Abstract:
A method and apparatus for extending the common mode range of a differential amplifier. A circuit has a common mode detection circuit, a common mode voltage inversion circuit, and a differential amplifier. The common mode detection circuit receives a differential signal and detects a common mode voltage. The common mode voltage inversion circuit is coupled to the common mode detection circuit. The common mode voltage inversion circuit has an input node that receives the common mode voltage and an output node that outputs body voltage, wherein the common mode voltage inversion circuit creates an inverse relationship between the common mode voltage and the body voltage. The differential amplifier includes a differential pair of transistors that have a pair of body terminals coupled to the output node of the common mode voltage inversion circuit.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The disclosure relates generally to differential amplifiers and, more specifically, to body biasing circuitry for differential amplifiers. 
         [0003]    2. Description of the Related Art 
         [0004]    As the demand for bandwidth and gain requirements in serial link receivers increases, complex problems continue to rise to the forefront. For example, input signals of varying ranges often lead to non-linear operation of differential amplifier stages. These non-linear responses are caused by limited common mode range of the differential amplifier. Design engineers continually seek to solve problems relating to common mode range of their devices. 
         [0005]    In a differential amplifier, the input common mode range refers to the range of differential input signals over which a differential amplifier maintains a linear response, including differential gain. In its simplest form, a differential amplifier has a pair of differential input transistors that receive a differential signal. Differential input signals have a common mode voltage that is the average of the differential voltage input signal received by the pair of transistors. Certain applications require a high common mode range. As common mode requirements go to extreme highs and lows, as compared to the power supply of the amplifier, biasing problems associated with the differential input transistors and current sources of the amplifier arise. These biasing problems lead to non-linear responses and inaccurate differential gain outputs of the differential amplifier. 
         [0006]    To solve the problem of lack of input common mode range, stages have often been added to amplifiers to shift the common mode range. One example of an added stage is an active level shifter. However, adding additional circuitry causes processing speed to decrease and the size of the devices to increase. Processing speed decreasing with device size increasing creates an even greater problem for applications having increased bandwidth and gain requirements. Further, level shifting does not extend common mode range. Level shifting only shifts a default common mode range to a desired level. Ultimately, design engineers have generally had to live with the common mode range present in the devices. 
         [0007]    Therefore, it would be advantageous to provide transistor biasing circuitry that can extend the input common mode range. 
       SUMMARY 
       [0008]    The illustrative embodiments provide a method and apparatus for extending common mode range. A circuit has a common mode detection circuit, a common mode voltage inversion circuit, and a differential amplifier. The common mode detection circuit receives a differential signal and detects a common mode voltage. The common mode voltage inversion circuit is coupled to the common mode detection circuit. The common mode voltage inversion circuit has an input node that receives the common mode voltage and an output node that outputs body voltage, wherein the common mode voltage inversion circuit creates an inverse relationship between the common mode voltage and the body voltage. The differential amplifier includes a differential pair of transistors that have a pair of body terminals coupled to the output node of the common mode voltage inversion circuit, wherein coupling the pair of body terminals to the output node of the common mode voltage inversion circuit extends a common mode range of the differential amplifier. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0009]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0010]      FIG. 1  is a block diagram illustrating a circuit system in accordance with an illustrative embodiment; 
           [0011]      FIG. 2  is a circuit diagram illustrating a body biasing circuit in connection with a differential amplifier in accordance with an illustrative embodiment; 
           [0012]      FIG. 3  is a graph illustrating a relationship between body voltage and the common mode voltage of the circuitry in accordance with an illustrative embodiment; 
           [0013]      FIG. 4  is a graph illustrating a relationship between threshold voltage and body-to-source voltage of the circuitry in accordance with an illustrative embodiment; 
           [0014]      FIG. 5  is a graph illustrating a relationship between threshold voltage and common mode voltage of the circuitry in accordance with an illustrative embodiment; 
           [0015]      FIG. 6  is a graph illustrating a relationship between gate-to-source voltage and common mode voltage of the circuitry in accordance with an illustrative embodiment; 
           [0016]      FIG. 7  is a circuit diagram illustrating behavior of a differential amplifier for a given low common mode voltage in accordance with an illustrative embodiment; 
           [0017]      FIG. 8  is a circuit diagram illustrating behavior of a differential amplifier for a given high common mode voltage in accordance with an illustrative embodiment; 
           [0018]      FIG. 9  is a graph depicting simulation results for increased common mode range at low common mode in accordance with an illustrative embodiment; and 
           [0019]      FIG. 10  is a graph depicting simulation results for increased common mode range at high common mode in accordance with an illustrative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    With reference now to the figures, and particularly with reference to  FIG. 1 , a block diagram of circuit system  116  is depicted in accordance with an illustrative embodiment. Differential signal  100  is received by both common mode detection circuit  102  and differential amplifier  110 . Common mode detection circuit  102  monitors differential signal  100  to determine common mode voltage  104  of differential signal  100 . Common mode voltage  104  passes through common mode inversion circuit  106  creating body voltage  108 . Body voltage  108  bears an inverse relationship to common mode voltage  104 , thereby extending the common mode range of differential amplifier  110 . Differential amplifier  110  produces output signal  114 . While circuit system  116  is comprised of common mode detection circuit  102 , common mode inversion circuit  106 , and differential amplifier  110  in this illustrative embodiment, additional circuitry may be added to circuit system  116 . 
