Abstract:
This package manufacturing method is a method for manufacturing a package using a rivet having a flat plate-shaped head portion and a core portion protruding from a rear surface of the head portion. The package includes: a plurality of substrates that are bonded to each other: a cavity that houses an object to be housed in an airtightly sealed state; the core portion which is disposed in a through hole that penetrates a base substrate and which electrically connects the object to be housed with the outside; and a glass frit that is filled between the through hole and the core portion and is fired to form a seal between the though hole and the core portion. The package manufacturing method includes: a rivet disposing step of inserting the core portion into the through hole until the rear surface of the head portion comes into contact with a first surface of the base substrate; a glass frit filling step of filling the glass frit in a paste form between the through hole and the core portion; and a firing step of integrally fixing the through hole, the rivet and the glass frit by firing the filled glass frit, and sealing between the through hole and the core portion. A gas relief passage, which leads from a base end of the core portion to a side surface or a front surface of the head portion, is formed on the rear surface of the head portion of the rivet.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2010-205700 filed on Sep. 14, 2010, the entire content of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a constant current circuit. 
         [0004]    2. Description of the Related Art 
         [0005]    A conventional constant current circuit is described.  FIG. 13  is a diagram illustrating a conventional constant current circuit. 
         [0006]    An increase in a current Iref that flows in a resistor  54  raises a voltage generated in the resistor  54  and accordingly raises the gate-source voltage of an NMOS transistor  52 , with the result that the conductance of the NMOS transistor  52  is increased. This reduces the gate voltage of an NMOS transistor  53 , which leads to a lower gate-source voltage of the NMOS transistor  53  and a smaller conductance of the NMOS transistor  53 . The current Iref is therefore reduced. A reduction in the current Iref that flows in the resistor  54  increases the current Iref because of the similar operation of the NMOS transistor  52  and the NMOS transistor  53 . The conventional constant current circuit keeps the current Iref constant by operating in the manner described above (see, for example, JP 06-132739 A ( FIG. 12 )). 
         [0007]    In prior art, when the power supply voltage is given as VDD, the gate-source voltage of a PMOS transistor  51  is given as Vgsp, the drain-source voltage of the NMOS transistor  53  is given as Vdsn, and the gate-source voltage of the NMOS transistor  52  is given as Vgsn, the constant current circuit needs to satisfy the following Expression (31) in order to operate properly: 
         [0000]        VDD&gt;|Vgsp|+Vdsn+Vgsn    (31)
 
