Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to (is a non-provisional filing of) U.S. Provisional Patent Application No. 60/870,836 entitled “PARALLEL BIPOLAR LOGIC DEVICES AND METHODS FOR USING SUCH” and filed Dec. 19, 2006 by Payne. The aforementioned application is assigned to an entity common hereto and is incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention is related to logic devices, and more particularly to differential logic devices. 
         [0003]    Differential Emitter coupled logic has been used to create multiple input stacked gates. As an example,  FIG. 1  shows a two input differential AND gate  100  implemented in emitter coupled logic. As shown, AND gate  100  includes two differential input pairs  107 ,  111 , with one stacked upon the other. One pair of differential inputs  132 ,  134  are applied to the respective bases of a transistor  106  and a transistor  108  of differential pair  107 . Another pair of differential inputs  136 ,  138  are applied to the respective bases of a transistor  110  and a transistor  112  of differential pair  111 . Both differential pair  107  and differential pair  111  are biased by the same current source generated by applying a bias voltage  140  to a transistor  116  that is electrically coupled to ground (VSS  142 ) via a resistor  114 . The collector of transistor  106  is electrically coupled to VDD  130  via a resistor  102 , and to the base of an emitter follower transistor  128 . The collector of transistor  108  is electrically coupled to VDD  130  via a resistor  104 , and to the base of an emitter follower transistor  126 . The collector of transistor  112  is also electrically coupled to the base of emitter follower transistor  126 . The collector of transistor  110  is electrically coupled to the emitters of transistor  106  and transistor  108 . Resistor  102  and resistor  104  are the same value. Emitter follower transistor  126  is biased by a transistor  120  and a resistor  118 , and emitter follower transistor  128  is biased by a transistor  124  and a resistor  122 . Two sets of differential outputs are provided from AND gate  100 : an upper biased pair including Y  184  and YZ  182 ; and a lower biased pair including YEFZ  186  and YEF  188 . As the input bias required into each stage may be slightly different, the two pairs of differential outputs are necessary depending upon the next stage to be driven. This is one of the significant disadvantages of differential emitter coupled logic. 
         [0004]    In operation, when both input  132  is asserted high relative to input  134 , and input  136  is asserted high relative to input  138 , output Y  184  is asserted high with respect to output YZ  182  and output YEF  188  is asserted high with respect to output YEFZ  186 . In this case, transistor  106  and transistor  110  are turned on, and the tail current sourced by transistor  114  traverses resistor  102 , transistor  106  and transistor  110 . No current traverses resistor  104  as transistor  108  and transistor  112  are turned off. Thus, the voltage level at the base of emitter follower transistor  128  (Vb is VDD−I tail *R 102 ) is lower than that at the base of emitter follower transistor  126  (Vb is near VDD  130 ). Thus, YEF  188  is greater than YEFZ  186  indicating a logic ‘1’ value. In contrast, where either or both of input  132  or input  136  are asserted low relative to input  134  or input  138 , respectively, the tail current sourced by transistor  114  traverses resistor  104 , and no current traverses resistor  102 . Thus, the voltage level at the base of emitter follower transistor  126  (Vb is VDD−I tail *R 104 ) is lower than that at the base of emitter follower transistor  128  (Vb is near VDD  130 ). 
         [0005]    One problem with the design of AND gate  100  is that considerable head room is needed between VDD  130  and VSS  142  as there are voltage drops through three transistors and through two resistors. This headroom limitation becomes more acute as additional inputs are added to a particular gate. For example, a three input AND gate includes an additional stacked differential input. In such a case, there are voltage drops through four transistors and through two resistors. Thus, such an approach to logic gates is severely limited in the number of inputs that may be handled in the same logic gate. Further, such an approach to logic gates requires the use of higher voltage power supplies, which can be a disadvantage in many design situations. 
