Abstract:
An adaptive biasing input stage includes pairs of differentially coupled amplifying and sensing field effect transistors having gates with differential inputs applied thereon. In addition, a static current source is coupled to sources of the amplifying and sensing field effect transistors at a predetermined node. Also, current mirrors are coupled to the sensing field effect transistors for forming loop mechanisms for increasing the current through the predetermined node when the differential inputs have a non-zero difference.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    The present application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 2007-42819 filed on May 2, 2007 and to Korean Patent Application No. 2007-76963 filed on Jul. 31, 2007, which are incorporated herein by reference in their entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates generally to integrated circuit amplifiers, and more particularly, to an adaptive biasing input stage having small area and high current efficiency, and to amplifiers including the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Analog circuits including analog integrated circuit amplifiers are common components in many electronic devices. As electronic devices are desired to be portable and small, analog integrated circuit amplifiers are desired to be formed with low area and to be operated with low power consumption. 
         [0004]    For minimizing current dissipation, an adaptive biasing amplifier increases biasing currents depending on the input.  FIG. 1  shows a circuit diagram of a prior art adaptive biasing amplifier  100  as disclosed in Degrauwe et al. “Adaptive Biasing CMOS Amplifier”, IEEE Journal of Solid-State Circuits, Vol. SC-17, No. 3, pages 522-528, June 1982. 
         [0005]    Referring to  FIG. 1 , the adaptive biasing amplifier  100  includes NMOSFETs (N-channel metal oxide semiconductor field effect transistors) MN 1  and MN 2  having gates with inputs Vin− and Vin+ applied thereon, respectively. In addition, diode connected PMOSFETs (P-channel metal oxide semiconductor field effect transistors) MP 1  and MP 2  are respectively connected to drains of the NMOSFETs MN 1  and MN 2 . The sources of the NMOSFETs MN 1  and MN 2  are connected to a static current source ISS. 
         [0006]    The adaptive biasing amplifier  100  also includes a first pair of NMOSFETs MN 3  and MN 4  forming a first current mirror, a second pair of NMOSFETs MN 5  and MN 6  forming a second current mirror, a third pair of NMOSFETs MN 7  and MN 8  forming a third current mirror, and a fourth pair of NMOSFETs MN 9  and MN 10  forming a fourth current mirror. The NMOSFETs MN 5  and MN 6  have area scaling with a ratio of 1:A, respectively, and the NMOSFETs MN 7  and MN 8  have area scaling with a ration of A:1, respectively. 
         [0007]    PMOSFETs MP 3  and MP 4  have gates connected to the gate of the PMOSFET MP 1 , and PMOSFETs MP 5  and MP 6  have gates connected to the gate of the PMOSFET MP 2 . The PMOSFETs MP 3 , MP 4 , MP 5 , and MP 6  are connected to the current mirrors formed by the NMOSFETs MN 3 , MN 4 , MN 5 , MN 6 , MN 7 , MN 8 , MN 9 , and MN 10 . 
         [0008]    In addition, the PMOSFETs MP 3 , MP 4 , MP 5 , and MP 6  have sources connected to a high supply voltage VDD, and the NMOSFETs MN 3 , MN 4 , MN 5 , MN 6 , MN 7 , MN 8 , MN 9 , and MN 10  have sources connected to a low supply voltage VSS which may be the ground voltage. As a difference between the inputs Vin− and Vin+ increases, the NMOSFETs MN 3 , MN 4 , MN 5 , MN 6 , MN 7 , MN 8 , MN 9 , and MN 10  and the PMOSFETs MP 3 , MP 4 , MP 5 , and MP 6  increase the respective biasing currents through the MOSFETs MN 1 , MN 2 , MP 1 , and MP 2 . 
         [0009]    The adaptive biasing amplifier  100  has numerous current mirrors formed by the NMOSFETs MN 3 , MN 4 , MN 5 , MN 6 , MN 7 , MN 8 , MN 9 , and MN 10  and numerous static current paths formed by the PMOSFETs MP 3 , MP 4 , MP 5 , and MP 6 . Such components dissipate relatively high static current resulting in low current efficiency. In addition, the input NMOSFETs MN 1  and MN 2  have diode-connected loads MP 1  and MP 2  resulting in low GBW (gain bandwidth). 
         [0010]      FIG. 2  shows a circuit diagram of another prior art adaptive biasing amplifier  110  as disclosed in Callewaert et al. “Class AB CMOS Amplifiers with High Efficiency”, IEEE Journal of Solid-State Circuits, Vol. 25, No. 6, pages 684-691, June 1990. The adaptive biasing amplifier  110  includes input NMOSFETs MN 11  and MN 12  and sensing NMOSFETs MN 13  and MN 14 . Such NMOSFETs MN 11 , MN 12 , MN 13 , and MN 14  have gates with inputs Vin− and Vin+ applied thereon as illustrated in  FIG. 2 . 
         [0011]    Also in  FIG. 2 , a first static current source ISS 1  is connected to the source of the NMOSFET MN 14 , and a second static current source ISS 2  is connected to the source of the NMOSFET MN 13 . The adaptive biasing amplifier  110  also includes a pair of NMOSFETs MN 15  and MN 16  forming a first current mirror and another pair of NMOSFETs MN 17  and MN 18  forming a second current mirror. The NMOSFETs MN 15  and MN 16  have area scaling with a ratio of 1:A, respectively, and the NMOSFETs MN 17  and MN 18  have area scaling with a ratio of A:1, respectively. 
