Abstract:
A fixed voltage generating circuit includes a current mirror, a differential pair, and a resistor coupled to the current mirror. A node of the resistor is coupled to a voltage source. The differential pair includes two resistors coupled to the voltage source for enabling the differential pair to output a fixed voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention is related to a fixed voltage generating circuit, and more particularly, to a fixed voltage generating circuit fabricated using a GaAs (GALLIUM ARSENIDE) process. 
         [0003]    2. Description of the Prior Art 
         [0004]    An RF power amplifier fabricated using a GaAs process has good performance and high efficiency, specifically, the RF power amplifier is less prone to signal distortion, has a lower noise to signal ratio, lower power consumption, higher gain, and smaller size. Thus the RF power amplifier gains advantages of shrinking sizes, increasing efficiency, and lowering power consumption of electronic components, and is suitable for use in mobile phones and all ranges of communication devices. 
         [0005]    In order that the RF power amplifier fabricated using a GaAs process can function normally under a wide input voltage range, a fixed voltage generated by a fixed voltage generating circuit is provided for operations of the RF power amplifier to ensure the RF power amplifier can function normally. 
         [0006]    However the fixed voltage generating circuit is usually fabricated using a CMOS (complementary metal-oxide-semiconductor) process, which includes PMOS (P-type metal-oxide-semiconductor) that is not suitable in a GaAs process. Thus the fixed voltage generating circuit cannot be integrated and fabricated in the same GaAs process when fabricating the RF power amplifier. Instead, an additional CMOS process is needed for fabricating the fixed voltage generating circuit to provide the fixed voltage to the RF power amplifier, thereby increasing sizes and lowering integration of related components. 
       SUMMARY OF THE INVENTION 
       [0007]    An embodiment of the present invention discloses a fixed voltage generating circuit. The fixed voltage generating circuit comprises a first resistor, a first transistor, a second transistor, a third transistor, a fourth transistor, a second resistor, and a third resistor. The first resistor has a first end and a second end, the second end being coupled to a voltage source. The first transistor has a control end coupled to the first end of the first resistor, a first end coupled to a ground node, and a second end coupled to the control end of the first transistor. The second transistor has a control end coupled to the first end of the first resistor and a first end coupled to the ground node. The third transistor has a control end for receiving a first differential voltage and a first end coupled to a second end of the second transistor. The fourth transistor has a control end for receiving a second differential voltage and a first end coupled to the second end of the second transistor. The second resistor has a first end coupled to a second end of the third transistor and a second end coupled to the voltage source. The third resistor has a first end coupled to a second end of the fourth transistor and a second end coupled to the voltage source. Resistance of the second resistor and resistance of the third resistor are substantially equal. 
         [0008]    Another embodiment of the present invention discloses a fixed voltage generating circuit. The A fixed voltage generating circuit comprises a first resistor, a first transistor, a second transistor, a third transistor, a fourth transistor, a second resistor, and a third resistor. The first transistor has a control end coupled to a first end of the first resistor, a first end coupled to a ground node, and a second end coupled to the control end of the first transistor. The second transistor has a control end coupled to the first end of the first resistor and a first end coupled to the ground node. The third transistor has a control end for receiving a first differential voltage and a first end coupled to a second end of the second transistor. The fourth transistor has a control end for receiving a second differential voltage and a first end coupled to the second end of the second transistor. The second resistor has a first end coupled to a second end of the third transistor, and a second end coupled to a voltage source. The third resistor has a first end coupled to a second end of the fourth transistor, and a second end coupled to the voltage source. Equivalent resistance of the second resistor and the third resistor is substantially equal to resistance of the first resistor and resistance of the second resistor and resistance of the third resistor are substantially equal. 