         [0006]    As another example of the same emitter coupled logic architecture,  FIG. 2  shows a three input differential OR gate  200 . As shown, OR gate  200  includes three differential input pairs  207 ,  211 ,  215  each stacked one upon the other. One pair of differential inputs  232 ,  234  is applied to the respective bases of a transistor  206  and a transistor  208  of differential pair  207 . Another pair of differential inputs  236 ,  238  is applied to the respective bases of a transistor  210  and a transistor  212  of differential pair  211 . Yet another pair of differential inputs  240 ,  242  is applied to the respective bases of a transistor  214  and a transistor  216  of differential pair  215 . Each of differential pair  207  differential pair,  211  and differential pair  215  are biased by the same current source generated by applying a bias voltage  244  to a transistor  218  that is electrically coupled to ground (VSS  246 ) via a resistor  220 . The collector of transistor  206  is electrically coupled to VDD  230  via a resistor  202 , and to the base of an emitter follower transistor  231 . The collector of transistor  208  is electrically coupled to VDD  230  via a resistor  204 , and to the base of an emitter follower transistor  229 . The collectors of transistor  212  and of transistor  216  are also electrically coupled to the base of emitter follower transistor  229 . The collector of transistor  210  is electrically coupled to the emitters of transistor  206  and transistor  208 . The collector of transistor  214  is electrically coupled to the emitters of transistor  210  and transistor  212 . Resistor  202  and resistor  204  are the same value. Emitter follower transistor  229  is biased by a transistor  224  and a resistor  222 , and emitter follower transistor  231  is biased by a transistor  228  and a resistor  226 . Two sets of differential outputs are provided from OR gate  200 : an upper biased pair including Y  282  and YZ  284 ; and a lower biased pair including YEFZ  288  and YEF  286 . Again, the need for two sets of differential outputs is a disadvantage of the existing emitter coupled logic architecture. The three pairs of input differential pairs  207 ,  211 ,  215  likewise require input signals that are offset at three differential common mode levels. 
         [0007]    In operation, when all of input  232 , input  236  and input  240  are asserted high with respect to inputs  234 ,  238 ,  242 , respectively; the tail current tail current sourced by transistor  218  traverses resistor  202 , and no current traverses resistor  204 . In this condition, the voltage level at the base of emitter follower transistor  231  (Vb is VDD−I tail *R 202 ) is lower than that at the base of emitter follower transistor  229  (Vb is near VDD  230 ). Thus, YEFZ  288  is greater than YEF  286 . In contrast, where any of input  232 , input  236  and/or input  240  is/are asserted low with respect to inputs  234 ,  238 ,  242 , respectively; the tail current sourced by transistor  218  traverses resistor  204 , and no current traverses resistor  202 . Thus, the voltage level at the base of emitter follower transistor  229  (Vb is VDD−I tail *R 104 ) is lower than that at the base of emitter follower transistor  231  (Vb is near VDD  130 ). 
         [0008]    Three input OR gate  200  has the same problem as the previously discussed two input AND gate in that considerable head room is needed between VDD  230  and VSS  246  as there are voltage drops through four transistors and through two resistors. This headroom limitation becomes more acute as additional inputs are added to a particular gate. For example, a four input OR gate includes an additional stacked differential input. In such a case, there are voltage drops through five transistors and through two resistors. Thus, such an approach to logic gates is severely limited in the number of inputs that may be handled in the same logic gate. Further, such an approach to logic gates requires the use of higher voltage power supplies, which can be a disadvantage in many design situations. 
         [0009]    Hence, for at least the aforementioned reasons, there exists a need in the art for advanced logic architectures. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The present invention is related to logic devices, and more particularly to differential logic devices. 
         [0011]    Some embodiments of the present invention provide methods for implementing logic. Such methods include providing a first differential input pair and a second differential pair. The collector of a first transistor of the first differential pair is electrically coupled to the collector of a first transistor of the second differential pair, and to an upper voltage via a first resistor. Further, the collector of a second transistor of the first differential pair is electrically coupled to the collector of a second transistor of the second differential pair, and to the upper voltage via a second resistor. The method further includes selecting a first resistive value associated with the first resistor to be different from a second resistive value associated with the second resistor. The difference between the first resistive value and the second resistive value yields a particular type of logic gate. In some cases, the particular type of logic gate is an AND gate, while in other cases, the particular type of logic gate is an OR gate. The aforementioned first and second resistors may be either passive resistors or active resistors. 