         [0012]    A pair of PMOSFETs MP 11  and MP 12  forms a third current mirror connected between a high supply voltage VDD and the NMOSFETs MN 11  and MN 14 . A pair of PMOSFETs MP 13  and MP 14  forms a fourth current mirror connected between the high supply voltage VDD and the NMOSFETs MN 13  and MN 12 . A first bias current source Ib 1  is connected between the high supply voltage VDD, the drain of the NMOSFET MN 11 , and a source of a biased PMOSFET MP 15 . Further referring to  FIG. 2 , PMOSFETs MP 17 ′ and MP 18 ′ that are diode-connected and an NMOSFET MN 19 ′ are formed for biasing the gate of the PMOSFET MP 15 . The PMOSFET MP 15  has a drain connected to the drain of the NMOSFET MN 15 . 
         [0013]    A second bias current source Ib 2  is connected between the high supply voltage VDD, the drain of the NMOSFET MN 12 , and a source of another biased PMOSFET MP 16 . A drain of the PMOSFET MP 16  is connected to a drain of the NMOSFET MN 18 . Two diode-connected PMOSFETs MP 17  and MP 18  are connected between the high supply voltage VDD and an NMOSFET MN 19 . The gate of the PMOSFET MP 16  is connected to the gate of the PMOSFET MP 18 . The gate of the NMOSFET MN 19  is connected to the gate of the NMOSFET MN 18 . The NMOSFETs MN 15 , MN 16 , MN 17 , MN 18 , and MN 19  have sources connected to a low supply voltage VSS which may be the ground voltage. 
         [0014]    As a difference between the input voltages Vin− and Vin+ increases, the components in the adaptive biasing amplifier  110  of  FIG. 2  increase the respective biasing currents through the MOSFETs MN 11 , MN 12 , MP 11 , and MP 14 . In addition, the GBW (gain bandwidth) of the adaptive biasing amplifier  110  of  FIG. 2  is improved from the adaptive biasing amplifier  100  of  FIG. 1 . 
         [0015]    However, the first series of diode-connected PMOSFETs MP 17  and MP 18  and the second series of diode-connected PMOSFETs MP 17 ′ and MP 18 ′ result in increased operating voltage in the adaptive biasing amplifier  110  that consequently has a limited minimum operating voltage. Lower operating voltage is desired for minimizing power consumption and portability with a lighter battery. In addition, the adaptive biasing amplifier  110  of  FIG. 2  includes four current sources ISS 1 , ISS 2 , Ib 1 , and Ib 2  that occupy large integrated circuit area. 
         [0016]    Thus, adaptive biasing amplifiers are desired to have high GBW (gain bandwidth), minimized area, high current efficiency, and low operating voltage. 
       SUMMARY OF THE INVENTION 
       [0017]    Accordingly, in a general aspect of the present invention, an adaptive biasing input stage is formed with a single static current source and without a series of multiple diode-connected MOSFETs. 
         [0018]    An adaptive biasing input stage according to an aspect of the present invention includes a pair of differentially coupled amplifying field effect transistors having gates with differential inputs applied thereon and includes a pair of differentially coupled sensing field effect transistors having gates with the differential inputs applied thereon. The adaptive biasing input stage further includes a static current source coupled between a first power supply node and sources of the amplifying and sensing field effect transistors at a predetermined node. Also, the adaptive biasing input stage includes a first current mirror coupled between one of the sensing field effect transistors and a second power supply node, and includes a second current mirror coupled between the first current mirror, the predetermined node, and the first power supply node. 
         [0019]    In an embodiment of the present invention, the sensing field effect transistors and the first and second current mirrors increase a current flowing from the predetermined node when a first differential input applied on such one of the sensing field effect transistors is greater than a second differential input applied on another of the sensing field effect transistors. 
         [0020]    In another embodiment of the present invention, the second current mirror includes a first mirror transistor having a first area and being coupled to the first current mirror, and includes a second mirror transistor having a second area that is A-times the first area and being coupled to the predetermined node. In an example embodiment of the present invention, A is between a range of from about 1 to about 2 with A being selected to maximize current efficiency of the adaptive biasing input stage. 
         [0021]    In another embodiment of the present invention, the adaptive biasing input stage includes a third current mirror coupled between another of the sensing field effect transistors and the second power supply node, and includes a fourth current mirror coupled between the third current mirror, the predetermined node, and the first power supply node. In that case, the sensing field effect transistors and the third and fourth current mirrors increase a current flowing from the predetermined node when a first differential input applied on the other of the sensing field effect transistors is greater than a second differential input applied on such one of the sensing field effect transistors. 
         [0022]    In a further embodiment of the present invention, the fourth current mirror includes a third mirror transistor having a third area and being coupled to the third current mirror, and includes a fourth mirror transistor having a fourth area that is A-times the third area and being coupled to the predetermined node. For example, A is between a range of from about 1 to about 2, with A being selected to maximize current efficiency of the adaptive biasing input stage. 