         [0008]    From Expression (31), the power supply voltage VDD needs to be higher than 1.6 V in order for the constant current circuit to operate properly when, for example, the gate-source voltage |Vgsp| and the gate-source voltage Vgsn are each 0.7 V and the drain-source voltage Vdsn is 0.2 V. In other words, the minimum operating power supply voltage is 1.6 V. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in view of the above, and an object of the present invention is therefore to provide a constant current circuit capable of operating on a lower power supply voltage. 
         [0010]    In order to attain the above object, a constant current circuit according to the present invention is structured as follows. 
         [0011]    A constant current circuit according to an aspect of the present invention includes: a first depletion type MOS transistor of a second conductivity type, which has a drain connected to a first power supply terminal and which serves as a current source; a first current mirror circuit which includes a first MOS transistor of the second conductivity type serving as an input-side transistor and having a source connected to a second power supply terminal and a second MOS transistor of the second conductivity type serving as an output-side transistor and having a source connected to the second power supply terminal, and which mirrors a current that flows in the first depletion type MOS transistor of the second conductivity type; a second current mirror circuit which includes a first MOS transistor of a first conductivity type serving as an input-side transistor and having a source connected to the first power supply terminal and a second MOS transistor of the first conductivity type serving as an output-side transistor and having a source connected to the first power supply terminal, and which mirrors a current that flows in the first current mirror circuit; a resistor which is provided between a source of the first depletion type MOS transistor of the second conductivity type and a drain of the first MOS transistor of the second conductivity type; and a third MOS transistor of the second conductivity type, which has a gate connected to one terminal of the resistor, a source connected to the second power supply terminal, and a drain connected to an output terminal of the second current mirror circuit, in which a gate of the first MOS transistor of the second conductivity type is connected to another terminal of the resistor, and a gate of the first depletion type MOS transistor of the second conductivity type is connected to the output terminal of the second current mirror circuit. 
         [0012]    Further, a constant current circuit according to another aspect of the present invention includes: a first depletion type MOS transistor of a second conductivity type, which has a drain connected to a first power supply terminal and which serves as a current source; a first current mirror circuit which includes a first MOS transistor of the second conductivity type serving as an input-side transistor and having a source connected to a second power supply terminal and a second MOS transistor of the second conductivity type serving as an output-side transistor and having a source connected to the second power supply terminal, and which mirrors a current that flows in the first depletion type MOS transistor of the second conductivity type; a resistor which is provided between a source of the first depletion type MOS transistor of the second conductivity type and a drain of the first MOS transistor of the second conductivity type; a third MOS transistor of the second conductivity type, which has a gate connected to one terminal of the resistor and a source connected to the second power supply terminal; and a second current mirror circuit which includes a first MOS transistor of a first conductivity type serving as an input-side transistor and having a source connected to the first power supply terminal and a second MOS transistor of the first conductivity type serving as an output-side transistor and having a source connected to the first power supply terminal, and which mirrors a current that flows in the third MOS transistor of the second conductivity type, in which a gate of the first MOS transistor of the second conductivity type is connected to another terminal of the resistor, and a gate of the first depletion type MOS transistor of the second conductivity type is connected to an output terminal of the second current mirror circuit. 
         [0013]    A constant current circuit of the present invention structured as above can operate if the power supply voltage is higher than a voltage that is the sum of the drain-source voltage of the first depletion type MOS transistor of the second conductivity type and the gate-source voltage of the second MOS transistor of the second conductivity type. The resultant effect is that a constant current circuit of the present invention is lower in minimum operating voltage than conventional constant current circuits. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the accompanying drawings: 
           [0015]      FIG. 1  is a diagram illustrating a constant current circuit according to an embodiment of the present invention; 
           [0016]      FIG. 2  is a diagram illustrating another example of the constant current circuit according to the embodiment; 
           [0017]      FIG. 3  is a diagram illustrating still another example of the constant current circuit according to the embodiment; 
           [0018]      FIG. 4  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0019]      FIG. 5  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0020]      FIG. 6  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0021]      FIG. 7  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0022]      FIG. 8  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0023]      FIG. 9  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0024]      FIG. 10  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0025]      FIG. 11  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; 
           [0026]      FIG. 12  is a diagram illustrating yet still another example of the constant current circuit according to the embodiment; and 
           [0027]      FIG. 13  is a diagram illustrating a conventional constant current circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    An embodiment of the present invention is described below with reference to the drawings. 
         [0029]    The structure of a constant current circuit is described first.  FIG. 1  is a diagram illustrating a constant current circuit according to the embodiment. 
         [0030]    The constant current circuit of this embodiment includes a depletion type NMOS transistor  10 , NMOS transistors  11  and  12 , PMOS transistors  13  and  14 , an NMOS transistor  15 , and a resistor  20 . 
         [0031]    A gate of the NMOS transistor  11  is connected to a drain of the NMOS transistor  11 , one end of the resistor  20 , and a gate of the NMOS transistor  12 . A source of the NMOS transistor  11  is connected to the ground terminal. The NMOS transistor  11  is wired in a saturated manner. A source of the NMOS transistor  12  is connected to a ground terminal. A gate of the PMOS transistor  13  is connected to a drain of the PMOS transistor  13 , a gate of the PMOS transistor  14 , and a drain of the NMOS transistor  12 . A source of the PMOS transistor  13  is connected to the power supply terminal The PMOS transistor  13  is wired in a saturated manner. A source of the PMOS transistor  14  is connected to the power supply terminal, and a drain of the PMOS transistor  14  is connected to a gate of the depletion type NMOS transistor  10  and a drain of the NMOS transistor  15 . A gate of the NMOS transistor  15  is connected to a source of the depletion type NMOS transistor  10  and the other end of the resistor  20 . A source of the NMOS transistor  15  is connected to the ground terminal. A drain of the depletion type NMOS transistor  10  is connected to the power supply terminal. 
         [0032]    The PMOS transistors  13  and  14  constitute a current mirror circuit, with the drain of the PMOS transistor  13  serving as an input terminal of the current mirror circuit and the drain of the PMOS transistor  14  serving as an output terminal of the current mirror circuit. The NMOS transistors  11  and  12  constitute a current mirror circuit, with the drain of the NMOS transistor  11  serving as an input terminal of the current mirror circuit and the drain of the NMOS transistor  12  serving as an output terminal of the current mirror circuit. 
         [0033]    The operation of the constant current circuit of this embodiment is described next. 
         [0034]    When the constant current circuit is powered on, the gate-source voltage of the depletion type NMOS transistor  10  is substantially 0 V, which causes a drain current to flow in the depletion type NMOS transistor  10 . The drain current starts up the constant current circuit. The constant current circuit therefore does not need a start-up circuit for starting up the constant current circuit. 
         [0035]    When the power supply voltage is given as VDD, the drain-source voltage of the depletion type NMOS transistor  10  is given as Vds 10 , and the gate-source voltage of the NMOS transistor  15  is given as Vgs 15 , the constant current circuit needs to satisfy the following Expression (1) to operate properly: 
         [0000]        VDD&gt;Vds 10+ Vgs 15   (1)
 