         [0012]    Other embodiments of the present invention provide differential logic gates. Such logic gates include a first differential input pair and a second differential input pair. The first differential pair receives a first differential input, and the second differential pair receives a second differential input. The collectors of each of a first transistor of the first differential pair and a first transistor of the second differential pair are each electrically coupled to an upper voltage via a first resistor. The collectors of each of a second transistor of the first differential pair and a second transistor of the second differential pair are each electrically coupled to an upper voltage via a second resistor. In some instances of the aforementioned embodiments, the first resistor has a resistive value that is different from that of the second resistor. Selection of this difference causes the logic gate to operate either as an AND gate or an OR gate. The first resistor and the second resistor may be either passive or active resistive loads. In some instances of the aforementioned embodiments, the logic gate includes a differential output at a single offset level, in contrast to the prior art gates that include two distinct outputs at different offset levels. 
         [0013]    Various instances of the aforementioned embodiments include a third differential pair that receives a third differential input. The collector of a first transistor of the third differential pair is electrically coupled to the upper voltage via the first resistor, and the collector of a second transistor of the third differential pair is electrically coupled to the upper voltage via the second resistor. In such cases, the resistive value of the first resistor and that of the second resistor may be selected to be different, with the difference between the resistive values being selected to yield either an OR gate function or an AND gate function that is applied to the three differential inputs. 
         [0014]    This summary provides only a general outline of some embodiments according to the present invention. Many other objects, features, advantages and other embodiments of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A further understanding of the various embodiments of the present invention may be realized by referenced to the figures which are described in remaining portions of the specification. In the figures, like reference numerals are used throughout several drawings to refer to similar components. In some instances, a sub-label consisting of a lower case letter is associated with a reference numeral to denote one of multiple similar components. When reference is made to a reference numeral without specification to an existing sub-label, it is intended to refer to all such multiple similar components. 
           [0016]      FIG. 1  shows a prior art two input differential AND gate; 
           [0017]      FIG. 2  depicts a prior art three input differential OR gate; 
           [0018]      FIG. 3  shows a two input differential AND/OR gate in accordance with one or more embodiments of the present invention; and 
           [0019]      FIG. 4  depicts a three input differential AND/OR gate in accordance with various embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The present invention is related to logic devices, and more particularly to differential logic devices. 
         [0021]    Turning to  FIG. 3 , a two input differential AND/OR gate  300  in accordance with various embodiments of the present invention is shown. As will be discussed more fully below, determination of whether gate  300  operates as an AND gate or as an OR gate is determined by proper selection of values for a resistor  302  and a resistor  304 . As shown, gate  300  includes two differential input pairs  307 ,  311  in parallel with each other. One set of differential inputs  332 ,  334  is applied to the respective bases of a transistor  306  and a transistor  308  of differential pair  307 . Another set of differential inputs  336 ,  338  is applied to the respective bases of a transistor  310  and a transistor  312  of differential pair  311 . Differential pair  307  is biased separate from differential pair  311 . In particular, differential pair  307  is biased by a current source generated by applying a bias voltage  340  to a transistor  320  that is electrically coupled to ground (VSS  342 ) via a resistor  341 , and differential pair  311  is biased by a current source generated by applying bias voltage  340  to a transistor  322  that is electrically coupled to VSS  342  via a resistor  343 . 
         [0022]    The collector of transistor  306  and the collector of transistor  310  are electrically coupled to VDD  330  via a resistor  302 , and to the base of an emitter follower transistor  331 . The collector of transistor  308  and the collector of transistor  312  are electrically coupled to VDD  330  via a resistor  304 , and to the base of an emitter follower transistor  329 . Emitter follower transistor  331  is biased by a transistor  326  and a resistor  347 , and emitter follower transistor  329  is biased by a transistor  324  and a resistor  345 . Only a single set of differential outputs (YEF  384  and YEFZ  386 ) are provided from gate  300 . 
         [0023]    Resistor  302  and resistor  304  create an input dependent IR voltage drop (i.e., current multiplied by load resistance) from VDD  330 . To create an AND gate from AND/OR gate  300 , resistor  302  and resistor  304  are purposely mismatched (or scaled) such that the following conditions must both be true in order for YEF  384 &gt;YEFZ  386 : 
         [0000]        V   INPUT 332   −V   INPUT 334 &gt;0; and 
         [0000]        V   INPUT 336   −V   INPUT 338 &gt;0. 
