Abstract:
A fully differential continuous-time current-mode high-speed complimentary metal oxide semiconductor comparator is disclosed. The comparator includes an input and an output; a pre-amplifier clement coupled to each respective one of the plurality of inverters; an application switch operative to couple the pre-amplifier element to the input of a corresponding one of the plurality of inverters, the application switch having a first duty cycle; a current source operative to provide a bias current; and a bias switch operative to couple the bias current to each of the plurality of inverters, the bias switch having a duty cycle that is complementary to the duty cycle of the application switch, wherein the output of each of the plurality of inverters is pulled to about one-half the maximum output voltage level before a comparison between input signals is performed. By maintaining the comparator output at a substantially predetermined voltage level during non-operating periods, the switching characteristics of the comparator are enhanced.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to integrated circuit devices and, more particularly, to a reduced form factor complementary metal oxide semiconductor comparator exhibiting high speed operating characteristics.  
         BACKGROUND OF THE INVENTION  
         [0002]    In presently available digital video display systems, an analog input signal, representing an image is converted into a digital signal for later presentation to a user on a suitable display device (i.e., computer monitor, LCD, flat panel display, etc.). To provide such functionality, the analog input signal is first converted into a digital signal, appropriately filtered, and then the filtered digital signal is transmitted to the display device for presentation to the user.  
           [0003]    The transmission of the digital signal from the converting and filtering device(s) to the display device is performed through the use of an interface. In particular, presently available digital display systems employ a digital video interface (DVI) interface which operates, for example, according to the standard promulgated by the Digital Display Working Group (DDWG). The DVI standard requires the use of transition-minimized differential-signaling (TMDS) to transmit graphic and other complementary data from a computing device, which performs the aforementioned conversion and filtering operations, to the display device. TMDS requires the use of a TMDS transmitter and a corresponding TMDS receiver.  
           [0004]    The TMDS receiver may include comparison circuitry and other operational components that provide the receiver&#39;s functionality. In order to meet the DVI standard, the receiver, in addition to other specifications, must be able to operate at frequencies up to about 1.65 GHz with 3-times oversampling. Consequently, any underlying comparator must also be able to operate at frequencies up to about 1.65 GHz with 3-times over sampling.  
         SUMMARY OF THE INVENTION  
         [0005]    The aforementioned and related drawbacks associated with conventional comparators and their application in digital systems are substantially reduced or eliminated by the present invention. The present invention is directed to a complementary metal oxide semiconductor (CMOS) comparator capable of operating at frequencies of about 5.0 GHz, having a 0.35 μm form factor layout. Thus, any device employing the comparator of the present invention will be in compliance with, and can be used, in conjunction with the DVI and other digital interfaces.  
           [0006]    The comparator of the present invention comprises a plurality of inverters, where each inverter includes an input and an ouput; a pre-amplifier element coupled to each of the plurality of inverters; an application switch operative to couple the pre-amplifier element to the input of a corresponding one of the plurality of inverters, the application switch having a first duty cycle; a current source operative to provide a bias current; and a bias switch operative to couple the bias current to each of the plurality of inverters, the bias switch having a duty cycle that is complementary to the duty cycle of the application switch, wherein the output of each of the plurality of inverters is pulled to about one-half the maximum output voltage level before a comparison between two input signals is performed. By maintaining the output of the comparator at one-half the maximum output level, the switching characteristics (i.e. the speed in which the inverters provide an output signal) is enhanced as the comparator does not have to be pulled from a low voltage to a high voltage at high frequencies after each comparison. In other words, the comparator does not have to traverse a full voltage swing of the underlying components in order to perform a comparison between the corresponding input signals.  
           [0007]    An advantage provided by the present invention is that the comparator can operate at high frequencies.  
           [0008]    Another advantage provided by the present invention is that it requires less real estate than conventional comparators.  
           [0009]    Yet another advantage provided by the present invention is that it exhibits a small peak-to-peak output voltage swing.  