         [0023]    In another embodiment of the present invention, a static current through each field effect transistor of the adaptive biasing input stage increases with A, and a respective dynamic current through each of the amplifying and sensing field effect transistors is a respective quadratic function of a difference between the differential inputs. 
         [0024]    In a further embodiment of the present invention, the adaptive biasing input stage further includes load field effect transistors coupled between the amplifying field effect transistors and the second power supply node and being biased by the first and third current mirrors. 
         [0025]    The adaptive biasing input stage of embodiments of the present invention may be used to particular advantage for forming amplifiers. In that case, such an amplifier includes the adaptive biasing input stage and an output stage coupled between the adaptive biasing input stage and an output node for generating an output signal at the output node. 
         [0026]    In an example embodiment of the present invention, the output stage includes first and second output field effect transistors. The first output field effect transistor is coupled between the second power supply node and the output node and is biased with one of the first and third current mirrors coupled to one of the sensing field effect transistors. The second output field effect transistor is coupled between the first power supply node and the output node and is biased with one of the second and fourth current mirrors coupled to such one of the sensing field effect transistors. 
         [0027]    In another embodiment of the present invention, the output stage includes first and second output field effect transistors, a compensating resistor, and a compensating capacitor. The first output field effect transistor is coupled between the second power supply node and the output node and has a gate coupled to a drain of one of the amplifying field effect transistors, with an active load being coupled to the drain of such one of the amplifying field effect transistors. The compensating resistor and the compensating capacitor are coupled in series between the gate of the first output field effect transistor and the output node. The second output field effect transistor is coupled between the first power supply node and the output node and is biased with one of the second and fourth current mirrors coupled to one of the sensing field effect transistors having a same one of the differential inputs applied thereon as such one of the amplifying field effect transistors. 
         [0028]    The adaptive biasing input stage of embodiments of the present invention may also be used to particular advantage for forming rail-to-rail amplifiers. In that case, such a rail-to-rail amplifier includes first and second adaptive biasing input stages, each implemented according to above described embodiments of the present invention and includes an output stage. 
         [0029]    The first adaptive biasing input stage receives differential inputs to generate a first intermediate signal to the output stage, and the second adaptive biasing input stage receives the differential inputs to generate a second intermediate signal to the output stage. In addition, respective field effect transistors of the first and second adaptive biasing input stages are mirror complements of each-other as P-type versus N-type field effect transistors. 
         [0030]    In this manner, the adaptive biasing input stage is formed with relatively few current mirrors, a single static current source, and without a series of multiple diode-connected field effect transistors. Thus, the adaptive biasing input stage occupies a small area, has high current efficiency, and high operating voltage range. 
         [0031]    These and other features and advantages of the present invention will be better understood by considering the following detailed description of the invention which is presented with the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIGS. 1 and 2  show circuit diagrams of adaptive biasing amplifiers according to the prior art; 
           [0033]      FIG. 3  shows an adaptive biasing input stage according to an embodiment of the present invention; 
           [0034]      FIG. 4  shows a dynamic current characteristic of the adaptive biasing input stage of  FIG. 3  according to an embodiment of the present invention; 
           [0035]      FIG. 5  shows a circuit diagram of an adaptive biasing amplifier advantageous for high load capacitance and having the adaptive biasing input stage of  FIG. 3  according to an embodiment of the present invention; 
           [0036]      FIG. 6  shows a plot of current efficiency versus area ratio of mirror field effect transistors in the adaptive biasing amplifier of  FIG. 5 , according to an embodiment of the present invention; 
           [0037]      FIG. 7  shows a circuit diagram of an adaptive biasing amplifier advantageous for high gain bandwidth and having the adaptive biasing input stage of  FIG. 3 , according to another embodiment of the present invention; and 
           [0038]      FIGS. 8 and 9  show circuit diagrams of rail-to-rail adaptive biasing amplifiers each having adaptive biasing input stages, according to an embodiment of the present invention. 
       