         [0000]    Due to the parallel nature of the logic architecture, three states exist at the output (YEF  384 , YEFZ  386 ). The three states are set forth in Table 1 below where it is assumed that the current (I T ) sourced via transistor  320  (I T1 ) is the same as the current sourced via transistor  322  (I T2 ): 
         [0000]                                          TABLE 1                   Equations for Resistive Values Used to Create an AND Gate            V INPUT 332  −   V INPUT 336  −                   V INPUT 334     V INPUT 338     V YEF     V YEFZ     V YEF  − V YEFZ                 &lt;0   &lt;0   VDD − 2 * I T  * R 304  − V BE     VDD − V BE     −2 * I T  * R 304         &lt;0   &gt;0   VDD − I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − R 304 )       &gt;0   &lt;0   VDD − I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − R 304 )       &gt;0   &gt;0   VDD − V BE     VDD − 2 * I T  * R 302  − V BE     2 * I T  * R 302                      
Using the above mentioned equations, values for resistor  302  and resistor  304  can be tabulated where an appropriate tail current (I T ) is assumed. For example, where the tail current is set at twenty microamperes, values of 4 kOhm for resistor  302  and 12 kOhm for resistor  304  satisfy the equations of Table 1 above. Table 2 below demonstrates this by solving each of the equations of Table 1 using the aforementioned values:
 
         [0000]                                  TABLE 2                   Solved Equations Using Exemplary Values            V INPUT 332  − V INPUT 334     V INPUT 336  − V INPUT 338     V YEF  − V YEFZ                 &lt;0   &lt;0   −480 mV       &lt;0   &gt;0   −160 mV       &gt;0   &lt;0   −160 mV       &gt;0   &gt;0   +160 mV                    
As can be seen, the conditions for an AND gate are satisfied where V YEF −V YEFZ  is only greater than zero where both V INPUT 332 −V INPUT 334 &gt;0 and V INPUT 336 −V INPUT 338 &gt;0 are true. It should be noted that other combinations of values for resistor  302 , resistor  304  and I T  may be used to create an AND gate in accordance with embodiments of the present invention.
 
         [0024]    In operation when an AND gate is created as set forth above, when input  332  is asserted high relative to input  334  and input  336  is asserted high relative to input  338 , output YEF  384  is asserted high relative to output YEFZ  386 . In this case, transistor  306  and transistor  310  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  and the tail current (I T2 ) sourced by transistor  322  both traverse resistor  302 , and no current traverses resistor  304 . Thus, the voltage at the base of emitter follower transistor  331  [VDD−(I T1 +I T2 )*R 302 ] is lower than the voltage at the base of emitter follower transistor  329  [VDD], and YEF  384  is at a higher voltage than YEFZ  386 . 
         [0025]    In contrast, when input  332  is asserted low relative to input  334  and input  336  is asserted low relative to input  338 , output YEF  384  is asserted low relative to output YEFZ  386 . In this case, transistor  308  and transistor  312  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  and the tail current (I T2 ) sourced by transistor  322  both traverse resistor  304 , and no current traverses resistor  302 . Thus, the voltage at the base of emitter follower transistor  331  [VDD] is higher than the voltage at the base of emitter follower transistor  329  [VDD−(I T1 +I T2 )*R 304 ], and YEFZ  386  is at a higher voltage than YEF  384 . 
         [0026]    In another condition, when input  332  is asserted high relative to input  334  and input  336  is asserted low relative to input  338 , output YEF  384  is asserted low relative to output YEFZ  386 . In this case, transistor  306  and transistor  312  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  traverses resistor  302 , and the tail current (I T2 ) sourced by transistor  322  traverse resistor  304 . Thus, the voltage at the base of emitter follower transistor  331  [VDD−I T1 *R 302 ] is higher than the voltage at the base of emitter follower transistor  329  [VDD−I T2 *R 304 ], as the value of resistor  302  is less than the value of resistor  304 , and YEFZ  386  is at a higher voltage than YEFZ  384 . The same condition occurs in the opposite condition where input  332  is asserted low relative to input  334  and input  336  is asserted high relative to input  338 . 