           [0010]    A feature of the present invention is that it operates over a wide input bandwidth range. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The aforementioned and related advantages and features of the present invention will become apparent upon review of the following detailed description of the invention, taken in conjunction with the following drawings, where like numerals represent like elements, in which:  
         [0012]    [0012]FIG. 1 is a block diagram of the comparator according to a preferred embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a schematic diagram of the comparator illustrated in FIG. 1;  
         [0014]    FIGS.  3 (A)- 3 (J) are timing diagrams illustrating the operation of the comparator illustrated in FIGS. 1 and 2;  
         [0015]    [0015]FIG. 4 is a block diagram of the comparator being used in a transition-minimized differential signal receiver according to an exemplary embodiment of the present invention; and  
         [0016]    FIGS.  5 (A)- 5 (C) are waveforms illustrating the output of the comparator illustrated in FIGS. 1 and 2 and  4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The CMOS comparator of the present invention will now be described with reference to FIGS.  1 - 5 . FIG. 1 is a block diagram of the comparator according to an exemplary embodiment of the present invention. As illustrated in FIG. 1, the comparator  10  includes a first inverter  22  and a second inverter  24 . The outputs of the respective inverters provide the result of a comparison between a corresponding set of input signals presented at inputs V INN  and V INP , respectively. As further illustrated in FIG. 1, the input of the first inverter  22  is coupled to node  41 . The input of the second inverter  24  is coupled to node  42 . The output of the first inverter  22  is coupled to the input of the second inverter  24  at node  42  on line  23 , and to a switch  25 . The output of the second inverter  24  is coupled to the input of the first inverter  22  at node  41  on line  21 , and to the switch  25 . The switch  25  is caused to alternate between an “on” state and an “off” state based on the application of a representative clock signal provided thereto. In addition, the output of the first inverter  22  may also be provided to any suitable device coupled to line  23 , as will be discussed in below. In corresponding fashion, the output of the second inverter  24  may be provided to any suitable device coupled to line  21 .  
         [0018]    As illustrated in greater detail in FIG. 2, the first inverter  22  is comprised of an n-channel MOSFET  122  having a source, drain and gate and a p-channel MOSFET  125  having a corresponding source, drain and gate. The gates of the n-channel MOSFET  122  and the p-channel MOSFET  125  are connected together. The drain of the n-channel MOSFET  122  is coupled to the source of the p-channel MOSFET  125  at node  126 . Node  126  represents the positive output (i.e. output) of the comparator  10  provided on pin  121   p . The source of the n-channel MOSFET  122  is coupled to ground. The drain of the p-channel MOSFET  125  is coupled to V DD  and the drain of a p-channel MOSFET  117 , which embodies the resistive element  17  of a first pre-amplifier element  20  (FIG. 1).  
         [0019]    The second inverter  24  is comprised of an n-channel MOSFET  124  having a source, drain and gate and a corresponding p-channel MOSFET  127 , also having a source, drain and gate. The gates of the n-channel MOSFET  124  and the p-channel MOSFET  127  are connected together. The inter-connected gates of the two MOSFETs are also connected to node  126 . The source of the n-channel MOSFET  124  is connected to ground. The drain of the n-channel MOSFET  124  is coupled to the source of the p-channel MOSFET  127  at node  128 . Node  128  is also connected to the inter-connected gates of n-channel MOSFET  122  and p-channel MOSFET  125 , which comprise the first inverter  22 . The drain of the p-channel MOSFET  127  is coupled to V DD  along the upper voltage rail of the comparator. Node  128  represents the negative output (outn) of the comparator  10  provided on pin  123   p.    
         [0020]    Switch  25  is connected the outputs of the inverters  22  and  24 , respectively, and is comprised of a parallel connected combination of an n-channel MOSFET  155  and a p-channel MOSFET  165 , where the source and drain of the respective MOSFETs are connected to the gates of the corresponding MOSFETs  122 ,  124 ,  125  and  128  that comprise the first and second inverters  22  and  24 , respectively at nodes  166  and  167 . The gate of the n-channel MOSFET  155  is connected to a clock signal (clk). Thus, the switchable “on” and “off” periods of the switch  25  are controlled by the main comparator operating clock (clk).  