    
    
       [0039]    The figures referred to herein are drawn for clarity of illustration and are not necessarily drawn to scale. Elements having the same reference number in  FIGS. 1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  9  refer to elements having similar structure and/or function. 
       DETAILED DESCRIPTION 
       [0040]      FIG. 3  shows a circuit diagram of an adaptive input stage  200  according to an embodiment of the present invention. Referring to  FIG. 3 , the adaptive input stage  200  includes a pair of differentially coupled amplifying NMOSFETs (N-channel metal oxide semiconductor field effect transistors) MN 21  and MN 22 . In addition, the adaptive input stage  200  includes a pair of differentially coupled sensing NMOSFETs MN 23  and MN 24 . 
         [0041]    A static current source  203  is coupled to sources of the amplifying and sensing NMOSFETs MN 21 , MN 22 , MN 23 , and MN 24  at a predetermined node  202 . In addition, the static current source  203  is coupled to a first power supply node that provides a low supply voltage VSS which may be the ground voltage. The gate of the amplifying NMOSFET MN 21  and the gate of the sensing NMOSFET MN 23  have a first input Vin− applied thereon. The gate of the amplifying NMOSFET MN 222  and the gate of the sensing NMOSFET MN 24  have a second input Vin+ applied thereon. The first and second inputs Vin− and Vin+ are differential inputs in an embodiment of the present invention. 
         [0042]    A first current mirror is formed by a pair of PMOSFETs (P-channel metal oxide semiconductor field effect transistors) MP 21  and MP 22 . The PMOSFET MP 21  is diode-connected between a second power supply node that provides a high supply voltage VDD and a drain of the sensing NMOSFET MN 23 . The PMOSFET MP 22  has a source coupled to the second power supply node VDD and has a gate coupled to the gate of the PMOSFET MP 21 . A load PMOSFET MP 23  has a gate coupled to the gate of the PMOSFET MP 21 , has a source coupled to the second power supply node VDD, and has a drain coupled to the drain of the amplifying NMOSFET MN 22 . 
         [0043]    A second current mirror is formed by a pair of NMOSFETs MN 25  and MN 26  coupled between the PMOSFET MP 22  of the first current mirror, the predetermined node  202 , and the first power supply node VSS. The NMOSFET MN 26  is diode-connected between the first power supply node VSS and a drain of the PMOSFET MP 22 . The NMOSFET MN 25  has a source coupled to the first power supply node VSS, has a gate coupled to the gate of the NMOSFET MN 26 , and has a drain coupled to the predetermined node  202 . 
         [0044]    A third current mirror is formed by a pair of PMOSFETs MP 24  and MP 25 . The PMOSFET MP 24  is diode-connected between the second power supply node VDD and a drain of the sensing NMOSFET MN 24 . The PMOSFET MP 25  has a source coupled to the second power supply node VDD and has a gate coupled to the gate of the PMOSFET MP 24 . A load PMOSFET MP 26  has a gate coupled to the gate of the PMOSFET MP 24 , has a source coupled to the second power supply node VDD, and has a drain coupled to the drain of the amplifying NMOSFET MN 21 . 
         [0045]    A fourth current mirror is formed by a pair of NMOSFETs MN 27  and MN 28  coupled between the PMOSFET MP 25  of the third current mirror, the predetermined node  202 , and the first power supply node VSS. The NMOSFET MN 28  is diode-connected between the first power supply node VSS and a drain of the PMOSFET MP 25 . The NMOSFET MN 27  has a source coupled to the first power supply node VSS, has a gate coupled to the gate of the NMOSFET MN 28 , and has a drain coupled to the predetermined node  202 . 
         [0046]    The NMOSFET MN 26  is a first mirror transistor having a first area, and the NMOSFET MN 25  is a second mirror transistor having a second area that is A-times the first area of the NMOSFET MN 26 . The NMOSFET MN 28  is a third mirror transistor having a third area, and the NMOSFET MN 27  is a fourth mirror transistor having a fourth area that is A-times the third area of the NMOSFET MN 28 . 
         [0047]    In an embodiment of the present invention, the PMOSFETs MP 21 , MP 22 , MP 23 , MP 24 , MP 25 , and MP 26  are equally sized. In addition, the NMOSFETs MN 21 , MN 22 , MN 23 , and MN 24  are fabricated to be matched according to an embodiment of the present invention. 
         [0048]    Operation of the adaptive input stage  200  of  FIG. 3  is now described with reference to a dynamic current characteristic as illustrated in  FIG. 4 . However, operation of the adaptive input stage  200  during a static state is first described. During the static state, Vin=0 when Vin+=Vin−=Vin,cm, with Vin,cm being a common mode voltage of the inputs Vin+ and Vin−. In that case, I 1 ,static=I 2 ,static=I 1 ′,static=I 2 ′,static=Istatic; with I 1 ,static being a static current for I 1 ; I 2 ,static being a static current for I 2 ; I 1 ′,static being a static current for I 1 ′; and I 2 ′,static being a static current for I 2 ′, during the static operation for the adaptive input stage  200  of  FIG. 3 . 
         [0049]    In that case, Istatic is expressed as follows in Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     static 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               I 
                               