         [0021]    Turning now to  FIG. 2 , a circuit diagram illustrating a body biasing circuit connected with a differential amplifier is depicted in accordance with an illustrative embodiment. Differential amplifier  252  includes input transistors  222  and  224 . In this example, input transistors  222  and  224  are n-type metal-oxide semiconductor (NMOS) field effect transistors. Input transistors  222  and  224  have their gate terminals connected to receive differential signal  200  and their source terminals tied to the drain terminal of transistor  244 . Tail current  248  passes through transistor  244 . The body terminals of input transistors  222  and  224  are tied together and connected to the output of operational amplifier  212  represented by body voltage  220 . Differential amplifier  252  further includes supply voltage  242  (V supply ), load resistors  236  and  238 , and output nodes  240  and  234  over which an output signal, such as output signal  114  in  FIG. 1 , is produced. While NMOS transistors are used in this example, PMOS transistors may also be used as input transistors for differential amplifier  252 . 
         [0022]    Input transistors  222  and  224  are input devices of differential amplifier  252 , and are critical to the common mode range and linear performance of differential amplifier  252 . Input transistors  222  and  224  need to stay biased and remain in saturation mode for linear response of differential amplifier  252 . The greater the range of input voltage signals that both input transistors  222  and  224  stay in saturation mode, the greater the common mode range of differential amplifier  252 . Saturation mode biasing of an NMOS transistor is determined by the voltage drop from the drain to the source of the transistor (V DS ) staying higher than the voltage drop from the gate to the source minus the threshold voltage (V TH ). For high common mode voltage ranges, differential amplifier  252  is biased better with a higher threshold voltage. For low common mode voltage ranges, differential amplifier  252  is biased better with lower threshold voltage. 
         [0023]    The threshold voltage (V TH ) of an NMOS transistor can be adjusted by the body effects of the transistor. The voltage drop from the body terminal of a transistor to the source terminal of the transistor (V BS ) bears an inverse relationship to the threshold voltage of the transistor. Thus, as V BS  decreases, V TH  increases, and vice versa. 
         [0024]    The body effect is used to extend the common mode range in this illustrative example. In this embodiment, body biasing circuit  250  is used to manipulate the V BS  of input transistors  222  and  224 . Body biasing circuit  250  includes resistors  202  and  204  which receive differential signal  200  and connect to a common node with capacitor  206  to form a detection circuit, such as common mode detection circuit  102  in  FIG. 1 . Common mode voltage  208  (V cm ) is detected as differential signal  200  is filtered by resistors  202  and  204  and capacitor  206 . Common mode voltage  208  passes through operational amplifier  212  to produce body voltage  220  (V body ). Resistors  214  and  215  alter the response of operational amplifier  212  into that of an inverting amplifier. Thus, body voltage  220  bears an inverse relationship to common mode voltage  208 . Body voltage  220  is connected to the body terminals of input transistors  222  and  224  and bears a direct relationship to V bs . Consequently, when body voltage  220  is created by inverting common mode voltage  208  and connected to the body terminals, V cm  and V TH  bear a direct relationship. In other words, the threshold voltage will move up with high common mode and down with low common mode. This direct relationship between threshold voltage and common mode voltage allows input transistors  222  and  224  to remain biased in saturation mode over a greater range of common mode voltage input, thus producing a wider common mode range for the differential amplifier. While body biasing circuit  250  and differential amplifier  252  are depicted in  FIG. 2  as an example of circuitry used in circuit system  116  in  FIG. 1 , additional circuitry may be added to circuit system  116  without departing from the scope of the present invention. 
         [0025]    Further, the circuitry of this illustrative embodiment may be optimized for a range of common mode voltages detected. Gain parameters for operational amplifier  212  are controlled by input resistor  214  (R 1 ), feedback resistor  215  (R 2 ), and reference voltage  218  (V ref ). Adjusting the parameters of these components may alter the relationship between body voltage  220  and common mode voltage  208 , allowing for a sharper or flatter response to changes in common mode voltage ranges detected. Adjustable responses to changes in common mode voltage ranges detected allows the circuitry to be tuned for different body effect parameters and create desired differential amplifier performance. 
         [0026]    The inverting amplifier formed from resistors  214 ,  215 , and operational amplifier  212  is used as an example of a circuit, such as common mode inversion circuit  106  in  FIG. 1 , to create the inverse relationship between common mode voltage  208  and body voltage  220 . Other types of inversion circuitry may be used to create this relationship, such as, for example, a common source amplifier, inverter, or comparator. 
         [0027]    Turning now to  FIGS. 3-6 , graphs illustrating voltage relationships between various nodes of the circuitry are depicted in accordance with an illustrative embodiment. Graph  300  in  FIG. 3  shows the inverse relationship between body voltage  302  and common mode voltage  304  for a reference voltage  306  (V ref ). This inverse relationship can be created by passing common mode voltage  304  through common mode inversion circuitry, such as operational amplifier  212  in  FIG. 2 . Common mode voltage  304  can be a common mode detected from an input differential signal, such as common mode voltage  208  detected from differential signal  200  in  FIG. 2 . This inverse relationship is represented by the following body to common mode equation: 
         [0000]    
       