         [0036]    From Expression (1), the power supply voltage VDD needs to be higher than 0.9 V in order for the constant current circuit to operate properly when, for example, the drain-source voltage Vds 10  is 0.2 V and the gate-source voltage Vgs 15  is 0.7 V. In other words, the constant current circuit has a minimum operating power supply voltage of 0.9 V, which is lower than the minimum operating power supply voltage in prior art. 
         [0037]    Designing the circuit in a manner that makes the NMOS transistor  15  higher in threshold voltage than the NMOS transistor  11 , and/or designing the circuit in a manner that makes the NMOS transistor  15  lower in driving performance than the NMOS transistor  11  gives the NMOS transistor  15  a gate-source voltage higher than that of the NMOS transistor  11 . A differential voltage between the gate-source voltage of the NMOS transistor  15  and the gate-source voltage of the NMOS transistor  11  is generated in the resistor  20 . A current Iref based on the differential voltage and the resistance value of the resistor  20  flows in the resistor  20 . The current mirror circuit constituted of the NMOS transistors  11  and  12  and the current mirror circuit constituted of the PMOS transistors  13  and  14  cause a current based on the current Iref to flow in the drain of the NMOS transistor  15 . 
         [0038]    The depletion type NMOS transistor  10  and the NMOS transistor  15  operate in conjunction with each other such that the current Iref and the drain current of the NMOS transistor  15  have a desired current ratio. Specifically, in the case where the current Iref that flows in the resistor  20  is large, a high voltage is generated in the resistor  20  and a voltage VA rises as well. This raises the gate-source voltage of the NMOS transistor  15  and increases the conductance of the NMOS transistor  15 . As a result, the gate voltage of the depletion type NMOS transistor  10  is lowered and the gate-source voltage of the depletion type NMOS transistor  10  drops as well, thereby reducing the conductance of the depletion type NMOS transistor  10 . Then the voltage VA drops and the current Iref is accordingly reduced. In the case where the current Iref that flows in the resistor  20  is small, the current Iref increases by the mechanism described above. The current Iref is thus kept constant. 
         [0039]    The current Iref that flows in the depletion type NMOS transistor  10 , the resistor  20 , and the NMOS transistor  11  is described next. 
         [0040]    When a voltage at the other end of the resistor  20  is given as VA, a voltage at the one end of the resistor  20  is given as VB, and the resistance value of the resistor  20  is given as Rb, the following Expression (2) is established: 
         [0000]    
       
         
           
             
               
                 
                   Iref 
                   = 
                   
                     
                       
                         V 
                          
                         
                             
                         
                          
                         A 
                       
                       - 
                       VB 
                     
                     Rb 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0041]    [When the Depletion Type NMOS Transistor  10  Operates in a Strong Inversion Mode and the Other Transistors Operate in a Strong Inversion Mode As Well] 
         [0042]    When the MOS transistor has a gate-source voltage Vgs, a drain current I, a threshold voltage Vth, a mobility μ n , a gate insulating film capacity per unit area C ox , a gate width W, and a gate length L, the following Expression (3) is established: 
         [0000]    
       
         
           
