Abstract:
An output driver circuit including a pre-driver stage that receives a first data signal, the pre-driver stage including a plurality of first differential pairs that perform current subtraction to output a second data signal based on the first data signal, and an output driver stage electrically coupled to the pre-driver stage that receives the second data signal from the pre-driver stage, the output driver stage including a plurality of second differential pairs that transmit an output signal along transmission lines.

Description:
[0001]    This is a continuation of application Ser. No. 11/712,914 filed Mar. 2, 2007. The disclosure of the prior application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Aspects of the invention generally relate to high-speed communication devices. More specifically, the invention can relate to a driver circuit with pre-emphasis. 
         [0003]    Gb/s-class high-speed communication devices that operate at a broad range of frequencies, consume a low amount of power, and reside on a small footprint of silicon are of great use. Such communication devices have advantages in serial communication applications, such as Peripheral Component Interconnect Express (PCIe), Serial Advanced Technology Attachment (SATA), and the like, as well as broader applications, such as networks, telecommunications, and other electronic systems. 
       SUMMARY 
       [0004]    In serial communication applications, integrated circuits (ICs) are typically mounted on a printed circuit board and are electrically coupled to each other by a plurality of printed circuit board (PCB) conductors, commonly known as traces. Traces provide a transmission media for signals, such as power, ground references, and input/output signals to and from each IC. Normally, high-frequency signals between ICs via the traces are adversely affected by parasitic resistance, inductance, and capacitance inherent in such interconnections. 
         [0005]    Further, impedance mismatches between an IC transmitter and the various signal transmission media (traces and other printed circuit board components) of the signal transmission path, as well as between an IC receiver and the various signal transmission media of the signal transmission path may produce signal reflections at the transmitter end and/or the receiver end of the signal transmission path. Such signal reflections may propagate along the transmission path and may potentially degrade system performance. As a result, it is highly desirable to closely match the output impedance of the transmitting IC to the various components including the signal transmission path, and the input impedance of the receiving IC. Generally, transmitters are designed for an output impedance of 50 Ohms to match the characteristic impedance of the PCB trace. 
         [0006]    Additionally, in order to compensate for the high-frequency signal attenuation inherent in the traces, a technique called pre-emphasis can be implemented. Pre-emphasis can be used to compensate for the high-frequency signal attenuation by emphasizing periods of transition and de-emphasizing periods of no transition in a time-varying serial data signal. 