         [0027]    At this point, it should be noted that an OR gate can be similarly created by appropriately selecting values for resistor  302 , resistor  304  and the tail current as before, but to satisfy the conditions of an OR gate. In particular, to create an OR gate from AND/OR gate  300 , resistor  302  and resistor  304  are purposely mismatched (or scaled) such that the following conditions must both be true in order for YEF  384 &lt;YEFZ  386 : 
         [0000]        V   INPUT 332   −V   INPUT 334 &lt;0; and 
         [0000]        V   INPUT 336   −V   INPUT 338 &lt;0. 
         [0000]    Again, due to the parallel nature of the logic architecture, three states exist at the output (YEF  384 , YEFZ  386 ). The three states are set forth in Table 3 below where it is assumed that the current (I T ) sourced via transistor  320  (I T1 ) is the same as the current sourced via transistor  322  (I T2 ): 
         [0000]                                          TABLE 3                   Equations for Resistive Values Used to Create an OR Gate            V INPUT 332  −   V INPUT 336  −                   V INPUT 334     V INPUT 338     V YEF     V YEFZ     V YEF  − V YEFZ                 &lt;0   &lt;0   VDD − 2 * I T  * R 304  − V BE     VDD − V BE     −2 * I T  * R 304         &lt;0   &gt;0   VDD − I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − R 304 )       &gt;0   &lt;0   VDD − I T  * R 304  − V BE     VDD − I T  * R 302  − V BE         &gt;0   &gt;0   VDD − V BE     VDD − 2 * I T  * R 302  − V BE     2 * I T  * R 302                      
Using the above mentioned equations, values for resistor  302  and resistor  304  can be tabulated where an appropriate tail current (I T ) is assumed. For example, where the tail current is set at twenty microamperes, values of 12 kOhm for resistor  302  and 4 kOhm for resistor  304  satisfy the equations of Table 3 above. Table 4 below demonstrates this by solving each of the equations of Table 3 using the aforementioned values:
 
         [0000]                                  TABLE 4                   Solved Equations Using Exemplary Values            V INPUT 332  − V INPUT 334     V INPUT 336  − V INPUT 338     V YEF  − V YEFZ                 &lt;0   &lt;0   −160 mV       &lt;0   &gt;0   +160 mV       &gt;0   &lt;0       &gt;0   &gt;0   +480 mV                    
As can be seen, the conditions for an OR gate are satisfied where V YEF −V YEFZ  is always greater than zero where either V INPUT 332 −V INPUT 334 &gt;0 or V INPUT 336 −V INPUT 338 &gt;0 are true. It should be noted that other combinations of values for resistor  302 , resistor  304  and I T  may be used to create an OR gate in accordance with embodiments of the present invention.
 
         [0028]    In operation when an OR gate is created as set forth above, when input  332  is asserted high relative to input  334  and input  336  is asserted high relative to input  338 , output YEF  384  is asserted high relative to output YEFZ  386 . In this case, transistor  306  and transistor  310  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  and the tail current (I T2 ) sourced by transistor  322  both traverse resistor  302 , and no current traverses resistor  304 . Thus, the voltage at the base of emitter follower transistor  331  [VDD−(I T1 +I T2 )*R 302 ] is lower than the voltage at the base of emitter follower transistor  329  [VDD], and YEF  384  is at a higher voltage than YEFZ  386 . 
         [0029]    In contrast, when input  332  is asserted low relative to input  334  and input  336  is asserted low relative to input  338 , output YEF  384  is asserted low relative to output YEFZ  386 . In this case, transistor  308  and transistor  312  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  and the tail current (I T2 ) sourced by transistor  322  both traverse resistor  304 , and no current traverses resistor  302 . Thus, the voltage at the base of emitter follower transistor  331  [VDD] is higher than the voltage at the base of emitter follower transistor  329  [VDD−(I T1 +I T2 )*R 304 ], and YEFZ  386  is at a higher voltage than YEF  384 . 
         [0030]    In another condition, when input  332  is asserted high relative to input  334  and input  336  is asserted low relative to input  338 , output YEF  384  is asserted high relative to output YEFZ  386 . In this case, transistor  306  and transistor  312  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320  traverses resistor  302 , and the tail current (I T2 ) sourced by transistor  322  traverse resistor  304 . Thus, the voltage at the base of emitter follower transistor  331  [VDD−I T1 *R 302 ] is lower than the voltage at the base of emitter follower transistor  329  [VDD−I T2 *R 304 ], as the value of resistor  302  is greater than the value of resistor  304 . Thus, YEF  384  is at a higher voltage than YEFZ  384 . The same condition occurs in the opposite condition where input  332  is asserted low relative to input  334  and input  336  is asserted high relative to input  338 . 