             
               
                 
                   Vgs 
                   = 
                   
                     
                       
                         
                           2 
                            
                           I 
                         
                         β 
                       
                     
                     + 
                     
                       Vth 
                        
                       
                         ( 
                         
                           β 
                           = 
                           
                             
                               μ 
                               n 
                             
                              
                             
                               C 
                               ox 
                             
                              
                             
                               W 
                               L 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0043]    When the NMOS transistor  11  has a drain current I 11  and a threshold voltage Vth 11  and the NMOS transistor  15  has a drain current I 15  and a threshold voltage Vth 15 , the following Expression (4) is established from Expressions (2) and (3): 
         [0000]    
       
         
           
             
               
                 
                   Iref 
                   = 
                   
                     
                       I 
                        
                       
                           
                       
                        
                       11 
                     
                     = 
                     
                       
                         
                           
                             V 
                              
                             
                                 
                             
                              
                             A 
                           
                           - 
                           VB 
                         
                         Rb 
                       
                       = 
                       
                         
                           
                             
                               
                                 2 
                                  
                                 I 
                                  
                                 
                                     
                                 
                                  
                                 15 
                               
                               
                                 β 
                                  
                                 
                                     
                                 
                                  
                                 15 
                               
                             
                           
                           - 
                           
                             
                               
                                 2 
                                  
                                 I 
                                  
                                 
                                     
                                 
                                  
                                 11 
                               
                               
                                 β 
                                  
                                 
                                     
                                 
                                  
                                 11 
                               
                             
                           
                           + 
                           
                             Vth 
                              
                             
                                 
                             
                              
                             15 
                           
                           - 
                           
                             V 
                              
                             
                                 
                             
                              
                             th 
                              
                             
                                 
                             
                              
                             11 
                           
                         
                         Rb 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0044]    In the case where the following Expression (5) and Vth 15 &gt;Vth 11  are satisfied, the following Expression (6) is established from Expression (4): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           2 
                            
                           I 
                            
                           
                               
                           
                            
                           15 
                         
                         
                           β 
                            
                           
                               
                           
                            
                           15 
                         
                       
                     
                     - 
                     
                       
                         
                           2 
                            
                           I 
                            
                           
                               
                           
                            
                           11 
                         
                         
                           β 
                            
                           
                               
                           
                            
                           11 
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
             
               
                 
                   Iref 
                   = 
                   
                     
                       
                         Vth 
                          
                         
                             
                         
                          
                         15 
                       
                       - 
                       
                         Vth 
                          
                         
                             
                         
                          
                         11 
                       
                     
                     Rb 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0045]    The NMOS transistor  11  and the NMOS transistor  15  are transistors having the same polarity, and the threshold voltage Vth 11  and the threshold voltage Vth 15  therefore have substantially the same temperature characteristics, which means that the temperature coefficient of (Vth 15 −Vth 11 ) is substantially 0. If the resistor  20  used has 0 as the temperature coefficient of the resistance value Rb, the temperature coefficient of the current Iref, too, is substantially 0. It is also concluded from Expression (6) that the current Iref is independent of the power supply voltage VDD. 
         [0046]    In the case where Vth 15 −Vth 11 =0, Iref=I 11 =I 15 , β 15 =β, and β 11 =αβ (α is a constant number that satisfies α&gt;1) are satisfied, the following Expression (7) is established from Expression (4). From Expression (7), the following Expression (8) is established. From Expression (8), the following Expression (9) is established: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Iref 
                         = 
                           
                          
                         
                           
                             
                               
                                 
                                   2 
                                    
                                   I 
                                    
                                   
                                       
                                   
                                    
                                   15 
                                 
                                 
                                   β 
                                    
                                   
                                       
                                   
                                    
                                   15 
                                 
                               
                             
                             - 
                             
                               
                                 
                                   2 
                                    
                                   I 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                                 
                                   β 
                                    
                                   
                                       
                                   
                                    
                                   11 
                                 
                               
                             
                           
                           Rb 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 
                                   2 
                                    
                                   Iref 
                                 
                                 β 
                               
                             
                             - 
                             
                               
                                 
                                   2 
                                    
                                   Iref 
                                 
                                 β 
                               
                             
                           
                           Rb 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 
                                   2 
                                    