         [0021]    Referring back to FIG. 1, a first pre-amplifier  20 , consisting of a resistive element  17  and a capacitive element  19  is coupled to node  15   a . The voltage provided by the first pre-amplifier element  20  is coupled to the input of the first inverter  22  at node  41  via a first application switch  18 . In corresponding fashion, a second pre-amplifier element  30 , consisting of a resistive element  27  and a capacitive element  29  is coupled to node  25   a . The voltage provided by the second pre-amplifier element  30  is coupled to the input of the second inverter  24  at node  42  via a second application switch  28 . As the first and second pre-amplifier elements are substantially identical in structure and operation, only first pre-amplifier element  20  will be described hereinafter.  
         [0022]    Referring back to FIG. 2, in an exemplary embodiment of the present invention, the resistive element  17  of the first pre-amplifier element  20  is comprised of a p-channel MOSFET  117 , having a source, drain and gate. The gate of the MOSFET  117  is connected to ground potential. The source of the MOSFET  117  is coupled to ground through the parasitic capacitance (represented as capacitor  19 ) and the gate-to-source capacitance (C gs ) of the MOSFET  117 . In an alternate embodiment, a suitable capacitor may be used in addition to the C gd  of the transistor to ensure proper capacitive load. With the MOSFET  117  having such a configuration, it effectively functions as a resistor. In addition, by using this configuration for the resistive element  17 , the RC constant, which directly corresponds to the charge/discharge rate of the nodes  15   a  and  25   a  of the comparator  10 , is maintained at a very small level; thereby, allowing the nodes  15   a  and  25   a  to be charged and discharged very quickly. In testing performed by the inventors, it was determined that the comparator  10  of the present invention can operate as switching frequencies as high as about 5.0 GHz. As a result, the comparator  10  is able to operate at higher switching frequencies than conventional comparators. This becomes very important when transmitting constantly varying signals between components, such as occurs in digital video systems.  
         [0023]    In addition to minimizing the RC time constant (i.e. switching time) of the inverters, the channel length and width of the transistors (i.e. MOSFETs) that comprise the inverters  22  and  24  has also been reduced. In an exemplary embodiment of the present invention, the channel length of transistors  122 ,  124 ,  125  and  128  is 0.4 μm. The channel width of transistors  122  and  124  is 4 μm. The channel width of transistors  125  and  128  is 5 μm. Reducing the channel length and width increases the drain-to-source current of the transistors, while at the same time reducing the gate-to-source (C gs ) and gate-to-drain (C gd ) capacitance of the transistors; thereby, resulting in a faster charge/discharge process.  
         [0024]    As further illustrated in FIG. 2, the first application switch is  18  comprised of an n-channel MOSFET  118  having a source drain and gate. The drain of the MOSFET  118  is coupled to the output of the second inverter on line  21 . The source of the MOSFET  118  is connected to node  15   a . The gate of the MOSFET  118  is coupled to clock signal (clkb). The function of the first application switch  18  is to couple the voltage provided by the first pre-amplifier element  20  to the input (node  41 ) of the first inverter  22  at a regular interval.  
         [0025]    The second application switch  28 , is used to couple the voltage provided by the second pre-amplifier element  30  to the input (node  42 ) of the second inverter  24 . The second application switch  28  is comprised of an n-channel MOSFET  128  having a source drain and gate. The drain of the MOSFET  128  is coupled to the output of the first inverter  22  on line  23 . The drain of the MOSFET  128  is coupled to node  25   a , which corresponds to a junction point of the second resistive element  27  and second capacitive element  29 . The gate of the MOSFET  128  is coupled to the second clock signal (clkb) that is used to control the on period and the off period of the MOSFET  118  (i.e., first application switch  18 ). Thus, the first application switch  18  and the second application switch  28  have the same duty cycle.  