         
           
             
               V 
               body 
             
             = 
             
               
                 
                   ( 
                   
                     
                       
                         R 
                          
                         
                             
                         
                          
                         2 
                       
                       
                         R 
                          
                         
                             
                         
                          
                         1 
                       
                     
                     + 
                     1 
                   
                   ) 
                 
                 * 
                 
                   V 
                   ref 
                 
               
               - 
               
                 
                   
                     R 
                      
                     
                         
                     
                      
                     2 
                   
                   
                     R 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 * 
                 
                   V 
                   
                     c 
                      
                     
                         
                     
                      
                     m 
                   
                 
               
             
           
         
       
     
         [0000]    with the slope of the inverse relationship being equal to −R 2 /R 1  for reference voltage  306 . 
         [0028]    Graph  400  in  FIG. 4  shows the body effect between threshold voltage  402  and body-to-source voltage  404  for an NMOS transistor, such as input transistors  222  and  224  in  FIG. 2 . The body effect creates an inverse relationship as well. Therefore, as body-to-source voltage  404  of a transistor increases, threshold voltage  402  decreases exponentially. This body effect is represented by the following body effect equation: 
         [0000]        V   TH   =V   T0 +γ(√{square root over (φ− V   BS )}−√{square root over (φ)})
 
         [0000]    where V T0    406  is threshold voltage for V BS =0. γ is the body effect parameter, and φ is the surface potential of the transistor. 
         [0029]    Graph  500  in  FIG. 5  shows the direct relationship between threshold voltage  402  and common mode voltage  304  for a transistor, such as input transistors  224  and  222  in  FIG. 2 . Thus, as the common mode voltage  304  increases, the threshold voltage  402  of a transistor is increased and the common mode range of the transistor is increased. These relationships are represented by the following equation: 
         [0000]      V TH ≅V T0 +γ(√{square root over (φ−V body   30  V cm −V GS )}−√{square root over (φ)})
 