         [0009]    Another embodiment of the present invention discloses a fixed voltage generating circuit. The fixed voltage generating circuit comprises a first resistor, a first transistor, a second transistor, a third transistor, a fourth transistor, a second resistor, a third resistor, and a fourth resistor. The first transistor has a control end coupled to a first end of the first resistor, a first end coupled to a ground node, and a second end coupled to the control end of the first transistor. The second transistor has a control end coupled to the first end of the first resistor and a first end coupled to the ground node. The third transistor has a control end for receiving a first differential voltage and a first end coupled to a second end of the second transistor. The fourth transistor has a control end for receiving a second differential voltage and a first end coupled to the second end of the second transistor. The second resistor has a first end coupled to a second end of the third transistor. The third resistor has a first end coupled to a second end of the fourth transistor. The fourth resistor has a first end coupled to a second end of the second resistor and a second end of the third resistor, and a second end coupled to a voltage source. Equivalent resistance of the second resistor, the third resistor, and the fourth resistor is substantially equal to resistance of the first resistor, and resistance of the second resistor and resistance of the third resistor are substantially equal. 
         [0010]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic illustrating fixed voltage generating circuit according to an embodiment of the present invention. 
           [0012]      FIG. 2  is a schematic illustrating fixed voltage generating circuit according to another embodiment of the present invention. 
           [0013]      FIG. 3  is a schematic illustrating fixed voltage generating circuit according to another embodiment of the present invention. 
           [0014]      FIG. 4  is a schematic illustrating fixed voltage generating circuit according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Please refer to  FIG. 1  which is a schematic illustrating a fixed voltage generating circuit  100  according to an embodiment of the present invention. The fixed voltage generating circuit  100  may include a first resistor  102 , a second resistor  104 , a third resistor  106 , a first transistor  108 , a second transistor  110 , a third transistor  112 , and a fourth transistor  114 . The first resistor  102  has a first end and a second end, the second end being coupled to a voltage source VDD. The first transistor  108  has a control end coupled to the first end of the first resistor  102 , a first end coupled to a ground node, and a second end coupled to the control end of the first transistor  108 . The second transistor  110  has a control end coupled to the first end of the first resistor  102  and a first end coupled to the ground node. The third transistor  112  has a control end for receiving a first differential voltage and a first end coupled to a second end of the second transistor  110 . The fourth transistor  114  has a control end for receiving a second differential voltage and a first end coupled to the second end of the second transistor  110 . The second resistor  104  has a first end coupled to a second end of the third transistor  112  and a second end coupled to the voltage source VDD. The third resistor  106  has a first end coupled to a second end of the fourth transistor  114  and a second end coupled to the voltage source VDD. A resistance ratio of the first resistor  102 , the second resistor  104 , and the third resistor  106  is substantially equal to 1:2:2, and resistance of the second resistor  104  and resistance of the third resistor  106  are substantially equal. 
         [0016]    In  FIG. 1 , a bias voltage is generated via the first resistor  102  at the control end of the first transistor  108  according to the voltage source VDD. The first transistor  108  and the second transistor  110  form a current mirror and a bias current I is generated at the second end of the second transistor  110  according to the bias voltage at the control end of the first transistor  108 , as in formula (1) (where a voltage difference between the control end of the first transistor  108  and the ground node is small and negligible). The bias current I also flows through a differential pair formed by the third transistor  112 , the fourth transistor  114 , the second resistor  104 , and the third resistor  106 . A current flowing through the right side of the differential pair, including the fourth transistor  114  and the third resistor  106 , is half the bias current I because components in the right side and components in the left side of the differential pair are substantially symmetrical. The differential pair is coupled to the voltage source VDD and thus a voltage VD is generated at the second end of the fourth transistor  114 , as in formula (2). 
         [0017]    In formula (1) and formula (3), R 1  is resistance of the first resistor  102 . In formula (2) and formula (4), R 3  is resistance of the third resistor  106 , and Re 1  is equivalent resistance of the second resistor  104  and the third resistor  106 , which is equal to parallel resistance of the second resistor  104  and the third resistor  106  because the right side and the left side of the differential pair are paralleled structure and the resistance of the second resistor  104  and the resistance of the third resistor  106  are substantially equal. Thus 
         [0000]    
       