         [0026]    A current source  32  (FIG. 1), coupled to the base of the comparator  10 , provides a constant level of current to the comparator  10 . The current source  32  is comprised, in part, of a current mirror consisting of a first n-channel MOSFET  132   a  having a source, drain and gate and a second n-channel MOSFET  132   b  also having a source, drain and gate. The gates of the corresponding current mirror transistors  132   a ,  132   b  are connected together at a node  133 . The respective sources of the current mirror transistors  132   a ,  132   b  are coupled to ground at node  135 . The drain of MOSFET  132   a  is coupled to the inputs of the first and second inverters  22  and  24 , respectively through a first bias switch  16  and a second bias switch  26 . The structure of the first and second bias switches will be described in greater detail below. The drain and gate of the MOSFET  132   b  are connected together. Thus the inter-connected drain and gate of MOSFET  132   b  is also coupled to the gate of the MOSFET  132   a  at node  133 . The drain of the MOSFET  132   b  is also coupled to a reference bias current source (ibias) at node  140 . Also coupled to node  133  is a third n-channel MOSFET  142 , which acts as a power down circuit for the comparator. The MOSFET  142  has a source drain and gate. The drain of the MOSFET  142  is coupled to node  133 . The source of the MOSFET  142  is coupled to ground at node  135  and the gate of the MOSFET  142  is coupled to an external power down signal PD. The current source  32  also includes a band gap circuit  143 , consisting of a plurality of parallel connected n-channel MOSFETS all coupled to node  135 .  
         [0027]    The current source of the present invention also includes an additional sink current source  50 , which is coupled to the inputs of the first and second inverters  22  and  24 , respectively, through a corresponding sink current application switch  52 . The sink current source  50  is comprised of a p-channel MOSFET  150 , having a corresponding source drain and gate. The drain of the sink current MOSFET  150  is coupled to V DD . The source of the sink current MOSFET  150  is coupled to the drain of the corresponding sink current application switch  52 . In addition, the source of the sink current MOSFET  150  is also coupled to its gate. The source of the sink current application switch  52  is also coupled to the respective sources of the first and second bias transistors  16  and  26 , respectively. The gate of the sink current application switch  52  is coupled to second clock signal (clkb); therefore, the on and off periods of the sink current application transistor  52 , is the same as those of the first and second application MOSFETS  118  and  128  and switch  25 . Thus, the sink current source  50  provides the bias current to the comparator during those operating periods when current source  32  is removed from the system.  
         [0028]    In an exemplary embodiment, the first bias transistor  16  and the second bias transistor  26  have substantially identical structures and modes of operation; therefore, only the structure of the first bias transistor  16  will be described hereinafter. As illustrated in FIG. 2, the first bias transistor  16  is comprised of an n-channel MOSFET having a source, drain and gate. The drain of the first bias transistor  16  is coupled to the source of first input transistor  12 . The source of the first bias transistor  16  is coupled to the drain of MOSFET  132   a  at node  136 . The gate of the first bias transistor  16  is coupled to the gate of the second bias transistor  26  and also to clock signal (clk). The duty cycle of the clock signal (clk) is complimentary to the second clock signal (clkb). Thus, when the first and second bias switches  16  and  26  are closed, the first and second application switches  18 ,  26 , switch  25  and the sink current application switch  52  are all open. Consequently, the bias switches and corresponding application switches have complementary duty cycles.  
         [0029]    The inputs to be compared are provided to the comparator of the present invention  10  by a first input transistor  12  and a second input transistor  14 . The first input transistor  12  is comprised of an n-channel MOSFET having a drain, source and gate. The drain of the first input transistor  12  is coupled to the source of the first application transistor  118  at node  15   a . The source of the first input transistor  12  is coupled to the drain of the first bias transistor  116 . The gate of the first input transistor  12  is coupled to the negative input of the comparator (V INN ). The second input to the comparator  10  is provided by a second input transistor  14  comprising an n-channel MOSFET having a drain, source and gate. The drain of the second input transistor  14  is coupled to the source of the second application transistor  26 . The source of the second input transistor  14  is coupled to the drain of the second application transistor  26 . The gate of the second input transistor  12  is coupled to the positive input of the comparator (V INP ).  