         [0007]      FIG. 1  shows an output driver with an exemplary analog current subtraction circuit that implements such a pre-emphasis technique. As shown, the current subtraction circuit includes two differential pairs A and B connected in parallel. Differential pair A includes transistors T 11  and T 12  with their sources connected to each other and to a first current source S 1 . The drain electrodes of transistors T 11  and T 12  are connected to output resistances R 1  and R 2 , respectively. Output resistances R 1  and R 2  represent transmission lines or traces on a printed circuit board, and are both typically 50 Ohms in most high-speed applications above 2 Gb/s. The differential pair B, which is added for pre-emphasis, includes transistors T 13  and T 14  with their source electrodes connected to a second current source S 2 . The drain electrodes of the transistors T 13  and T 14  are also connected to output resistances R 1  and R 2 , respectively. Furthermore, drain electrode of transistor T 11  is connected to drain of transistor T 13 , forming an output node V OUT     —     B . Likewise, drain of transistor T 12  is connected to drain of transistor T 14 , forming an output node V OUT . Input nodes V IN , V IN     —     B , EV IN     —     B , and EV IN  are located at the gates of transistors T 11 , T 12 , T 13  and T 14 , respectively. 
         [0008]      FIG. 2  shows a wave-form diagram depicting the relationship between the signals applied to input nodes V IN , V IN     —     B , EV IN     —     B , and EV IN  and the output signals measured at output nodes V OUT  and V OUT     —     B .  FIG. 2  is used to generally explain the concept of pre-emphasis. As shown in  FIG. 2 , a data signal is applied to input node V IN , and, in an inverted form, is synchronously applied to input node V IN     —     B . Further, the data signal, delayed by a unit interval or by one period of the transmitted data rate, is applied to input node EV IN , and, in an inverted and similarly delayed form, is applied to input node EV IN     —     B . All data signals applied to input nodes V IN , V IN     —     B , EV IN , and EV IN     —     B  have a set amplitude value ‘a.’ As described below, the data signals can be emphasized by an amount ‘b’ over the set amplitude value ‘a’, or de-emphasized by an amount ‘b’ below the set amplitude value ‘a.’ 
         [0009]    In  FIG. 2 , as the data signal at input node V IN  transitions from either “low” to “high” or “high” to “low,” the output signal at output node V OUT  is emphasized by an amount ‘b.’ During periods of no transition in the data signal at input node V IN , the output signal at output node V OUT  is de-emphasized by amount ‘b.’ Referring to  FIG. 2 , as the input signal at input node V IN  transitions from “high” to “low” at t2, the output signal at output node V OUT  transitions to an “emphasized low,” which is a “low” signal emphasized by an amount ‘b.’ However, at time t3, since the signal at V IN  does not transition and remains “low,” the “emphasized low” signal at V OUT  is de-emphasized by an amount ‘b,’ and becomes a “de-emphasized low.” When the signal at V IN  transitions from “low” to “high” at time t4, the signal at V OUT  transitions from a “de-emphasized low” to an “emphasized high,” where the “emphasized high” is a “high” signal that is emphasized by an amount ‘b.’ At time t5, since there is no change in the signal at V IN , the signal at V OUT  is de-emphasized from an “emphasized high” to a “de-emphasized high.” At time t6, as the signal at V IN  transitions from “high” to “low,” the signal at V OUT  transitions from a “de-emphasized high” to an “emphasized low.” And finally at time t7, as the signal at V IN  transitions from “low” to “high,” the signal at V OUT  transitions from an “emphasized low” to an “emphasized high.” 
         [0010]    One exemplary type of pre-emphasis scheme applied to the circuit topology of  FIG. 1  is described below with reference to Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Channel 
                 Channel 
                   