         
           
             
               Re 
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             = 
             
               
                 
                   R 
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                   3 
                 
                 2 
               
               . 
             
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   I 
                   = 
                   
                     VDD 
                     
                       R 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       
                         VD 
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             
                               I 
                               2 
                             
                             × 
                             R 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             I 
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             
                               VDD 
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     2 
                     ) 
                   
                 
               
             
           
         
       
     
         [0018]    According to formula (1), when the voltage source VDD varies, the bias current I flowing through the differential pair changes accordingly. Assuming the voltage source VDD varies by a voltage variation dVDD which causes the bias current I to change by a current variation dI, as in formula (3). At this time, the voltage VD at the second end of the fourth transistor  114  changes by a voltage deviation dVD, as in formula (4). In formula (4), if R 1 =Re 1 , that is, the resistance of the first resistor  102  is substantially equal to the equivalent resistance Re 1  of the second resistor  104  and the third resistor  106 , the change of the voltage deviation dVD of the voltage VD is substantially zero, namely, the voltage VD is fixed and does not change with the voltage variation dVDD of the voltage source VDD. 
         [0000]    
       
         
           
             
               
                 
                   
                      
                     I 
                   
                   = 
                   
                     
                        
                       VDD 
                     
                     
                       R 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     3 
                     ) 
                   
                 
               
             
             
               
                 
                   
                     
                       
                         
                            
                           VD 
                         
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                               
                                  
                                 I 
                               
                               2 
                             
                             × 
                             R 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                                
                               I 
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                               
                                  
                                 VDD 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     4 
                     ) 
                   
                 
               
             
           
         
       
     
         [0019]    As illustrated above in  FIG. 1 , a fixed voltage VD which does not change with the voltage source VDD is generated so that the fixed voltage generating circuit  100  may work under a wide range of input voltages. In the prior art, the second resistor  104  and the third resistor  106  are replaced with PMOSs in CMOS process which are not suitable for a GaAs process, thus adapting circuit structure used in CMOS process for a GaAa process is not practical. However by adjusting a resistance ratio of multiple resistors and implementing a circuit structure of connecting resistors to the voltage source VDD as described in the embodiment of the present invention, the fixed voltage VD can be generated in a GaAs process without using an additional CMOS process to provide a fixed voltage so as to increase sizes and lowering integration of related components. 
         [0020]    Please refer to  FIG. 2  which is a schematic illustrating a fixed voltage generating circuit  200  according to another embodiment of the present invention. The fixed voltage generating circuit  200  may include all components the fixed voltage generating circuit  100  and may further include a fourth resistor  202 . The second resistor  104  and the third resistor  106  of  FIG. 2  are not coupled directly to the voltage source VDD but are coupled to the voltage source VDD via the fourth resistor  202 . The fourth resistor  202  has a first end coupled to the second end of the second resistor  104  and the second end of the third resistor  106 , and a second end coupled to the voltage source VDD. Equivalent resistance of the second resistor  104 , the third resistor  106 , and the fourth resistor  202  is substantially equal to the resistance of the first resistor  102 . The resistance of the second resistor  104  and the resistance of the third resistor  106  are substantially equal. 
         [0021]    The same principle of formula (1) and formula (3) may be applied in  FIG. 2 . The bias current I flows through the fourth resistor  202  and the differential pair formed by the third transistor  112 , the fourth transistor  114 , the second resistor  104 , and the third resistor  106 . The current flowing through the right side of the differential pair, including the fourth transistor  114  and the third resistor  106 , is half the bias current I because the components in the right side and the components in the left side of the differential pair are substantially symmetrical. The fourth resistor  202  is coupled to the voltage source VDD thus a voltage VD is generated at the second end of the fourth transistor  114 , as in formula (5). 
         [0022]    In formula (5) and formula (6), R 3  is the resistance of the third resistor  106 , R 4  is resistance of the fourth resistor  202 , and Re 2  is equivalent resistance of the second resistor  104 , the third resistor  106 , and the fourth resistor  202 . The equivalent resistance of the second resistor  104  and the third resistor  106  is equal to the parallel resistance of the second resistor  104  and the third resistor  106 . The equivalent resistance of the second resistor  104 , the third resistor  106 , and the fourth resistor  202  is equal to the equivalent resistance of the second resistor  104  and the third resistor  106  plus the resistance of the fourth resistor  202 . The resistance of the second resistor  104  and the resistance of the third resistor  106  are substantially equal. Thus 
         [0000]    
       
         
           
             
               Re 
                
               
                   
               
                
               2 
             
             = 
             
               
                 
                   R 
                    
                   
                       
                   
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                   3 
                 
                 2 
               
               + 
               
                 R 
                  
                 
                     
                 
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                 4. 
               
