, and the PMOS transistors 105 and 106 have the same gate-source potential, the same current flows in each transistor. Because of this, the same current flows in the resistors 601 and 602, and the reference voltage Vref of the output terminal 401 is such that Vref=VSS+Vref2×(R602/R601), whereby any reference voltage level obtained by multiplying the voltage Vref2 by a resistance ratio R602/R601 can be output. Generally, a difference in the resistance ratio in the same chip can be made small to such an extent that the difference can be disregarded, and therefore, any reference voltage which is not affected by process variation due to resistance can be obtained.
In a case of a P-type substrate, the first embodiment causes a back-gate bias on the NMOS transistor 302 and a back-gate bias effect of the NMOS transistor 302 is included in factors to determine a reference voltage level, thereby resulting in that variability factors due to process variation increase. However, the second embodiment does not cause any back-gate bias on a transistor for determining a reference voltage level even when a P-type substrate is used, so that a reference voltage level is determined only by the values of Vtnl, Vtnh, V101, K201, K202, K301, and K302. Therefore, if the configuration according to the second embodiment of the present invention is used, the number of variability factors of a reference voltage due to process variation is small even with the use of a P-type substrate, and further, corrected values of a reference voltage level and its temperature characteristic can be made small.
Here, the NMOS transistors 201 to 204 use transistors having the same threshold voltage Vtnh. However, if the NMOS transistors 203 and 204 can constitute a current mirror circuit as a pair, the threshold value thereof may be different from that of the NMOS transistors 201 and 202. Further, the NMOS transistors 301 to 303 use transistors having the same threshold voltage Vtnl, but the NMOS transistor 303 may use a transistor having a threshold voltage which is appropriate for an operation power-supply voltage and different from threshold voltages of the others.
Note that the above description takes, as an example, a method in which on the premise that K values in a current mirror circuit are equal to each other, a reference voltage is corrected by adjusting the K values of respective transistors. However, it is also possible to correct a reference voltage level by adjusting a ratio of drain terminal currents of the respective transistors by changing K values of a mirrored pair of the current mirror circuit.
As such, it is possible to obtain a reference voltage which can be easily corrected by adjusting only the values of K201, K202, K301, and K302 so as to correct the temperature characteristic with no variability depending on process variation of resistance.
Embodiment 3
FIG. 4 is a circuit diagram illustrating a voltage reference circuit according to the third embodiment.
The voltage reference circuit according to the third embodiment includes PMOS transistors 101, 701, and 702, NMOS transistors 201 and 202, an output terminal 401, a power supply terminal 501, and an earth terminal 502. An absolute value |Vtpl| of a threshold voltage (hereinafter referred to as Vtpl) of the PMOS transistors 701 and 702 is lower than an absolute value |Vtph| of a threshold voltage (hereinafter referred to as Vtph) of the PMOS transistor 101. Respective K values of the PMOS transistors 101, 701, and 702 are K101, K701, and K702. The NMOS transistors 201 and 202 constitute a current mirror circuit.
Next will be explained connections of the voltage reference circuit according to the third embodiment. Source terminals of the NMOS transistors 201 and 202 are connected to the earth terminal 502. A gate terminal of the NMOS transistor 202 is connected to a gate terminal and a drain terminal of the NMOS transistor 201 and a drain terminal of the PMOS transistor 701. Gate terminals of the PMOS transistors 101 and 701 are connected to a drain terminal and a gate terminal of the PMOS transistor 702 and a drain terminal of the NMOS transistor 202, and source terminals thereof are connected to the power supply terminal 501. The output terminal 401 is connected to a drain terminal of the PMOS transistor 101 and a source terminal of the PMOS transistor 702.
Next will be explained an operation of the voltage reference circuit according to the third embodiment. The voltage reference circuit according to the third embodiment is a circuit to form a reference voltage on the basis of a power supply terminal voltage (VDD). A circuit operation is such that roles of the PMOS transistors and the NMOS transistors in the first embodiment are reversed. A current (hereinafter referred to as IB) flowing in the NMOS transistors 201 and 202 is a constant current determined by Vtph, Vtpl, K101, and K701, when a start circuit is provided so that the current is not stable at 0 A at an intersection between VGS−ID curves of the PMOS transistors 101 and 701. When respective gate-source voltages necessary for the PMOS transistors 101 and 702 to flow the current IB are assumed VGS101B and VGS702B, a reference voltage Vref appearing at the output terminal 401 is such that Vref=VDD−(|VGS101B|−|VGS702B|), and its value is determined by IB, Vtph, Vtpl, K101, and K702. Here, since the current IB is determined by Vtph, Vtpl, K101, and K701, a reference voltage level Vref4 is determined only by Vtph, Vtpl, K101, K701, and K702. Thus, a reference voltage which does not vary depending on process variation of resistance can be obtained.
Further, by setting the values of K101 and K702, a temperature characteristic of the reference voltage level Vref can be corrected so as to be flat with respect to temperature characteristics of IB, VGS101B, and VGS702B.
When all the transistors work in a saturation region, the constant current IB and the reference voltage Vref are represented by the following formula:




  
    
      
        
          
            I
            B
          
          =
          
            
              
                (
                
                  
                    
                    
                      V
                      tph
                    
                    
                  
                  -
                  
                    
                    
                      V
                      pnl
                    
                    
                  
                
                )
              
              2
            
            
              
                (
                
                  
                    1
                    
                      
                        K
                        701
                      
                    
                  
                  -
                  
                    1
                    
                      
                        K
                        101
                      
                    
                  
                
                )
              
              2
            
          
        
        ⁢
        
          

        
        ⁢
        
          
            where
            ⁢
            
              
            
            ⁢
            
              K
              101
            
          
          >
          
            
              K
              701
            
            .
          
        
      
    
    
      
        (
        7
        )
      
    
  
  
    
      
        
          
            
              
                V
                ref
              
              =
              
              ⁢
              
                VDD
                -
                
                  (
                  
                    
                      
                      
                        V
                        
                          GS
                          ⁢
                          
                            
                          
                          ⁢
                          101
                          ⁢
                          B
                        
                      
                      
                    
                    -
                    
                      
                      
                        V
                        
                          GS
                          ⁢
                          
                            
                          
                          ⁢
                          702
                          ⁢
                          B
                        
                      
                      
                    
                  
                  )
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                VDD
                -
                
                  
                    
                      I
                      B
                    
                  
                  ⁢
                  
                    (
                    
                      
                        1
                        
                          
                            K
                            101
                          
                        
                      
                      -
                      
                        1
                        
                          
                            K
                            702
                          
                        
                      
                    
                    )
                  
                
                -
                
                  (
                  
                    
                      
                      
                        V
                        tph
                      
                      
                    
                    -
                    
                      
                      
                        V
                        tpl
                      
                      
                    
                  
                  )
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                VDD
                -
                
                  
                    (
                    
                      
                        
                          1
                          
                            
                              K
                              701
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              702
                            
                          
                        
                      
                      
                        
                          1
                          
                            
                              K
                              701
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              101
                            
                          
                        
                      
                    
                    )
                  
                  ⁢
                  
                    (
                    
                      
                        
                        
                          V
                          tph
                        
                        
                      
                      -
                      
                        
                        
                          V
                          tpl
                        
                        
                      
                    
                    )
                  
                
              
            
          
        
      
    
    
      
        (
        8
        )
      
    
  
  
    
      
        
          where
          ⁢
          
            
          
          ⁢
          
            K
            101
          
        
        >
        
          
            K
            701
          
          .
        