                                 S 
                                  
                                 
                                     
                                 
                                  
                                 S 
                               
                             
                             
                               2 
                                
                               
                                 ( 
                                 
                                   2 
                                   - 
                                   A 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             0 
                             ≤ 
                             A 
                             &lt; 
                             2 
                           
                         
                       
                       
                         
                           ∞ 
                         
                         
                           
                             2 
                             ≤ 
                             A 
                           
                         
                       
                     
                     } 
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    ISS is the level of the static current provided by the static current source  203 , and A is the ratio of the areas of the NMOSFETs MN 25  and MN 26  or of the NMOSFETs MN 27  and MN 28 . When A≧2, the static current Istatic does not reach infinity but rather saturation results. 
         [0050]    Next, operation of the adaptive input stage  200  during a dynamic state is described. During the dynamic state, Vin≠0 with Vin=Vin+−Vin−. Assuming that the MOSFETs of  FIG. 3  operate in strong inversion, the Istatic component of the currents I 1 , I 1 ′, I 2 , and I 2 ′ is expressed as follows in Equation 2 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     I 
                     static 
                   
                   = 
                   
                     
                       K 
                       2 
                     
                      
                     
                       
                         ( 
                         
                           
                             V 
                             
                               in 
                               , 
                               cm 
                             
                           
                           - 
                           
                             V 
                             
                               x 
                               , 
                               static 
                             
                           
                           - 
                           
                             V 
                             thn 
                           
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    K=μn*Cox*(W/L), with μn being the electron mobility, Cox being a capacitance per unit area, and W/L being the width to length ratio of each of the NMOSFETs MN 21 , MN 22 , MN 23 , and MN 24 . In addition, Vx,static is a voltage at the predetermined node  202  during the static state, and Vthn is the threshold voltage of each of the NMOSFETs MN 21 , MN 22 , MN 23 , and MN 24 . Equation 2 may then also be expressed as Equation 3 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       V 
                       
                         in 
                         , 
                         cm 
                       
                     
                     - 
                     
                       V 
                       
                         x 
                         , 
                         static 
                       
                     
                     - 
                     
                       V 
                       thn 
                     
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                             
                         
                          
                         
                           I 
                           static 
                         
                       
                       K 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
         [0051]    Furthermore, Vx which is the voltage at the predetermined node  202  may be approximated to Equation 4 below: 
         [0000]      V x ≈V x,static −α|V in |(α≈√{square root over (A)})   [Equation 4] 
         [0052]    If Vin&gt;0 when Vin+&gt;Vin− during the dynamic state, then I 1 ,dynamic is expressed as Equation 5 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             1 
                             , 
                             dynamic 
                           
                         
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     + 
                                   
                                 
                                 - 
                                 
                                   V 
                                   X 
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     , 
                                     cm 
                                   
                                 
                                 + 
                                 
                                   
                                     V 
                                     in 
                                   
                                   2 
                                 
                                 - 
                                 
                                   V 
                                   
                                     X 
                                     , 
                                     static 
                                   
                                 
                                 + 
                                 
                                   α 
                                    
                                   
                                      
                                     
                                       V 
                                       in 
                                     
                                      
                                   
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               { 
                               
                                 
                                   
                                     ( 
                                     
                                       α 
                                       + 
                                       
                                         1 
                                         2 
                                       
                                     
                                     ) 
                                   
                                    
                                   
                                     V 
                                     in 
                                   
                                 
                                 + 
                                 
                                   
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       
                                         I 
                                         static 
                                       
                                     
                                     K 
                                   
                                 
                               
                               } 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Such I 1 ,dynamic is illustrated by a quadratic function A versus Vin in  FIG. 4 . When Vin&gt;0, a large amount of dynamic current I 1 ,dynamic flows from the current loop formed by MOSFETs MN 24 , MP 24 , MP 25 , MN 28 , and MN 27 . Thus, the current mirrors formed by MOSFETs MN 28 , MN 27 , MP 24  and MP 25  increase the current flowing through the predetermined node  202  when Vin&gt;0. 
         [0053]    In addition in that case, I 2 ,dynamic, I 1 ′,dynamic, and I 2 ′,dynamic are expressed as Equation 6 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             2 
                             , 
                             dynamic 
                           
                         
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     - 
                                   
                                 
                                 - 
                                 
                                   V 
                                   X 
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     , 
                                     cm 
                                   
                                 
                                 - 
                                 
                                   
                                     V 
                                     in 
                                   
                                   2 
                                 
                                 - 
                                 
                                   V 
                                   
                                     X 
                                     , 
                                     static 
                                   
                                 
                                 + 
                                 
                                   α 
                                    
                                   
                                      
                                     
                                       V 
                                       in 
                                     
                                      
                                   
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               K 
                               2 
                             
                              
                             
                               
                                 { 
                                 
                                   
                                     
                                       ( 
                                       
                                         α 
                                         - 
                                         
                                           1 
                                           2 
                                         
                                       
                                       ) 
                                     
                                      
                                     
                                       V 
                                       in 
                                     
                                   
                                   + 
                                   
                                     
                                       
                                         2 
                                          
                                         
                                             
                                         
                                          
                                         
                                           I 
                                           static 
                                         
                                       
                                       K 
                                     
                                   
                                 
                                 } 
                               
                               2 
                             
                           
                           = 
                           
                             I 
                             
                               
                                 1 
                                 ′ 
                               
                               , 
                               dynamic 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           I 
                           
                             
                               2 
                               ′ 
                             
                             , 
                             dynamic 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Such I 2 ,dynamic, I 1 ′,dynamic, and I 2 ′,dynamic are illustrated by current characteristic B versus Vin in  FIG. 4 . 
         [0054]    If Vin&lt;0 when Vin+&lt;Vin− during the dynamic state, then I 1 ,dynamic, I 1 ′,dynamic, and I 2 ′,dynamic are expressed as Equation 7 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             1 
                             , 
                             dynamic 
                           