                   
                   
                   
               
               
                 width of 
                 width of 
               
               
                 differential 
                 differential 
                 Total load 
               
               
                 pair A 
                 pair B 
                 capacitance 
                 I 1   
                 I 2   
                 I 2 /I 1   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 100% 
                 0% 
                 100% 
                 100% 
                 0% 
                 0% 
               
               
                 100% 
                 10% 
                 110% 
                 100% 
                 10% 
                 10% 
               
               
                 100% 
                 20% 
                 120% 
                 100% 
                 20% 
                 20% 
               
               
                 100% 
                 30% 
                 130% 
                 100% 
                 30% 
                 30% 
               
               
                 100% 
                 40% 
                 140% 
                 100% 
                 40% 
                 40% 
               
               
                   
               
             
          
         
       
     
         [0011]    Table 1 shows currents I 1  and I 2  passing through differential pairs A and B, respectively, as well as the pre-emphasis ratio I 2 /I 1 . The channel width, capacitance, and current values are in terms of a percentage of respective arbitrary values. The current subtraction scheme of  FIG. 1  maintains a constant current value of 100% for I 1 . Differential pair B is added for pre-emphasis. Differential pair B can be set to maintain a maximum current value of 40% for I 2 . However, because I 1  is at a constant 100%, adding differential pair B for pre-emphasis can create severe bandwidth limitations because of the additional load capacitance introduced by differential pair B. For example, in order to maintain a pre-emphasis ratio of 40%, the current I 2  flowing through differential pair B must be maintained at 40% of current I 1 . Thus, the total channel width of transistors T 13  and T 14  of differential pair B must be 40% of the total channel width of transistors T 11  and T 12  of differential pair A. The 40% increase of the total channel width area in the output driver amounts to a 40% increase in total load capacitance, which lowers the impedance of the output driver, and results in mismatch at high frequencies, causing severe bandwidth limitations. 
         [0012]    The total load capacitance issue of  FIG. 1 , however, can be somewhat eased by maintaining the total amount of current I 1 +I 2  constant. Another circuit topology schematically identical to the current subtraction circuit of  FIG. 1 , but implementing a different pre-emphasis scheme, is described below with respect to Table 2. The channel width, capacitance, and current values in Table 2 are also in terms of a percentage of respective arbitrary values. Specifically, as shown in Table 2, the total current I 1 +I 2  through both differential pairs A and B can be maintained at the same level by adjusting the current flow through both differential pairs A and B. Conventionally, it is desirable to design a tunable differential pair A with a channel width of 100% so as to be able to tune current I 1  passing through differential pair A. By decreasing current I 1  to 71%, it is possible to maintain a pre-emphasis ratio of 40% by adding differential pair B having current I 2  at 29%. However, because the total channel width of differential pair A remains at 100% and the total channel width of differential pair B is at 29%, the total load capacitance is still too large at 129%. Thus, Although this scheme allows for the same amount of pre-emphasis ratio I 2 /I 1  with differential pair B having a smaller channel width, this type of pre-emphasis scheme implementation is only a marginal improvement over the previous pre-emphasis scheme implementation. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Channel width 
                 Channel width 
                   
                   
                   
                   
               
               
                 of differential 
                 of differential 
                 Total load 
               
               
                 pair A 
                 pair B 
                 capacitance 
                 I 1   
                 I 2   
                 I 2 /I 1   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 100% 
                 0% 
                 100% 
                 100% 
                 0% 
                 0% 
               
               
                 100% 
                 9% 
                 109% 
                 91% 
                 9% 
                 10% 
               
               
                 100% 
                 17% 
                 117% 
                 83% 
                 17% 
                 20% 
               
               
                 100% 
                 23% 
                 123% 
                 77% 
                 23% 
                 30% 
               
               
                 100% 
                 29% 
                 129% 
                 71% 
                 29% 
                 40% 
               
               
                   
               
             
          
         
       
     