      
    
    
      
        
      
    
  




From the formula (8), it is found that the value of the reference voltage Vref is a reference voltage determined by Vtph, Vtpl, K101, K701, and K702. Further, in order to correct the temperature characteristic, only the values of K101, K701, and K702 may be adjusted.
Note that the above description takes, as an example, a method in which on the premise that K values in a current mirror circuit are equal to each other, a reference voltage is corrected by adjusting the K values of respective transistors. However, it is also possible to correct a reference voltage level by adjusting a ratio of drain terminal currents of the respective transistors by changing K values of a mirrored pair of the current mirror circuit.
As such, it is possible to obtain a reference voltage which can be easily corrected by adjusting only the values of K101, K701, and K702 so as to correct the temperature characteristic with no variability depending on process variation of resistance.
Embodiment 4
FIG. 5 is a circuit diagram illustrating a voltage reference circuit according to the fourth embodiment.
The voltage reference circuit according to the fourth embodiment includes PMOS transistors 101 to 104 and 701 to 703, NMOS transistors 201 to 206, an output terminal 401, a power supply terminal 501, an earth terminal 502, and resistors 601 and 602. An absolute value |Vtpl| of a threshold voltage of the PMOS transistors 701 and 702 is lower than an absolute value |Vtph| of a threshold voltage of the PMOS transistors 101 and 102. Respective K values of the PMOS transistors 101, 102, 701, and 702 are K101, K102, K701, and K702. Respective resistance values of the resistors 601 and 602 are R601 and R602. The PMOS transistors 103 and 104 constitute a current mirror circuit, and the NMOS transistor 201 and the NMOS transistors 202, 203, and 204 constitute current mirror circuits.
Next will be explained connections in the voltage reference circuit according to the fourth embodiment. Source terminals of the NMOS transistors 201 to 206 are connected to the earth terminal 502. Gate terminals of the NMOS transistors 202 to 204 are connected to a gate terminal and a drain terminal of the NMOS transistor 201 and a drain terminal of the PMOS transistor 701. Gate terminals of the PMOS transistors 101 and 701 are connected to a drain terminal of the PMOS transistor 101 and a drain terminal of the NMOS transistor 202, and source terminals thereof are connected to the power supply terminal 501. One end of the resistor 601 is connected to a gate terminal of the PMOS transistor 102 and a source terminal of the PMOS transistor 703, and the other end thereof is connected to a drain terminal of the PMOS transistor 104 and a gate terminal of the PMOS transistor 702. A drain terminal of the PMOS transistor 102 is connected to a drain terminal of the NMOS transistor 203 and a gate terminal of the PMOS transistor 703, and a source terminal thereof is connected to the power supply terminal 501. A drain terminal of the PMOS transistor 703 is connected to the earth terminal 502. A drain terminal of the PMOS transistor 702 is connected to a drain terminal of the NMOS transistor 204 and gate terminals of the NMOS transistors 205 and 206, and a source terminal thereof is connected to the power supply terminal 501. Gate terminals of the PMOS transistors 103 and 104 are connected to a drain terminal of the PMOS transistor 103 and a drain terminal of the NMOS transistor 205, and source terminals thereof are connected to the power supply terminal 501. One end of the resistor 602 is connected to a drain terminal of the NMOS transistor 206 and the output terminal 401, and the other end thereof is connected to the power supply terminal 501.
Next will be explained an operation of the voltage reference circuit according to the fourth embodiment. The voltage reference circuit according to the fourth embodiment is such that roles of the PMOS transistors and the NMOS transistors in the second embodiment are reversed. A current flowing in the NMOS transistors 201 to 204 is the constant current (IB) determined by Vtph, Vtph, K101, and K701 as described in the third embodiment. When respective voltages VGS necessary for the PMOS transistors 102 and 702 to flow the current IB are assumed VGS102B and VGS702B, a reference voltage Vref5 appearing at both ends of the resistor 601 is such that Vref5=|VGS102B|−|VGS702B|, and its value is determined by IB, Vtpl, Vtph, K102, and K702. Since the current IB is determined by Vtph, Vtpl, K101, and K701, it is possible to obtain, by taking out the voltage Vref5, a reference voltage which is determined by the values of Vtph, Vtpl, K101, K102, K701, and K702 and which does not vary depending on process variation due to resistance. Further, by adjusting the values of K102 and K702, a temperature characteristic of the voltage Vref5 can be corrected so as to be flat with respect to temperature characteristics of IB, VGS102B, and VGS702B.
When all the transistors work in a saturation region, the voltage Vref5 is represented by the following formula:




  
    
      
        
          
            
              
                V
                
                  ref
                  ⁢
                  
                    
                  
                  ⁢
                  5
                
              
              =
              
              ⁢
              
                
                  
                  
                    V
                    
                      GS
                      ⁢
                      
                        
                      
                      ⁢
                      102
                      ⁢
                      B
                    
                  
                  
                
                -
                
                  
                  
                    V
                    
                      GS
                      ⁢
                      
                        
                      
                      ⁢
                      702
                      ⁢
                      B
                    
                  
                  
                
              
            
          
        
        
          
            
              =
              
              ⁢
              
                
                  (
                  
                    
                      
                        
                          1
                          
                            
                              K
                              102
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              702
                            
                          
                        
                      
                      
                        
                          1
                          
                            
                              K
                              702
                            
                          
                        
                        -
                        
                          1
                          
                            
                              K
                              101
                            
                          
                        
                      
                    
                    +
                    1
                  
                  )
                
                ⁢
                
                  (
                  
                    
                      
                      Vtph
                      
                    
                    -
                    
                      
                      Vtpl
                      
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        9
        )
      
    
  
  
    
      
        
          where
          ⁢
          
            
          
          ⁢
          
            K
            101
          
        
        >
        
          
            K
            701
          
          .
        
      
    
    
      
        
      
    
  




From the formula (9), it is found that the value of the voltage Vref5 is a reference voltage determined by Vtph, Vtpl, K101, K102, K701, and K702. Further, in order to correct the temperature characteristic, only the values of K101, K102, K701, and K702 may be adjusted.
Since the same current flows in the PMOS transistor 104 and the NMOS transistor 206, the reference voltage Vref of the output terminal 401 is such that Vref=VDD−Vref5×(R602/R601), whereby any reference voltage level which is based on a power supply terminal voltage and which is obtained by multiplying the voltage Vref5 by R602/R601 can be output. Generally, because a difference in the resistance ratio in the same chip can be made small to such an extent that the difference can be disregarded, any reference voltage which is not affected by process variation due to resistance can be obtained.
The voltage reference circuit according to the fourth embodiment is a circuit to form a reference voltage on the basis of a power supply terminal voltage (VDD), and is a circuit in which a reference voltage level is not affected by a back-gate bias effect when an N-type substrate is used. The circuit according to the third embodiment causes a back-gate bias on the PMOS transistor 702 in FIG. 4, and a back-gate bias effect of the PMOS transistor 702 is included in factors to determine a reference voltage level, thereby resulting in that variability factors due to process variation increase. However, the fourth embodiment does not cause any back-gate bias on a transistor for determining a reference voltage level even when an N-type substrate is used, so that the reference voltage level is determined only by the values of Vtpl, Vtph, K101, K102, K701, and K702. Therefore, if the configuration according to the fourth embodiment of the present invention is used, the number of variability factors due to process variation is small even with the use of an N-type substrate, and further, corrected values of a reference voltage level and its temperature characteristic can be made small.
Here, the PMOS transistors 101 to 104 use transistors having the same threshold voltage Vtph. However, if the PMOS transistors 103 and 104 constitute a current mirror circuit, the threshold value thereof may be different from that of the NMOS transistors 101 and 102. Further, the PMOS transistors 701 to 703 use transistors having the same threshold voltage Vtpl, but the PMOS transistor 703 may use a transistor having a threshold voltage which is appropriate and different from threshold voltages of the others, according to an operation power-supply voltage.
Note that the above description takes, as an example, a method in which on the premise that K values in a current mirror circuit are equal to each other, a reference voltage is corrected by adjusting the K values of respective transistors. However, it is also possible to correct a reference voltage level by adjusting a ratio of drain terminal currents of respective transistors by changing K values of a mirrored pair of the current mirror circuit.
As such, it is possible to obtain a reference voltage which can be easily corrected by adjusting only the values of K101, K102, K701, and K702 so as to correct the temperature characteristic with no variability depending on process variation of resistance.
As discussed above, a voltage reference circuit according to the present invention may include: a first MOS transistor; a second MOS transistor including a gate terminal connected to a gate terminal of the first MOS transistor and having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value and a K value of the first MOS transistor; a current mirror circuit flowing a current based on a difference between the absolute values of the threshold values of the first MOS transistor and the second MOS transistor; a third MOS transistor flowing the current of the current mirror circuit; and a fourth MOS transistor having an absolute value of a threshold value and a K value higher than an absolute value of a threshold value and a K value of the third MOS transistor and flowing the current of the current mirror circuit, and the voltage reference circuit may be configured to output, as a reference voltage, a constant voltage based on the absolute values of the threshold values and the K values of the third MOS transistor and the fourth MOS transistor.
Accordingly, circuits which generate a constant voltage of a voltage reference circuit as shown in the embodiments and circuit which output the constant voltage as a reference voltage are only examples, and the present invention is not limited to these circuits.