                                   Iref 
                                 
                                 β 
                               
                             
                             Rb 
                           
                            
                           
                             ( 
                             
                               1 
                               - 
                               
                                 1 
                                 
                                   a 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       Iref 
                     
                      
                     
                       ( 
                       
                         
                           Iref 
                         
                         - 
                         
                           
                             
                               
                                 
                                   2 
                                   β 
                                 
                                  
                                 
                                     
                                 
                               
                             
                             Rb 
                           
                            
                           
                             ( 
                             
                               1 
                               - 
                               
                                 1 
                                 
                                   a 
                                 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   Iref 
                   = 
                   
                     
                       1 
                       
                         Rb 
                         2 
                       
                     
                     · 
                     
                       
                         2 
                         β 
                       
                     
                     · 
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             1 
                             
                               a 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0047]    If the resistor  20  used has the resistance value Rb whose temperature characteristics cancel out the temperature characteristics of β, the temperature coefficient of the current Iref is 0 as well. It is also concluded from Expression (9) that the current Iref is independent of the power supply voltage VDD. 
         [0048]    [When the Depletion Type NMOS Transistor  10  Operates in a Strong Inversion Mode and the Other Transistors Operate in a Weak Inversion Mode] 
         [0049]    When the MOS transistor has a slope factor n, a Boltzmann constant k, a temperature T, an electronic charge q, and a process-dependent parameter I 0 , the following Expression (10) is established: 
         [0000]    
       
         
           
             
               
                 
                   Vgs 
                   = 
                   
                     
                       
                         nkT 
                         q 
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           
                             I 
                             
                               
                                 I 
                                 0 
                               
                                
                               K 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       Vth 
                        
                       
                         
 
                       
                       ( 
                       
                         K 
                         = 
                         
                           W 
                           L 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0050]    The following Expression (11) is established from Expressions (2) and (10): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Iref 
                         = 
                           
                          
                         
                           I 
                            
                           
                               
                           
                            
                           11 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               V 
                                
                               
                                   
                               
                                
                               A 
                             
                             - 
                             VB 
                           
                           Rb 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 nkT 
                                 q 
                               
                                
                               
                                 ln 
                                  
                                 
                                   ( 
                                   
                                     
                                       I 
                                        
                                       
                                           
                                       
                                        
                                       15 
                                     
                                     
                                       
                                         I 
                                         0 
                                       
                                        
                                       K 
                                        
                                       
                                           
                                       
                                        
                                       15 
                                     
                                   
                                   ) 
                                 
                               
                             
                             - 
                             
                               
                                 nkT 
                                 q 
                               
                                
                               
                                 ln 
                                  
                                 
                                   ( 
                                   
                                     
                                       I 
                                        
                                       
                                           
                                       
                                        
                                       11 
                                     
                                     
                                       
                                         I 
                                         0 
                                       
                                        
                                       K 
                                        
                                       
                                           
                                       
                                        
                                       11 
                                     
                                   
                                   ) 
                                 
                               
                             
                             + 
                             
                               Vth 
                                
                               
                                   
                               
                                
                               15 
                             
                             - 
                             
                               Vth 
                                
                               
                                   
                               
                                
                               11 
                             
                           
                           Rb 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0051]    In the case where the following Expression (12) and Vth 15 &gt;Vth 11  are satisfied, the following Expression (13) is established from Expression (11): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         nkT 
                         q 
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               I 
                                
                               
                                   
                               
                                
                               15 
                             
                             
                               
                                 I 
                                 0 
                               
                                
                               K 
                                
                               
                                   
                               
                                
                               15 
                             
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         nkT 
                         q 
                       
                        
                       
                         ln 
                          
                         
                           ( 
                           
                             
                               I 
                                
                               
                                   
                               
                                
                               11 
                             
                             
                               
                                 I 
                                 0 
                               
                                
                               K 
                                
                               
                                   
                               
                                
                               11 
                             
                           
                           ) 
                         
                       
                     
                   
                   = 
                   0 
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   Iref 
                   = 
                   
                     
                       
                         Vth 
                          
                         
                             
                         
                          
                         15 
                       
                       - 
                       
                         Vth 
                          
                         
                             
                         
                          