         [0013]    One method of circumventing the load capacitance issue of the previous two pre-emphasis schemes of Tables 1 and 2 as applied to  FIG. 1  is to use a Digital to Analog Converter (DAC) configuration for the pre-emphasis scheme.  FIG. 3  shows such a DAC pre-emphasis scheme with current subtraction performed in the output stage. This DAC pre-emphasis scheme was presented in 2005 ISSCC, digest of technical papers, vol. 48 3.4  “A Transmit Architecture with  4- Tap Feedfoward Equalization for  6.25/12.5 Gb/s Serial Backplane Communications ,” Paul Landman et. al. By using a DAC configuration, it is possible to use all output stages at all times, with or without pre-emphasis. Utilizing all output stages (slices) eliminates redundancy at the output and thus eliminates any unwanted capacitance. 
         [0014]    As shown in  FIG. 3 , each of the ten slices  401  to  410  contains a differential pair. Each differential pair includes two transistors T 41  and T 42 , with their sources connected to each other and to a current source S 41 . The drains of each of the two transistors T 41  and T 42  are connected to output resistances R 41  and R 42 , respectively. Moreover, all of the differential pairs of each respective slice  401  to  410  are connected in parallel, and therefore drive the same output resistances R 41  and R 42 . Each pair of transistors of each slice  401  to  410  are connected to their respective pre-driver circuit  421  to  430 . Each pre-driver circuit  421  to  430  can either supply a non-delayed data signal or a delayed data signal and drive each respective slice  401  to  410  of the output driver. Each pre-driver circuit  421  to  430  also supplies the delayed or non-delayed data in an inverted and non-inverted form to the appropriate transistors. 
         [0015]    In the case of  FIG. 3 , six pre-driver circuits  421  to  426  supply non-delayed data, and four pre-driver circuits  427  to  430  supply delayed data. Each current source S 41  of every slice  401  to  410  draws an equal portion of the total current. Accordingly, if there are ten slices  401  to  410  in the output driver, each current source S 41  will draw one tenth of the total current. Without any pre-emphasis, each of the slices  401  to  410  is driven by its respective pre-driver circuit set to supply non-delayed data, with 100% of the total current being pulled from all slices  401  to  410 . For example, in order to create a pre-emphasis ratio of 66.6%, six of the ten pre-driver circuits  421  to  426  are set to drive six respective slices  401  to  406  with a non-delayed data signal, and the remaining four of the ten pre-driver circuits  427  to  430  are set to drive the other four respective slices  407  to  410  with a delayed data signal. Thus, 60% of the total current is pulled from the six slices  401  to  406  driven with a non-delayed data signal, and 40% of the total current is pulled from the four slices  407  to  410  driven with a delayed data signal. As a result, the total current pulled remains 100%, without any increase in capacitance. 
         [0016]    However, the DAC pre-emphasis scheme has its own drawbacks, such as redundancy in the pre-driver stage. With N slices  401  to N, N pre-driver circuits are required to drive the N slices in the output stage. However, as the number of pre-driver circuits increases to drive more slices, the size of pre-driver circuits must be reduced in order to meet power and area constraints. As the size of each pre-driver circuit decreases, the effect of parasitic capacitance becomes more prominent, resulting in bandwidth limitation. In order to compensate for these detrimental effects, each slice  401  to N can be designed to pull more than 1/N of the total current from each slice. For example, when driving ten slices  401  to  410 , to compensate for parasitic capacitance, each slice  401  to  410  is fabricated having a total channel width much larger than that required to pull only 1/10 of the total current. Increasing the channel width of the slices in the output driver stage, however, results in greater current consumption, higher power dissipation, higher capacitive loads, and a larger silicon area. 