             
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         VD 
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             I 
                             × 
                             R 
                              
                             
                                 
                             
                              
                             4 
                           
                           - 
                           
                             
                               I 
                               2 
                             
                             × 
                             R 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             I 
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           VDD 
                           - 
                           
                             
                               VDD 
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     5 
                     ) 
                   
                 
               
             
           
         
       
     
         [0023]    According to formula (1), when the voltage source VDD varies, the bias current I changes accordingly. The bias current I changes by a current variation dI, as in formula (3) and the voltage VD at the second end of the fourth transistor  114  changes by a voltage deviation dVD, as in formula (6). In formula (6), if R 1 =Re 2 , that is, the resistance of the first resistor  102  is substantially equal to the equivalent resistance Re 2  of the second resistor  104 , the third resistor  106 , and the fourth resistor  202 , the change of the voltage deviation dVD of the voltage VD is substantially zero, namely, the voltage VD is fixed and does not change with the voltage variation dVDD of the voltage source VDD. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                            
                           VD 
                         
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                                
                               I 
                             
                             × 
                             R 
                              
                             
                                 
                             
                              
                             4 
                           
                           - 
                           
                             
                               
                                  
                                 I 
                               
                               2 
                             
                             × 
                             R 
                              
                             
                                 
                             
                              
                             3 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                                
                               I 
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                              
                             VDD 
                           
                           - 
                           
                             
                               
                                  
                                 VDD 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             × 
                             Re 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                    
                   
                       
                   
                    
                   
                     ( 
                     6 
                     ) 
                   
                 
               
             
           
         
       
     
         [0024]    As illustrated above in  FIG. 2 , the fixed voltage VD which does not change with the voltage source VDD is generated in a GaAs process so that the fixed voltage generating circuit  200  may work under a wide range of input voltages without using additional CMOS process to provide a fixed voltage. 
         [0025]    Please refer to  FIG. 3  which is a schematic illustrating fixed voltage generating circuit  300  according to another embodiment of the present invention. The fixed voltage generating circuit  300  may include all components of the fixed voltage generating circuit  200  and may further include a fifth resistor  302  coupled between the first end of the third transistor  112  and the second end of the second transistor  110 , and a sixth resistor  304  coupled between the first end of the fourth transistor  114  and the second end of the second transistor  110 . 
         [0026]    In  FIG. 3 , the bias current I flows through the fourth resistor  202 , the fifth resistor  302 , the sixth resistor  304 , and the differential pair. The current flowing through the right side of the differential pair is half the bias current I. The same principle of formula (1), (3), (5), (6) may be applied in  FIG. 3 . As long as R 1 =Re 2 , that is, the resistance of the first resistor  102  is substantially equal to the equivalent resistance Re 2  of the second resistor  104 , the third resistor  106 , and the fourth resistor  202 , the change of the voltage deviation dVD of the voltage VD is substantially zero, namely, the voltage VD is fixed and does not change with the voltage variation dVDD of the voltage source VDD. 
         [0027]    Please refer to  FIG. 4  which is a schematic illustrating fixed voltage generating circuit  400  according to an embodiment of the present invention. The fixed voltage generating circuit  400  may include all components of the fixed voltage generating circuit  100  and may further include a fifth resistor  302  coupled between the first end of the third transistor  112  and the second end of the second transistor  110 , and a sixth resistor  304  coupled between the first end of the fourth transistor  114  and the second end of the second transistor  110 . 
         [0028]    In  FIG. 4 , the bias current I flows through the fifth resistor  302 , the sixth resistor  304 , and the differential pair. The current flowing through the right side of the differential pair is half the bias current I. The same principles of formula (1) to (4) may be applied in  FIG. 4 . As long as R 1 =Re 1 , that is, the resistance of the first resistor  102  is substantially equal to the equivalent resistance Re 1  of the second resistor  104  and the third resistor, the change of the voltage deviation dVD of the voltage VD is substantially zero, namely, the voltage VD is fixed and does not change with the voltage variation dVDD of the voltage source VDD. 
         [0029]    In summary, by adjusting a resistance ratio of multiple resistors and implementing a circuit structure of connecting resistors to the voltage source VDD as described in the embodiment of the present invention, the fixed voltage VD can be generated in a GaAs process without using an additional CMOS process to provide a fixed voltage so as to increase sizes and lowering integration of related components in fabrication. 
         [0030]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.