         [0000]    which is the body effect equation substituting common mode voltage  304  for body-to-source voltage  404  where V GS  is the gate-to-source voltage of a transistor which is equal to common mode voltage  304  minus the source voltage for a transistor in circuitry in accordance with an illustrative embodiment, such as input transistors  224  and  222  in  FIG. 2 . Next, placing the previous equation in terms of R 1 , R 2  and V ref , such as those of body biasing circuit  250  in  FIG. 2 , result in the following equation: 
         [0000]    
       
         
           
             
               V 
               TH 
             
             ≅ 
             
               
                 V 
                 
                   T 
                    
                   
                       
                   
                    
                   0 
                 
               
               + 
               
                 γ 
                 ( 
                 
                   
                     
                       ϕ 
                       + 
                       
                         
                           ( 
                           
                             
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 2 
                               
                               
                                 R 
                                  
                                 
                                     
                                 
                                  
                                 1 
                               
                             
                             + 
                             1 
                           
                           ) 
                         
                         * 
                         
                           ( 
                           
                             
                               V 
                               cm 
                             
                             - 
                             
                               V 
                               ref 
                             
                           
                           ) 
                         
                       
                       - 
                       
                         V 
                         GS 
                       
                     
                   
                   - 
                   
                     ϕ 
                   
                 
                 ) 
               
             
           
         
       
     
         [0000]    This equation further provides for control over optimization of the circuitry such as the gain parameters described above. These equations are approximate because they ignore the slight effects of V GS  dependence on V TH . 
         [0030]    Graph  600  in  FIG. 6  shows the direct relationship between gate-to-source voltage  602  and common mode voltage  304  for transistors, such as input transistors  224  and  222  in  FIG. 2 , for a constant drain-to-source current. This direct relationship is represented by the following equations: 
         [0000]    
       
         
           
             
               
                 I 
                 DS 
               
               ≅ 
               
                 
                   β 
                   2 
                 
                 * 
                 
                   
                     ( 
                     
                       
                         V 
                         GS 
                       
                       - 
                       
                         V 
                         TH 
                       
                     
                     ) 
                   
                   2 
                 
               
             
             ; 
           
         
       
       
         
           and 
         
       
       
         
           
             
               V 
               GS 
             
             ≅ 
             
               
                 
                   2 
                   * 
                   
                     
                       I 
                       DS 
                     
                   
                 
                 β 
               
               + 
               
                 V 
                 TH 
               
             
           
         
       