         [0017]    Aspects of the invention reduce or remove the area and power penalties, as well as the extra capacitive loading issues described above by implementing a pre-emphasis scheme with the current subtraction performed in the pre-driver stage. By performing current subtraction in the pre-driver stage instead of the output driver stage, the invention can minimize or remove the additional capacitive loading due to large differential pairs in the output driver of  FIG. 1  and the segmented pre-driver circuits of  FIG. 3 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The embodiments of the invention will be described with reference to the following drawings, wherein like numerals designate like elements, and wherein: 
           [0019]      FIG. 1  shows a block diagram of a pre-emphasis scheme with analog current subtraction performed in an output driver stage; 
           [0020]      FIG. 2  shows a signal wave-form showing pre-emphasis by subtraction; 
           [0021]      FIG. 3  shows an exemplary DAC configuration of an analog current subtraction scheme; 
           [0022]      FIG. 4  shows a first exemplary pre-emphasis scheme with current subtraction performed in the pre-driver stage; 
           [0023]      FIG. 5  shows an exemplary schematic of the second and third buffers BUF 2  and BUF 3  in the pre-driver stage of the first exemplary pre-emphasis scheme; 
           [0024]      FIG. 6  shows a second exemplary pre-emphasis scheme with multiple input delays; 
           [0025]      FIG. 7  shows an exemplary schematic of the second, third, and fourth buffers BUF 2 , BUF 3 , and BUF 4  in the pre-driver stage of the second exemplary pre-emphasis scheme; and 
           [0026]      FIG. 8  shows a flow-chart of an exemplary pre-emphasis process. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]    In the following descriptions, many of the exemplary circuits are shown to include n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) in a variety of configurations. While MOSFET devices are used by example, the disclosed circuits may be implemented using any number of other transistor types, such as J-FETs, bipolar transistors, and the like. Additionally, while n-channel devices are used in the following examples, the same general approaches may also apply to circuits incorporating p-channel FETs or PNP bipolar transistors, for example. 
         [0028]    Still further, while the terms “drain” and “source” are used for ease of explanation and to adhere to traditional engineering usage, it should be recognized that a drain and source of a FET transistor may be considered interchangeable, and for the following descriptions merely thought of as a first end and a second end of a semiconductor channel unless otherwise stated or apparent to one of ordinary skill in the art. 
         [0029]      FIG. 4  shows a first exemplary embodiment of a pre-emphasis circuit having current subtraction performed in the pre-driver stage. As shown in  FIG. 4 , the output driver stage can include a current addition circuit having two differential pairs A 6  and B 6  that are connected in parallel. Differential pair A 6  can include transistors T 61  and T 62  with their source electrodes connected to each other and to a first current source S 61 . The drain electrodes of the transistors T 61  and T 62  are connected to output resistances R 61  and R 62 , respectively. Nodes V A1 , V A2 , V B1 , and V B2  are located at the gate electrodes of transistors T 61 , T 62 , T 63 , and T 64 , respectively. The output resistances R 61  and R 62  are the termination resistors for impedance matching. Differential pair B 6  can include transistors T 63  and T 64  with their sources connected to a second current source S 62 . The drains of the transistors T 63  and T 64  are also connected to output resistances R 61  and R 62 , respectively. Furthermore, the drain of transistor T 61  is connected to the drain of transistor T 63 . Likewise, the drain of transistor T 62  is connected to the drain of transistor T 64 . 
         [0030]    Differential pairs A 6  and B 6  can be current mode differential pairs whose inputs are given by either small-signal current-mode logic or rail to rail CMOS logic. 