         [0030]    The operation of the comparator  10  will now be described with reference to FIGS.  3 A- 3 J. FIGS.  3 A- 3 J are timing diagrams illustrating the operation of the comparator  10 . As illustrated in FIG. 3, the clock signals clk and clkb, that control the operation of the comparator, are complementary in nature. Thus, when the main clock signal (clk) is high, the second secondary clock signal (clkb), which controls the “on” and “off” periods of the application MOSFETS  118 , 128 , sink current application switch  52  and the switch  25  is low. In application, the comparator has two operating modes: (1) a standby mode, where the outputs of the comparator are maintained substantially at a predetermined voltage level; and (2) a comparison mode, where the outputs of the comparator reflect the relative difference between the signals (i.e. voltages) provided at the inputs of the comparator at V INN  and V INP , respectively.  
         [0031]    In the standby mode, for example, during time interval t 0 -t 1 , clk (FIG. 3A) is high. This results in the first bias transistor  116  (FIG. 3C) and the second bias transistors  126  (FIG. 3D) being closed (i.e., on), wherein the bias current provided by the current source  32  is supplied to the comparator. In addition, the switch  25  (FIG. 3G) is also closed. During this period, the corresponding outputs OUTN (FIG. 31) and OUTP (FIG. 3J) of the comparator  10  are pulled to about one-half the maximum output voltage level as the corresponding input nodes  15   a  and  25   a  are charged to a suitable level by the bias current flowing through the first and second bias transistors. More specifically, in an exemplary embodiment, when the first and second bias transistors are both on (i.e. conducting), the voltage at the output(s) of the comparator  10  is maintained at about 0.5V DD .  
         [0032]    In the comparison mode, for example, during time interval t 1 -t 2 , clk is low; thus, clkb (FIG. 3B) is high. This causes the first bias MOSFET  116 , the second bias MOSFET  126  and the switch  25  to become opened (i.e. non-conducting). During the comparison mode, the first application switch  18  (FIG. 3E), the second application switch  28  (FIG. 3F) and the sink current application switch  52  (FIG. 3H) are all closed (i.e. conducting). This causes the two input nodes  15   a  and  25   a , respectively, to become charged by the corresponding input signals provided at V INN  and V INP  and the output(s) of the comparator no longer being actively clamped by the bias current source  32  at 0.5V DD . During the comparison mode, the output(s) of the comparator represent the relative difference between the voltages present at inputs V INN  and V INP , respectively. For example, if the input voltage (V INN ) present or applied at node  15   a  is greater than the input voltage (V INP ) present or applied at node  25   a , the voltage present at the corresponding output of inverter  22  (signal on pin  123   p ) will be greater than the voltage present at the output of inverter  24  (i.e. signal on pin  121   p ). Correspondingly, if the input voltage (V INP ) present or applied at node  25   a  is greater than the input voltage (V INN ) present or applied at node  15   a , the voltage present at the corresponding output of inverter  22  (i.e. signal at pin  123   p ) will be less than the voltage present at the output of the inverter  24  (i.e. signal at pin  121   p ). In operation, this voltage comparison is completed at substantially the same rate as the clock frequency; thus, the comparator can operate at frequencies of about 5.0 GHz.  