                         11 
                       
                     
                     Rb 
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0052]    The temperature coefficient of the current Iref is substantially 0 as in the case where the other transistors operate in a strong inversion mode. It is also concluded from Expression (13) that the current Iref is independent of the power supply voltage VDD. 
         [0053]    Further, in the case where Vth 15 −Vth 11 =0 and Iref=I 11 =γ 15  (γ&gt;0) are satisfied, the following Expression (14) is established from Expression (11): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Iref 
                         = 
                           
                          
                         
                           
                             
                               
                                 nkT 
                                 q 
                               
                                
                               
                                 ln 
                                  
                                 
                                   ( 
                                   
                                     
                                       I 
                                        
                                       
                                           
                                       
                                        
                                       15 
                                     
                                     
                                       
                                         I 
                                         0 
                                       
                                        
                                       K 
                                        
                                       
                                           
                                       
                                        
                                       15 
                                     
                                   
                                   ) 
                                 
                               
                             
                             - 
                             
                               
                                 nkT 
                                 q 
                               
                                
                               
                                 ln 
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         [0054]    If the resistor  20  used has the resistance value Rb whose temperature characteristics cancel out the temperature characteristics of the numerator of Expression (14), the temperature coefficient of the current Iref is 0 as well. It is also concluded from Expression (14) that the current Iref is independent of the power supply voltage VDD. 
         [0055]    Structured in this manner, the constant current circuit can operate if the power supply voltage VDD is higher than a voltage that is the sum of the drain-source voltage Vds 10  of the depletion type NMOS transistor  10  and the gate-source voltage Vgs 15  of the NMOS transistor  15 . The constant current circuit needs as the power supply voltage VDD a voltage that is the sum of one drain-source voltage and one gate-source voltage, instead of a voltage that is the sum of one drain-source voltage and two gate-source voltages, and therefore is reduced in minimum operating power supply voltage. 
         [0056]    The constant current circuit structured as above also does not need a start-up circuit for starting up the constant current circuit. 
       MODIFICATION EXAMPLE 1 
       [0057]      FIG. 2  is a diagram illustrating another example of the constant current circuit according to this embodiment. Compared to  FIG. 1 , an impedance element  21 , which is constituted of a resistor, a MOS transistor wired in a saturated manner, a diode, and others, is added in  FIG. 2 . The impedance element  21  is provided between the source of the depletion type NMOS transistor  10  and a point where the other end of the resistor  20  and the gate of the NMOS transistor  15  are connected to each other. 
         [0058]    With this structure, a voltage is generated in the impedance element  21  based on the current Iref, and the source voltage and the gate voltage of the depletion type NMOS transistor  10  are consequently higher than in the circuit of  FIG. 1 . The drain-source voltage of the NMOS transistor  15  is therefore high, which prompts the NMOS transistor  15  to operate in a saturation mode. 
       MODIFICATION EXAMPLE 2 
       [0059]      FIG. 3  is a diagram illustrating still another example of the constant current circuit according to this embodiment. Compared to  FIG. 1 , a depletion type NMOS transistor  22  is added in  FIG. 3  as a cascode circuit of the NMOS transistor  12 . The depletion type NMOS transistor  22  has a gate connected to the ground terminal, a source connected to the drain of the NMOS transistor  12 , and a drain connected to the drain of the PMOS transistor  13 . 
         [0060]    With this circuit structure, a fluctuation in the power supply voltage VDD which causes a fluctuation in the drain voltage of the PMOS transistor  13  hardly changes the drain voltage of the NMOS transistor  12 . The current mirror circuit constituted of the NMOS transistors  11  and  12  thus maintains the desired current ratio. Other circuit structures, too, can have a cascode circuit added to the drain of the NMOS transistor  12 . 