                         
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     + 
                                   
                                 
                                 - 
                                 
                                   V 
                                   X 
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               { 
                               
                                 
                                   
                                     ( 
                                     
                                       α 
                                       - 
                                       
                                         1 
                                         2 
                                       
                                     
                                     ) 
                                   
                                    
                                   
                                     V 
                                     in 
                                   
                                 
                                 - 
                                 
                                   
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       
                                         I 
                                         static 
                                       
                                     
                                     K 
                                   
                                 
                               
                               } 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             I 
                             
                               
                                 1 
                                 ′ 
                               
                               , 
                               dynamic 
                             
                           
                           = 
                           
                             I 
                             
                               
                                 2 
                                 ′ 
                               
                               , 
                               dynamic 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     7 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Such I 1 ,dynamic, I 1 ′,dynamic, and I 2 ′,dynamic are illustrated by current characteristic D versus Vin in  FIG. 4 . 
         [0055]    Also in that case, I 2 ,dynamic is expressed as Equation 8 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           I 
                           
                             2 
                             , 
                             dynamic 
                           
                         
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               ( 
                               
                                 
                                   V 
                                   
                                     in 
                                     - 
                                   
                                 
                                 - 
                                 
                                   V 
                                   X 
                                 
                                 - 
                                 
                                   V 
                                   thn 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             K 
                             2 
                           
                            
                           
                             
                               { 
                               
                                 
                                   
                                     ( 
                                     
                                       α 
                                       + 
                                       
                                         1 
                                         2 
                                       
                                     
                                     ) 
                                   
                                    
                                   
                                     V 
                                     in 
                                   
                                 
                                 - 
                                 
                                   
                                     
                                       2 
                                        
                                       
                                           
                                       
                                        
                                       
                                         I 
                                         static 
                                       
                                     
                                     K 
                                   
                                 
                               
                               } 
                             
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    Such I 2 ,dynamic is illustrated by a quadratic function C versus Vin in  FIG. 4 . When Vin&lt;0, a large amount of dynamic current I 2 ,dynamic flows from the current loop formed by MOSFETs MN 23 , MP 21 , MP 22 , MN 26 , and MN 25 . Thus, the current mirrors formed by MOSFETs MN 23 , MP 21 , MP 22 , MN 26 , and MN 25  increase the current flowing through the predetermined node  202  when Vin&lt;0. 
         [0056]    Additionally,  FIG. 4  shows that at an example Vin=V 1 , I 1 ,dynamic=Ia, and I 2 ,dynamic=Ib. In that case, (A− 1 )Ia+(A− 3 )Ib+ISS&gt;0 which approximates to (A− 1 )Ia≧0 because Ib is much less than Ia. Thus, the value of A (which is the area ratio for the NMOSFETs MN 25  and MN 26  or for the NMOSFETs MN 27  and MN 28 ) that satisfies the dynamic characteristic has the range of A≧1. Such a dynamic characteristic of A≧1 results in providing large dynamic currents I 1 ,dynamic or I 2 ,dynamic when Vin≠0 in the dynamic state. Also given Equation 1 above, for simultaneously satisfying both the static condition of maintaining a small static current and the dynamic condition of a fast slew rate, A is desired to be in the range of 1≦A&lt;2. 
         [0057]      FIG. 5  shows an amplifier  300  formed with the adaptive input stage  200  of  FIG. 3  according to an embodiment of the present invention. Elements having the same reference number in  FIGS. 3 and 5  refer to elements having similar structure and/or function. The amplifier  300  includes the adaptive input stage  200  and an output stage  302  coupled between the adaptive input stage  200  and an output node  304 . 
         [0058]    The output stage  302  includes an output PMOSFET MP 31  having a source coupled to the second power supply node VDD, a gate coupled to the gate of the PMOSFET MP 24 , and a drain coupled to the output node  304 . The output stage  302  also includes an output NMOSFET MN 31  having a source coupled to the first power supply node VSS, a gate coupled to the gate of the NMOSFET MN 26 , and a drain coupled to the output node  304 . A load capacitor CL is coupled between the output node  304  and the first power supply node VSS. 
         [0059]    Furthermore, the area ratio A may be optimized for maximizing current efficiency (CE) of the adaptive biasing amplifier  300  of  FIG. 5 . CE is expressed as Equation 9 below: 
         [0000]        CE=I   load,dynamic   /I   total   =SR×C   L   /I   total    [Equation 9] 
         [0000]    Itotal is the total quiescent current in the adaptive biasing amplifier  300  and SR is the slew rate for charging or discharging the load capacitor CL. 
         [0060]    Iload,dynamic which is the dynamic current to the load capacitor CL is expressed as Equation 10 below: 
         [0000]        I   load,dynamic   =I   1,dynamic   −I   2,dynamic   =±√{square root over (K)} (α √{square root over (K)}V   in   2 +2 I   static   |V   in |)   [Equation 10] 
         [0000]    The plus sign for the Iload,dynamic in Equation 10 is for a current charging the load capacitor CL when Vin&gt;0, and the minus sign is for the current discharging the load capacitor CL when Vin&lt;0. The slew rate SR is expressed as Equation 11 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     S 
                      