         [0031]    Turning now to  FIG. 4 , the logic gate of  FIG. 3  is extended to be a three input differential AND/OR gate  400  in accordance with various embodiments of the present invention. Based on the description of gate  400 , one of ordinary skill in the art will appreciate the extensibility of the logic architecture of the present invention. Based on this, one of ordinary skill in the art will recognize that logic gates of four or more differential inputs may be created without impacting the head room available through the selection of VDD. 
         [0032]    Gate  400  includes three differential input pairs  307 ,  311 ,  415  in parallel with each other. One set of differential inputs  332 ,  334  are applied to the respective bases of transistor  306  and transistor  308  of differential pair  307 . Another set of differential inputs  336 ,  338  are applied to the respective bases of transistor  310  and transistor  312  of differential pair  311 ; and yet another set of differential inputs  440 ,  442  are applied to the respective bases of a transistor  414  and a transistor  416  of differential pair  415 . Differential pair  307  is biased by a current source generated by applying bias voltage  340  to transistor  320  that is electrically coupled to ground (VSS  342 ) via resistor  341 ; differential pair  311  is biased by a current source generated by applying bias voltage  340  to transistor  322  that is electrically coupled to VSS  342  via resistor  343 ; and differential pair  415  is biased by a current source generated by applying bias voltage  340  to a transistor  426  that is electrically coupled to VSS  342  via a resistor  449 . 
         [0033]    The collector of transistor  306 , the collector of transistor  310  and the collector of transistor  414  are electrically coupled to VDD  330  via resistor  302 , and to the base of emitter follower transistor  331 . The collector of transistor  308 , the collector of transistor  312  and the collector of transistor  416  are electrically coupled to VDD  330  via resistor  304 , and to the base of emitter follower transistor  329 . Emitter follower transistor  331  is biased by transistor  324  and resistor  347 , and emitter follower transistor  329  is biased by transistor  324  and resistor  345 . Only a single set of differential outputs (YEF  384  and YEFZ  386 ) are provided from gate  300 . 
         [0034]    As with the two input gate of  FIG. 3 , resistor  302  and resistor  304  create an input dependent IR voltage drop from VDD  330 . To create an AND gate from AND/OR gate  400 , resistor  302  and resistor  304  are purposely mismatched (or scaled) such that the following conditions must both be true in order for YEF  384 &gt;YEFZ  386 : 
         [0000]        V   INPUT 332   −V   INPUT 334 &gt;0; 
         [0000]        V   INPUT 336   −V   INPUT 338&gt; 0; and 
         [0000]        V   INPUT 440   −V   INPUT 442 &gt;0. 
         [0000]    Again, due to the parallel nature of the logic architecture, five states exist at the output (YEF  384 , YEFZ  386 ). The five states are set forth in Table 5 below where it is assumed that the current (I T ) sourced via transistor  320  (I T1 ) is the same as the current sourced via transistor  322  (I T2 ) and that (I T3 ) sourced via transistor  426 : 
         [0000]                                              TABLE 5                   Equations for Resistive Values Used to Create an AND Gate            V INPUT 332  −                           V INPUT 334     V INPUT 336  − V INPUT 338     V INPUT 440  − V INPUT 442     V YEF     V YEFZ     V YEF  − V YEFZ                 &lt;0   &lt;0   &lt;0   VDD − 3 * I T  * R 304  − V BE     VDD − V BE     −3 * I T  * R 304         &lt;0   &lt;0   &gt;0   VDD − 2 * I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − 2 * R 304 )       &lt;0   &gt;0   &lt;0   VDD − 2 * I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − 2 * R 304 )       &lt;0   &gt;0   &gt;0   VDD − I T  * R 304  − V BE     VDD − 2 * I T  * R 302  − V BE     I T  * (2 * R 302  − R 304 )       &gt;0   &lt;0   &lt;0   VDD − 2 * I T  * R 304  − V BE     VDD − I T  * R 302  − V BE     I T  * (R 302  − 2 * R 304 )       &gt;0   &lt;0   &gt;0   VDD − I T  * R 304  − V BE     VDD − 2 * I T  * R 302  − V BE     I T  * (2 * R 302  − R 304 )       &gt;0   &gt;0   &lt;0   VDD − I T  * R 304  − V BE     VDD − 2 * I T  * R 302  − V BE     I T  * (2 * R 302  − R 304 )       &gt;0   &gt;0   &gt;0   VDD − V BE     VDD − 3 * I T  * R 302  − V BE     3 * I T  * R 302                      