         [0031]    As shown in  FIG. 4 , first buffer BUF 1 , second buffer BUF 2 , and third buffer BUF 3  make up the pre-driver stage and drive differential pairs A 6  and B 6 , respectively. Specifically, BUF 1  drives differential pair A 6 , and BUF 2  and BUF 3  drive second differential pair B 6 . Buffers BUF 1  and BUF 2  each receive a non-inverted data signal at node V IN6  and an inverted data signal at node V IN6     —     B . BUF 3  receives a non-inverted delayed data signal at node EV IN6 , and an inverted delayed data signal at node EV IN6     —     B . BUF 1  drives differential pair A 6  by supplying the non-inverted data signal to node V A1  and the inverted data signal to node V A2 . BUF 2  and BUF 3  are described in greater detail below with respect to  FIG. 5 . 
         [0032]      FIG. 5  shows an exemplary schematic diagram of second and third buffers BUF 2  and BUF 3  in the pre-driver stage. As shown in  FIG. 5 , BUF 2  and BUF 3  are in the form of two differential pairs connected in parallel. BUF 2  includes transistors T 91  and T 92  with their sources connected to each other and to a variable current source S 92 . The drains of transistors T 91  and T 92  are connected to resistors R 91  and R 92 , respectively. BUF 3  includes transistors T 93  and T 94  with their sources connected to another variable current source S 93 . The drains of the transistors T 93  and T 94  are also connected to resistors R 91  and R 92 , respectively. Furthermore, the drain of transistor T 91  is connected to the drain of transistor T 93 , forming an output node V B2 , which is the same node located at the gate of transistor T 64  of differential pair B6. Likewise, the drain of transistor T 92  is connected to the drain of transistor T 94 , forming an output node V B2 , which is the same node located at the gate of transistor T 63  of differential pair B 6 . Input nodes V IN6 , V IN6     —     B , EV IN6     —     B , and EV IN6  are located at the gates of transistors T 91 , T 92 , T 93 , and T 94 , respectively, and correspond to input nodes V IN6 , V IN6     —     B , EV IN6     —     B , and EV IN6  of BUF 1 , BUF 2 , and BUF 3  in  FIG. 4 . 
         [0033]    Variable current source S 92  can be tuned to pull a predetermined fraction of current I BUF2  from BUF 2 . Likewise, variable current source S 93  can be tuned to pull a predetermined fraction of current I BUF3  from BUF 3 . While  FIG. 5  only shows one variable current source for each buffer, it should be understood that any number and type of variable current sources may be provided for each buffer. For example, variable current source S 93  can be an n-channel transistor having the voltage at its gate electrode regulated by an external circuit, or the like. 
         [0034]    Each of the current sources S 92  and S 93  can be tuned to regulate the amount of current pulled from each of BUF 2  and BUF 3 . By regulating currents I BUF2  and I BUF3 , it is possible to tune the pre-emphasis value in the pre-driver stage. 
         [0035]    With reference to  FIG. 4  and Table 3 below, for purposes of illustration, differential pair A 6  is sized to pull a maximum of 60% of the overall current in the output driver stage and differential pair B 6  is sized to pull a maximum of 40% of the overall current in the output driver stage. An overall ratio will be determined by tuning currents I BUF2  and I BUF3  in the pre-driver stage. The overall ratio is determined by the following equation: 
         [0000]      A od +(B od )(C 2 ):(B od )(C 3 ) 
         [0036]    where A od  is the current ratio of the current I 61  passing through differential pair A 6  to the overall current in the output driver stage; B od  is the current ratio of current I 62  passing through the differential pair B 6  to the overall current in the output driver stage; C 2  is the current ratio of current I BUF2  passing through BUF 2  to the overall current in the pre-driver stage; and C 3  is the current ratio of the current I BUF3  passing through BUF 3  to the overall current in the pre-driver stage. 
         [0037]    Table 3 shows current ratios C 2  and C 3  with A od  set to 60% and B od  set to 40%. A percentage amount by which a data signal is emphasized or de-emphasized is the percentage amount substantially equivalent to (B od )(C 3 ). 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 A od   
                 B od   
                 C 2   
                 C 3   
                 (A od ) + (B od )(C 2 ) 
                 (B od )(C 3 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 60% 
                 40% 
                 100% 
                 0% 
                 100% 
                 0% 
               