       MODIFICATION EXAMPLE 3 
       [0061]      FIG. 4  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 4  differs from  FIG. 1  in that the gate of the depletion type NMOS transistor  10  is connected to the drain of the PMOS transistor  13  and that the gates of the PMOS transistors  13  and  14  are connected to the drain of the PMOS transistor  14 . 
         [0062]    With the transistors connected in this manner, the gate voltage of the depletion type NMOS transistor  10  is controlled based on a relation between a current of the NMOS transistor  12  which mirrors the current Iref and a current of the PMOS transistor  13  which mirrors a current caused by the voltage VA to flow in the NMOS transistor  15 . As in other examples, the circuit of this Modification Example 3 operates in a manner that keeps the current Iref constant even if the current Iref changes. 
       MODIFICATION EXAMPLE 4 
       [0063]      FIG. 5  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment. Compared to  FIG. 4 , the impedance element  21  is added in  FIG. 5 . The impedance element  21  is provided between the source of the depletion type NMOS transistor  10  and a point where the other end of the resistor  20  and the gate of the NMOS transistor  15  are connected to each other. This way, the NMOS transistor  15  is prompted to operate in a saturation mode as in Modification Example 1. 
       MODIFICATION EXAMPLE 5 
       [0064]      FIG. 6  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment. Compared to  FIG. 4 , the depletion type NMOS transistor  22  is added in  FIG. 6  as a cascode circuit of the NMOS transistor  15 . The depletion type NMOS transistor  22  has a gate connected to the ground terminal, a source connected to the drain of the NMOS transistor  15 , and a drain connected to the drain of the PMOS transistor  14 . 
         [0065]    With this circuit structure, a fluctuation in the power supply voltage VDD which causes a fluctuation in the drain voltage of the PMOS transistor  14  hardly changes the drain voltage of the NMOS transistor  15 . Therefore, the drain current of the NMOS transistor  15  does not change as well. Other circuit structures can have a cascode circuit added to the drain of the NMOS transistor  15 . 
       MODIFICATION EXAMPLE 6 
       [0066]      FIG. 7  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 7  differs from  FIG. 1  in that the gate of the NMOS transistor  15  is connected to a point where the drain of the NMOS transistor  11  and the resistor  20  are connected to each other, and that the gates of the NMOS transistors  11  and  12  are connected to a point where the source of the depletion type NMOS transistor  10  and the resistor  20  are connected to each other. The gate-source voltage of the NMOS transistor  15  which, in the circuit design of  FIG. 1 , is higher than the gate-source voltage of the NMOS transistor  11  is lower than the gate-source voltage of the NMOS transistor  11  in  FIG. 7 . 
       MODIFICATION EXAMPLE 7 
       [0067]      FIG. 8  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 8  differs from  FIG. 2  in that the gates of the NMOS transistors  11  and  12  and the gate of the NMOS transistor  15  are connected in the manner described in Modification Example 6. The circuit design of  FIG. 8  is such that the gate-source voltage of the NMOS transistor  15  is lower than the gate-source voltage of the NMOS transistor  11 . 
       MODIFICATION EXAMPLE 8 
       [0068]      FIG. 9  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 9  differs from  FIG. 3  in that the gates of the NMOS transistors  11  and  12  and the gate of the NMOS transistor  15  are connected in the manner described in Modification Example 6. The circuit design of  FIG. 9  is such that the gate-source voltage of the NMOS transistor  15  is lower than the gate-source voltage of the NMOS transistor  11 . 
       MODIFICATION EXAMPLE 9 
       [0069]      FIG. 10  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 10  differs from  FIG. 4  in that the gates of the NMOS transistors  11  and  12  and the gate of the NMOS transistor  15  are connected in the manner described in Modification Example 6. The circuit design of  FIG. 10  is such that the gate-source voltage of the NMOS transistor  15  is lower than the gate-source voltage of the NMOS transistor  11 . 
       MODIFICATION EXAMPLE 10 
       [0070]      FIG. 11  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 11  differs from  FIG. 5  in that the gates of the NMOS transistors  11  and  12  and the gate of the NMOS transistor  15  are connected in the manner described in Modification Example 6. The circuit design of  FIG. 11  is such that the gate-source voltage of the NMOS transistor  15  is lower than the gate-source voltage of the NMOS transistor  11 . 
       MODIFICATION EXAMPLE 11 
       [0071]      FIG. 12  is a diagram illustrating yet still another example of the constant current circuit according to this embodiment.  FIG. 12  differs from  FIG. 6  in that the gates of the NMOS transistors  11  and  12  and the gate of the NMOS transistor  15  are connected in the manner described in Modification Example 6. The circuit design of  FIG. 12  is such that the gate-source voltage of the NMOS transistor  15  is lower than the gate-source voltage of the NMOS transistor  11 .