     
         [0000]    where V DS  represents the drain-to-source current across the transistors and β is a process parameter. As threshold voltage  402  increases with increasing input common mode voltage  304 , as shown in  FIG. 5 , the gate-to-source voltage  602  increases. Increasing the gate-to-source voltage  602  improves the biasing of the input transistors, such as input transistors  224  and  222  in  FIG. 2 , at high common mode because the drain-to-source voltage is increased. Conversely, at lower common mode voltage  304 , threshold voltage  402  decreases which decreases gate-to-source voltage  602  of input transistors  224  and  222 . Decreasing gate-to-source voltage  602  improves the biasing of the current source, such as transistor  244  in  FIG. 2 , at low common mode because the drain-to-source voltage of the current source is increased. These equations are also approximate because they ignore the slight effects of V GS  dependence on V TH . 
         [0031]    Turning now to  FIGS. 7 and 8 , circuit diagrams illustrating behavior of a differential amplifier for different given common mode voltages are depicted in accordance with an illustrative embodiment. The circuit diagrams illustrate differential amplifier  700  performing at low common mode in  FIG. 7  and at high common mode in  FIG. 8 . Low common mode  702  in  FIG. 7  and high common mode  802  in  FIG. 8  symbolize the common mode voltage of the differential signal input. 
         [0032]    When the common mode input is low (low common mode  702 ), body voltage  704  rises due to the inverse relationship created, such as by body biasing circuit  250  in  FIG. 2 . The body effect, represented by the body effect equation, lowers the threshold voltage of transistors  722  and  724 . Lowering the threshold voltage lowers the gate-to-source voltage  706 . The lowered gate-to-source voltage  706  results from the direct relationship created with common mode voltage, such as illustrated by graph  600  in  FIG. 6 . Lower gate-to-source voltage  706  allows for a higher drain-to-source voltage  708  (V DS3 ) across transistor  744 . Transistor  744  acts as a current source for differential amplifier  700 , with tail current  710  representing the current. Higher drain-to-source voltage  708  across transistor  744  allows it to remain biased in saturation mode despite low common mode  702  input. The bias of transistor  744  in saturation mode allows differential amplifier  700  to perform properly at low common mode  702 , thereby extending the common mode range of differential amplifier  700  for low common mode voltage ranges. 
         [0033]    When the common mode input is high (high common mode  802 ) the inverse relationship created lowers body voltage  804 , such as by body biasing circuit  250  in  FIG. 2 . The body effect, represented by the body effect equation, raises the threshold voltage of transistors  722  and  724 . Raising the threshold voltage allows transistors  722  and  724  to stay biased and remain in saturation mode for the higher range of common mode voltages detected. The gate-to-source voltage  806  rises as a result of the direct relationship created with common mode voltage, as illustrated by graph  600  in  FIG. 6 . Raising gate-to-source voltage  806  lowers drain-to-source voltage  808  across transistor  744 . The lower drain-to-source voltage  808  increases drain-to-source voltage  804  of transistors  722  and  724 , keeping them biased in saturation mode. Because they stay biased in saturation mode, the small signal gain of transistors  722  and  724  remains relatively constant. Higher gate-to-source source voltage  806  lowers drain-to-source voltage  808  across transistor  744 . At high common mode  802 , bias of transistor  744  is not much of an issue as drain-to-source voltage  808  is generally high for high common mode  802 . Lowering drain-to-source voltage  808  allows for higher drain-to-source voltage  804  across transistors  722  and  724 . Due to the higher drain-to-source voltage  810  across transistors  722  and  724  allows transistors  722  and  724  to remain biased in saturation mode for high common mode  802  and the gain performance of differential amplifier  700  increases. 
         [0034]    Turning now to  FIGS. 9 and 10 , graphs illustrating simulation results for increased common mode range for given common mode voltages are depicted in accordance with an illustrative embodiment.  FIG. 9  shows the DC gain  904  for common mode voltage  902  of signals  906  and  908  at low common mode. Signal  906  illustrates a graph of a signal from a differential amplifier with body terminals connected to a body biasing circuit, such as illustrated by  FIG. 2 . Signal  908  illustrates a signal from a differential amplifier that does not use a body biasing circuit. Rather, the body terminal is tied to ground, as commonly used in high common mode differential amplifier settings. For an application having maximum acceptable level of DC gain  910 , the increase of the common mode range detected during the simulation at low common mode is over 50%. Likewise,  FIG. 10  shows the DC gain  1004  for common mode voltage  1002  of signals  1006  and  1008  at high common mode. Signal  1006  illustrates a graph of a signal from a differential amplifier with body terminals connected to a body biasing circuit, such as illustrated by  FIG. 2 . Signal  1008  illustrates a signal form a differential amplifier that does not use a body biasing circuit. Rather, the body terminal is set to 600 mV, as commonly used in low common mode differential amplifier settings. For an application having minimum acceptable level of DC gain  1010 , the increase of the common mode range detected during the simulation at high common mode is over 25%. 
         [0035]    There are several additional benefits to the described embodiments of the invention besides extending the common mode range of the differential amplifier. The differential amplifier performance improves because the input devices and current source devices are biased in a more ideal location. Thus, the DC gain accuracy of the amplifier improves, the AC response is flatter, and the jitter performance is improved. The common mode rejection of the differential amplifier is also improved because the current source impedance is higher with better saturation mode biasing across the input common mode range. There is an additional benefit to increasing the current source impedance from this body biasing approach. If source degeneration is implemented in the differential amplifier, the resulting DC gain of the differential amplifier is more accurate. This is especially true at very low DC gain applications where the source degeneration resistance is very high. 
         [0036]    The circuit, as described above, is part of the design for an integrated circuit chip. The chip design is created in a graphical computer programming language and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive, such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
         [0037]    The description of the present invention has been presented for purposes of illustration and description, and it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.