                     
                         
                     
                      
                     R 
                   
                   = 
                   
                     
                       
                         ± 
                         
                           K 
                         
                       
                        
                       
                         ( 
                         
                           
                             α 
                              
                             
                                 
                             
                              
                             
                               K 
                             
                              
                             
                               V 
                               in 
                               2 
                             
                           
                           + 
                           
                             2 
                              
                             
                                 
                             
                              
                             
                               I 
                               static 
                             
                              
                             
                                
                               
                                 V 
                                 in 
                               
                                
                             
                           
                         
                         ) 
                       
                     
                     
                       C 
                       L 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     11 
                   
                   ] 
                 
               
             
           
         
       
     
         [0000]    The CE versus A may be plotted as illustrated in  FIG. 6  and analyzed for determining an optimum area ratio A=1.5 at point  215  in  FIG. 6  when the current efficiency CE is maximized. 
         [0061]    The adaptive biasing amplifier  300  of  FIG. 5  is advantageous for the case of a large capacitance of the load capacitor CL or for the case of a variable capacitance of the load capacitor CL. Sufficient phase margin may be obtained with the adaptive biasing amplifier  300  of  FIG. 5 . However, the adaptive biasing amplifier  300  of  FIG. 5  may have relatively low gain. 
         [0062]      FIG. 7  shows a circuit diagram of an adaptive biasing amplifier  350  having relatively high gain and formed with the adaptive input stage  200  of  FIG. 3  according to another embodiment of the present invention. Elements having the same reference number in  FIGS. 3 and 7  refer to elements having similar structure and/or function. The amplifier  350  includes the adaptive input stage  200  and an output stage  352  coupled between the adaptive input stage  200  and an output node  354 . 
         [0063]    The output stage  352  includes an output PMOSFET MP 32  having a source coupled to the second power supply node VDD, a gate coupled to the drain of the NMOSFET MN 22 , and a drain coupled to the output node  354 . The output stage  352  also includes an output NMOSFET MN 32  having a source coupled to the first power supply node VSS, a gate coupled to the gate of the NMOSFET MN 26 , and a drain coupled to the output node  354 . A load capacitor CL is coupled between the output node  354  and the first power supply node VSS. 
         [0064]    In addition, the output stage  352  includes a compensating resistor Rz and a compensating capacitor Cc coupled in series between the gate of PMOSFET MP 32  and the output node  354 . The adaptive biasing amplifier  350  of  FIG. 7  provides relatively high gain and high GBW (gain bandwidth) and is advantageous when the load capacitor CL has relatively low capacitance. 
         [0065]      FIG. 8  shows a circuit diagram of an adaptive biasing rail-to-rail amplifier  400  according to another embodiment of the present invention. The rail-to-rail amplifier  400  includes a first adaptive biasing input stage  402 , a second adaptive biasing input stage  404 , and an output stage  406 . 
         [0066]    The first adaptive biasing input stage  402  receives the differential inputs Vin+ and Vin− to generate a first intermediate signal at a first intermediate node  408 . The second adaptive biasing input stage  404  receives the differential inputs Vin+ and Vin− to generate a second intermediate signal at a second intermediate node  410 . The output stage  406  receives the first and second intermediate signals at the first and second intermediate nodes  408  and  410  to generate an output signal Vout at an output node  412 . 
         [0067]    The first adaptive biasing input stage  402  is implemented similarly to the adaptive biasing input stage  200  of  FIG. 3  with elements having the same reference number in  FIGS. 3 and 8  referring to elements having similar structure and/or function. In addition, the second adaptive biasing input stage  404  includes MOSFETs that are mirror complements of the MOSFETs of the first adaptive biasing input stage  402 . 
         [0068]    Thus, the second adaptive biasing input stage  404  includes NMOSFETs MN 45 , MN 46 , MN 44 , MN 41 , MN 43 , and MN 42  that are configured as mirror complements of the PMOSFETs MP 25 , MP 26 , MP 24 , MP 21 , MP 23 , and MP 22 , respectively, of the first adaptive biasing input stage  402 . In addition, the second adaptive biasing input stage  404  includes PMOSFETs MP 48 , MP 47 , MP 45 , MP 46 , MP 41 , MP 43 , MP 44 , and MP 42  that are configured as mirror complements of the NMOSFETs MN 28 , MN 27 , MN 25 , MN 26 , MN 21 , MN 23 , MN 24 , and MN 22 , respectively, of the first adaptive biasing input stage  402 . The second adaptive biasing input stage  404  includes a respective static current source  414  that is configured as a mirror complement of the respective static current source  203  of the first adaptive biasing input stage  402 . 