         [0033]    During a subsequent standby mode (i.e. time interval t 2 -t 3 ), the respective output(s) of the comparator  10  are once again pulled to about 0.5V DD , wherein the comparator awaits new inputs being provided to the input transistors  12  and  14 , respectively to be compared. By maintaining the output level of the comparator  10  at substantially the middle of the maximum voltage swing during the standby mode, the amount of time required to perform a subsequent comparison operation (i.e. switching from an idle state to a comparison state) is minimized. In fact, in an exemplary embodiment of the present invention, by maintaining the output level of the comparator during the standby mode to about 0.5V DD , the comparator can operate at speeds in excess of about 5.0 GHz. Thus, the comparator of the present invention can be used in devices capable of handling digital switching operations. In particular, the comparator  10  can be used in conjunction with TMDS receivers operating in conformance with the DVI standard.  
         [0034]    [0034]FIG. 4 is a block diagram of the comparator  10  being used in combination with a set-reset flip flop  60  to implement the operation of a transition-minimized differential-signaling receiver according to an exemplary embodiment of the present invention. The set-reset flip flop is used to prevent undefined output voltage levels from negatively affecting comparator operation or performance when switching between the standby and comparison modes. As illustrated in FIG. 4, a first (i.e., negative) output (OUTN) of the comparator is provided to the reset pin (R) of the set-reset flip flop  60  on line  123 . In corresponding fashion, the second (i.e., positive) output (OUTP) of the comparator is provided to the set pin (S) of the set-reset flip flop  60  via line  121 . The output (Q) of the set-reset flip flop  60  is provided at pin  61 , while an inverted version of the output (Q) is provided on line  62 . The set-reset flip flop  60  of the present invention operates as illustrated in Table 1 below.  
                                       TABLE 1                                   S   R   Q   {overscore (Q)}   MODE                           0   0   0   0   Avoid           0   1   0   1   Normal                           Operation           1   0   1   0   Normal                           Operation           1   1   Q   {overscore (Q)}   Data Unchanged                      
 
         [0035]    As discussed above in greater detail with respect to FIGS.  1 - 3 , the comparator  10  can operate at speeds of about 5.0 GHz; therefore, the output provided by the set-reset flip flop  60  can also be provided at speeds upwards to about 5.0 GHz. Therefore, a TMDS receiver incorporating the comparator of the present invention can be used in conjunction with any type of suitable display device operating in accordance with, for example, the DVI standard. In addition, by having a 0.35 μm form layout the comparator  10  of the present invention can be used in many other types of devices, including flat panel display devices.  
         [0036]    The improved operating characteristics of the comparator  10  of the present invention will now be discusssed with reference to FIGS.  5 A- 5 C. FIG. 5A is a plot of voltage versus time illustrating the set up period of the comparator  10  during one operating condition. For purposes of example, assume the voltage present at the positive input (V INP ) of the comparator  10  is greater than the voltage present at the negative input (V INN ) of the comparator. In this situation, node  25   a  charges to a higher level than corresponding node  15   a . In other words, the magnitude of the voltage present at node  25   a  is greater than the magnitude of the voltage present at node  15   a . When the comparator enters its next comparison mode (i.e. clock signal being transmitted to the application transistors  118  and  128 ), the difference between the voltage present at nodes  15   a  and  25   a  will be provided as the output of the comparator  10  on line  121  (FIG. 5B). This signal is then transmitted to the flip-flop  60  (FIG. 4).  
         [0037]    Correspondingly, as illustrated in FIG. 5C, when V INN  is greater in magnitude with respect to V INP , the charge present at node  15   a  is greater than the magnitude of the charge present at node  25   a . In this situation, the flip-flop  60  will receive the output of the comparator on line  123  (FIG. 4) during the next falling edge of the clk. This represents the next comparison mode of the comparator.  
         [0038]    As illustrated in FIG. 5A, the differential input signal provided to the comparator is very small. Consequently, shifting from the standby mode to the comparison mode is performed very quickly.  
         [0039]    The above detailed description of the present invention has been provided for the purposed of illustration and description. Although the present invention has been described with respect to several specific embodiments, various changes and modifications may be suggested to persons of ordinary skill in the art, and it is intended that the present invention encompass such changes and modifications that fall within the scope of the claims appended hereto.