               
                 60% 
                 40% 
                 80% 
                 20% 
                 92% 
                 8% 
               
               
                 60% 
                 40% 
                 60% 
                 40% 
                 84% 
                 16% 
               
               
                 60% 
                 40% 
                 40% 
                 60% 
                 76% 
                 24% 
               
               
                 60% 
                 40% 
                 20% 
                 80% 
                 68% 
                 32% 
               
               
                 60% 
                 40% 
                 0% 
                 100% 
                 60% 
                 40% 
               
               
                   
               
             
          
         
       
     
         [0038]    During operation, BUF 1  is constantly driving differential pair A 6 . BUF 2  and BUF 3  perform subtraction between the data signal and the delayed data signal. In order to tune the pre-driver stage so as to have no pre-emphasis, variable current source S 92  can be tuned to pull 100% of current I BUF2 , and variable current source S 93  can be tuned to pull 0% of current I BUF3 . In such a case, since no current is passing through BUF 3 , and all current is passing through BUF 2 , the overall current through the pre-driver is equivalent to current I BUF2  passing through BUF 2 . Thus, the current ratio C 2  of current I BUF2  to the overall current in the pre-driver stage will be 100%, and the current ratio C 3  of current I BUF3  to the overall current in the pre-driver stage will be 0%. 
         [0039]    This pre-emphasis technique allows for any amount of pre-emphasis with little or no performance degradation. Some pre-emphasis settings are shown in Table 3. For example, as shown in Table 3, if one desires for the data signal to be emphasized/de-emphasized by 24%, one can tune current source S 92  to pull 40% of current I BUF2  and tune current source S 93  to pull 60% of current I BUF3 . As a result, an output signal will be emphasized by 24% during periods of transition, and de-emphasized by 24% during periods of no transition. 
         [0040]    During implementation of this exemplary embodiment, because no extra load capacitance is added at the outputs V OUT6  and V OUT6     —     B  of the output driver stage, the output driver stage maintains a low level of capacitance, differential pairs A 6  and B 6  accommodate 100% of the maximum current at the output driver stage, the number of signals from the pre-driver stage remains small, and the absence of numerous small pre-drivers reduces the performance degradation due to excessive parasitic capacitance. 
         [0041]    This technique can also be applied to the inputs with multiple delays, so as to compensate for complex response over the transmission medium.  FIG. 6  shows a possible implementation for such cases. 
         [0042]      FIG. 6  shows a second exemplary embodiment, which is  FIG. 4  enhanced with an additional buffer BUF 4  in the pre-driver stage for a multiple bit-delayed input signal. 
         [0043]    As shown in  FIG. 7 , buffer BUF 4  includes transistors T 95  and T 96  with their sources connected to each other and to a variable current source S 94 . The drains of transistors T 95  and T 96  are connected to resistors R 91  and R 92 , respectively. Furthermore, the drains of transistors T 91  and T 93  are connected to the drain of transistor T 95 , forming output node V B1 , which is the same node located at the gate of transistor T 63  of differential pair B 6 . Likewise, the drains of transistors T 92  and T 94  are connected to the drain of transistor T 96 , forming an output node V B2 , which is the same node located at the gate of transistor T 64  of differential pair B 6 . Input nodes E2V IN6     —     B , and E2V IN6  are located at the gates of transistors T 95  and T 96 , respectively, and correspond to the input nodes E2V IN6     —     B  and E2V IN6  of BUF 4  in  FIG. 6 . 
         [0044]    BUF 4  receives a non-inverted delayed data signal at node E2V IN6 , and an inverted delayed data signal at node E2V IN6     —     B . The non-inverted delayed data signal and the inverted delayed data signal received at nodes E2V IN6  and E2V IN6     —     B  of BUF 4 , respectively, are the original non-inverted and inverted data signals V IN6  and V IN6     —     B  that are delayed by a second delay time. The second delay time can be equivalent to two unit intervals or two periods of the transmitted data rate and is greater than the delay time of the data signals received at nodes EV IN6  and EV IN6     —     B  of BUF 3 . 
         [0045]    BUF 4  allows the output signal to be additionally de-emphasized during a period of no transition. For example, after signal V IN6  transitions to either a “high” or a “low” and then does not transition for a period equal to the second delay, the output signal V OUT6  is de-emphasized by an additional amount proportional to the current ratio C 4  of current I BUF4  to the overall current in the pre-driver stage. 
         [0046]    Table 4 shows current ratios C 2 , C 3 , and C 4  with A od  set to 60% and B od  set to 40% for illustrative purposes. A percentage amount by which a data signal is emphasized or de-emphasized after the first delay is the percentage amount substantially equivalent to half of (B od )(C 3 ). A percentage amount by which a data signal is additionally de-emphasized after the second delay is the percentage amount substantially equivalent to (B od )(C 4 ). 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 (B od ) 
               
               
                 A od   
                 B od   
                 C 2   
                 C 3   
                 C 4   
                 (A od ) + (B od )(C 2 ) 
                 (B od )(C 3 ) 
                 (C 4 ) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 60% 
                 40% 
                 100% 
                 0% 
                 0% 
                 100% 
                 0% 
                 0% 
               
               
                 60% 
                 40% 
                 50% 
                 50% 
                 0% 
                 80% 
                 20% 
                 0% 
               
               
                 60% 
                 40% 
                 50% 
                 40% 
                 10% 
                 80% 
                 16% 
                 4% 
               
               
                 60% 
                 40% 
                 25% 
                 60% 
                 15% 
                 70% 
                 24% 
                 6% 
               
               
                 60% 
                 40% 
                 0% 
                 75% 
                 25% 
                 60% 
                 30% 
                 10% 
               
               
                   
               
             
          
         
       
     