         [0069]    The output stage  406  includes a first bias current source IBIAS 1  coupled between the second power supply node VDD and the first intermediate node  408  that is coupled to the drain of the NMOSFET MN 22 . The output stage  406  also includes a second bias current source IBIAS 2  coupled between the first power supply node VSS and the second intermediate node  410  that is coupled to the drain of the PMOSFET MP 42 . The bias current sources IBIAS 1  and IBIAS 2  provide equal currents according to one embodiment of the present invention. 
         [0070]    The output stage  406  further includes a first output PMOSFET MP 51  having a source coupled to the second power supply node VDD, a gate coupled to the first intermediate node  408 , and a drain coupled to the output node  412 . A first compensating capacitor Cc 1  is coupled between the gate and the drain of the PMOSFET MP 51 . 
         [0071]    The output stage  406  also includes a first output NMOSFET MN 51  having a source coupled to the first power supply node VSS, a gate coupled to the second intermediate node  410 , and a drain coupled to the output node  412 . A second compensating capacitor Cc 2  is coupled between the gate and the drain of the NMOSFET MN 51 . The compensating capacitors Cc 1  and Cc 2  have equal capacitances according to one embodiment of the present invention. 
         [0072]    The output stage  406  further includes a second output PMOSFET MP 52  having a source coupled to the first intermediate node  408 , a gate having a first bias voltage VBP applied thereon, and a drain coupled to the second intermediate node  410 . The output stage  406  also includes a second output NMOSFET MN 52  having a drain coupled to the first intermediate node  408 , a gate having a second bias voltage VBN applied thereon, and a source coupled to the second intermediate node  410 . 
         [0073]    The first and second adaptive biasing input stages  402  and  404  operate similarly to the adaptive biasing input stage  200  of  FIG. 3 . By having both the first and second adaptive biasing input stages  402  and  404  that are configured as mirror complements, the rail-to-rail amplifier  400  operates for each of the inputs Vin+ and Vin− being in the rail-to-rail range of VDD to VSS. 
         [0074]      FIG. 9  shows a circuit diagram of an adaptive biasing rail-to-rail amplifier  500  with further minimization of the number of components according to another embodiment of the present invention. The rail-to-rail amplifier  500  includes a first adaptive biasing input stage  502 , a second adaptive biasing input stage  504 , and an output stage  506 . The output stage  506  of  FIG. 9  is implemented similarly to the output stage  406  of  FIG. 8 . 
         [0075]    In addition, the first adaptive biasing input stage  502  of  FIG. 9  is implemented similarly to the first adaptive biasing input stage  402  of  FIG. 8 . However, the PMOSFET MP 22  and the current mirror formed by the NMOSFETs MN 25  and MN 26  in the first adaptive biasing input stage  402  of  FIG. 8  are eliminated in the first adaptive biasing input stage  502  of  FIG. 9 . Rather, the first adaptive biasing input stage  502  of  FIG. 9  includes a mirroring PMOSFET MP 53  having a source coupled to the second power supply node VDD, a gate coupled to the gate of the PMOSFET MP 21 , and a drain coupled to the drain of the PMOSFET MP 25 . 
         [0076]    Similarly, the second adaptive biasing input stage  504  of  FIG. 9  is implemented similarly to the second adaptive biasing input stage  404  of  FIG. 8 . However, the NMOSFET MP 42  and the current mirror formed by the PMOSFETs MP 45  and MP 46  in the second adaptive biasing input stage  404  of  FIG. 8  are eliminated in the second adaptive biasing input stage  504  of  FIG. 9 . Rather, the second adaptive biasing input stage  504  of  FIG. 9  includes a mirroring NMOSFET MN 53  having a source coupled to the first power supply node VSS, a gate coupled to the gate of the NMOSFET MN 41 , and a drain coupled to the drain of the NMOSFET MN 45 . 
         [0077]    In  FIG. 9 , by having both the first and second adaptive biasing input stages  502  and  504  that are configured as mirror complements, the rail-to-rail amplifier  500  operates for each of the inputs Vin+ and Vin− being in the rail-to-rail range of VDD to VSS. In addition, the rail-to-rail amplifier  500  of  FIG. 9  is implemented with less MOSFETs in the first and second adaptive biasing input stages  502  and  504  than the rail-to-rail amplifier  400  of  FIG. 8 . Thus, the rail-to-rail amplifier  500  of  FIG. 9  advantageously has minimized integrated circuit area. 
         [0078]    In this manner, the adaptive biasing input stage  200  of  FIG. 3  and the amplifiers  300 ,  350 ,  400 , and  500  of  FIGS. 5 ,  7 ,  8 , and  9  including such an adaptive biasing input stage has high current efficiency with a minimized number of current sources and current mirrors. Thus, such integrated circuits operate with minimized power consumption and integrated circuit area. 
         [0079]    The foregoing is by way of example only and is not intended to be limiting. Thus, any number of elements as illustrated and described herein is by way of example only. In addition, the adaptive biasing input stage  200  of  FIG. 3  may be used for implementing other types of amplifiers aside from the examples of  FIGS. 5 ,  7 ,  8 , and  9 . 
         [0080]    The present invention is limited only as defined in the following claims and equivalents thereof.