## Claims

1. A voltage reference circuit comprising: a first MOS transistor including a source terminal connected to a first power supply terminal; a second MOS transistor including a source terminal connected to the first power supply terminal and a gate terminal connected to a gate terminal of the first MOS transistor, the second MOS transistor having an absolute value of a threshold value higher than an absolute value of a threshold value of the first MOS transistor; a current mirror circuit flowing a current based on a difference between the absolute values of the threshold values of the first MOS transistor and the second MOS transistor; a third MOS transistor flowing the current of the current mirror circuit; a fourth MOS transistor having an absolute value of a threshold value higher than an absolute value of a threshold value of the third MOS transistor and flowing the current of the current mirror circuit, wherein the voltage reference circuit outputs, as a reference voltage, a constant voltage based on the absolute values of the threshold values of the third MOS transistor and the fourth MOS transistor; wherein the current mirror circuit includes: a fifth MOS transistor including a drain terminal and a gate terminal connected to a drain terminal of the first MOS transistor; a sixth MOS transistor including a gate terminal connected to the gate terminal of the fifth MOS transistor and a drain terminal connected to the gate terminal and a drain terminal of the second MOS transistor; a seventh MOS transistor including a gate terminal connected to the gate terminal of the fifth MOS transistor and a drain terminal connected to a drain terminal of the third MOS transistor; an eighth MOS transistor including a gate terminal connected to the gate terminal of the fifth MOS transistor and a drain terminal connected to a drain terminal of the fourth MOS transistor; and a resistor including one terminal connected to a gate terminal of the third MOS transistor and another terminal connected to a gate terminal of the fourth MOS transistor, wherein the voltage reference circuit outputs, as a reference voltage, a constant voltage based on a voltage at both ends of the resistor.

2. A voltage reference circuit comprising: a first MOS transistor including a source terminal connected to a first power supply terminal; a second MOS transistor including a source terminal connected to the first power supply terminal and a gate terminal connected to a gate terminal of the first MOS transistor, the second MOS transistor having an absolute value of a threshold value higher than an absolute value of a threshold value of the first MOS transistor; a current mirror circuit flowing a current based on a difference between the absolute values of the threshold values of the first MOS transistor and the second MOS transistor; a third MOS transistor flowing the current of the current mirror circuit; wherein the current mirror circuit includes: a fourth MOS transistor including a drain terminal and a gate terminal connected to a drain terminal of the first MOS transistor; and a fifth MOS transistor including a gate terminal connected to the gate terminal of the fourth MOS transistor and a drain terminal connected to a gate terminal and a drain terminal of the third MOS transistor, wherein the third MOS transistor is configured to include a gate terminal connected to the gate terminal of the second MOS transistor and a source terminal connected to a drain terminal of the second MOS transistor, so that the voltage reference circuit outputs the reference voltage from a connecting point of the source terminal of the third MOS transistor and the drain terminal of the second MOS transistor; and wherein the voltage reference circuit outputs, as a reference voltage, a constant voltage based on the absolute values of the threshold values of the third MOS transistor and the second MOS transistor.