         [0047]    During operation of the second exemplary embodiment shown in  FIG. 6 , BUF 1  is constantly driving differential pair A 6 . BUF 2 , BUF 3 , and BUF 4  perform subtraction between the data signal, the delayed data signal, and the multiple bit-delayed signal. In order to tune the pre-driver stage so as to have no pre-emphasis, current source S 92  can be tuned to pull 100% of current I BUF2 , current source S 93  can be tuned to pull 0% of current I BUF3 , and current source S 94  can also be tuned to pull 0% of current I BUF4 . In such a case, since no current is passing through BUF 3  and BUF 4 , and 100% of current is passing through BUF 2 , the overall current through the pre-driver is equivalent to current I BUF2  passing through BUF 2 . Thus, the current ratio C 2  of current I BUF2  to the overall current in the pre-driver stage will be 100%, the current ratio C 3  of current I BUF3  to the overall current in the pre-driver stage will be 0%, and the current ratio C 4  of current I BUF4  to the overall current in the pre-driver stage will be 0%. 
         [0048]    Some pre-emphasis settings for the pre-emphasis scheme depicted in  FIG. 6  are shown in Table 4. For example, as shown in Table 4, if one desires for the data signal to be emphasized/de-emphasized by 12% with a second delay de-emphasis during no transition of 6%, one can tune current source S 92  to pull 25% of current I BUF2 , tune current source S 93  to pull 60% of current I BUF3 , and tune current source S 94  to pull 15% of current I BUF4 . As a result, an output signal will be emphasized by 12% during periods of transition, and de-emphasized by 12% during periods of no transition after the first delay. After the second delay during a period of no transition, the signal will be additionally de-emphasized by 6%. 
         [0049]    During implementation of this second exemplary embodiment, because no extra load capacitance is added at the outputs V OUT6  and V OUT6     —     B  of the output driver stage, the output driver stage maintains a low level of capacitance, differential pairs A 6  and B 6  accommodate 100% of the maximum current at the output driver stage, the number of signals from the pre-driver stage remains small, and the absence of numerous small pre-drivers reduces the performance degradation due to excessive parasitic capacitance. 
         [0050]    Of course, more buffers may be added in order to compensate for high-loss and more complex responses over the transmission medium. 
         [0051]      FIG. 8  shows a flow-chart of an exemplary pre-emphasis process. The exemplary pre-emphasis process can be implemented in a circuit where current subtraction is performed in the pre-driver stage, such as the pre-emphasis scheme shown in  FIG. 4 . 
         [0052]    The process begins and proceeds to step S 1002  where the process receives an initial data signal, and proceeds to step S 1004 . For example, in the pre-emphasis scheme shown in  FIG. 4 , buffers BUF 1 , BUF 2 , and BUF 3  in the pre-driver stage receive input signals at nodes V IN6 , V IN6     —     B , EV IN6     —     B , and EV IN6 . 
         [0053]    In step S 1004 , the process performs current subtraction on the initial data signal, and proceeds to step S 1006 . For example, in the pre-emphasis scheme shown in  FIG. 4 , buffers BUF 2 , and BUF 3  perform current subtraction on input signals received at nodes V IN6 , V IN6     —     B , EV IN6     —     B , and EV IN6 . 
         [0054]    In step S 1006 , the process outputs an intermediate data signal based on the initial data signal, and proceeds to step S 1008 . For example, in the pre-emphasis scheme shown in  FIG. 4 , the signal processed by the buffers in the pre-driver stage is outputted to differential pairs A 6  and B 6  in the output driver stage. 
         [0055]    In step S 1008 , the process receives the intermediate data signal, and proceeds to step S 1010 . For example, in the pre-emphasis scheme shown in  FIG. 4 , differential pairs A 6  and B 6  in the output driver stage receive signals at nodes V A1 , V A2 , V B1 , and V B2 . 
         [0056]    In step S 1010 , the process performs current summation on the intermediate data signal, and proceeds to step S 1012 . For example, in the pre-emphasis scheme shown in  FIG. 4 , differential pairs A 6  and B 6  perform current summation on input signals received at nodes V A1 , V A2 , V B1 , and V B2 . 
         [0057]    In step S 1012 , the process transmits a final data signal based on the intermediate data signal, and proceeds to step S 1014 . For example, in the pre-emphasis scheme shown in  FIG. 4 , the output driver outputs signals at nodes V UT6  and V OUT6     —     B . 
         [0058]    In step S 1014 , the process determines whether the circuit implementing the pre-emphasis process is powered off. If the circuit is not powered off, then the process returns to step S 1002 . Otherwise, the process ends. 
         [0059]    As has been shown, the invention introduces the flexibility in setting the pre-emphasis ratio without adding extra capacitance at the outputs. The present invention makes this possible by handling the current subtraction in the pre-driver stage and current summation at the output driver stage. 
         [0060]    While this invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.