Using the above mentioned equations, values for resistor  302  and resistor  304  can be tabulated where an appropriate tail current (I T ) is assumed. For example, where the tail current is set at fifteen microamperes, values of 4 kOhm for resistor  302  and 12 kOhm for resistor  304  satisfy the equations of Table 5 above to yield a logical AND function. Table 6 below demonstrates this by solving each of the equations of Table 5 using the aforementioned values:
 
         [0000]                                          TABLE 6                   Solved Equations Using Exemplary Values                V INPUT 332  −   V INPUT 336  −   V INPUT 440  −               V INPUT 334     V INPUT 338     V INPUT 442     V YEF  − V YEFZ                         &lt;0   &lt;0   &lt;0   −540 mV           &lt;0   &lt;0   &gt;0   −300 mV           &lt;0   &lt;0   &lt;0   −300 mV           &lt;0   &gt;0   &gt;0    −60 mV           &gt;0   &lt;0   &lt;0   −300 mV           &gt;0   &lt;0   &gt;0    −60 mV           &gt;0   &lt;0   &lt;0    −60 mV           &gt;0   &gt;0   &gt;0   +180 mV                        
As can be seen, the conditions for an AND gate are satisfied where V YEF −V YEFZ  is only greater than zero where all of V INPUT 332 −V INPUT 334 &gt;0, V INPUT 336 −V INPUT 338 &gt;0 and V INPUT 440 −V INPUT 442 &gt;0 are true. It should be noted that other combinations of values for resistor  302 , resistor  304  and I T  may be used to create an AND gate in accordance with embodiments of the present invention.
 
         [0035]    In operation when an AND gate is created as set forth above, when input  332  is asserted high relative to input  334 , input  336  is asserted high relative to input  338 , and input  440  is asserted high relative to input  442 , output YEF  384  is asserted high relative to output YEFZ  386 . In this case, transistor  306 , transistor  310  and transistor  414  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320 , the tail current (I T2 ) sourced by transistor  322 , and the tail current (I T3 ) sourced by transistor  426  all traverse resistor  302 , and no current traverses resistor  304 . Thus, the voltage at the base of emitter follower transistor  331  [VDD−(I T1 +I T2 +I T3 )*R 302 ] is lower than the voltage at the base of emitter follower transistor  329  [VDD], and YEF  384  is at a higher voltage than YEFZ  386 . 
         [0036]    In contrast, when all of input  332  is asserted low relative to input  334 , input  336  is asserted low relative to input  338 , and input  440  is asserted low relative to input  442 , output YEF  384  is asserted low relative to output YEFZ  386 . In this case, transistor  308 , transistor  312  and transistor  416  are turned on. In such a condition, the tail current (I T1 ) sourced by transistor  320 , the tail current (I T2 ) sourced by transistor  322 , and the tail current (I T3 ) sourced by transistor  426  all traverse resistor  304 , and no current traverses resistor  302 . Thus, the voltage at the base of emitter follower transistor  331  [VDD] is higher than the voltage at the base of emitter follower transistor  329  [VDD−(I T1 +I T2 +I T3 )*R 304 ], and YEFZ  386  is at a higher voltage than YEFZ  384 . 
         [0037]    In all other conditions, output YEF  384  is asserted low relative to output YEFZ  386 . In this case, one or two of transistor  306 , transistor  310  and transistor  414  are turned on, while one or two of transistor  308 , transistor  312  and transistor  416  are turned off. In such a condition, one or two of the tail currents (I T1 , I T2 , I T3 ) traverse resistor  302  and one or two of the tail currents (I T1 , I T2 , I T3 ) traverses resistor  304 . Thus, where it is assumed that each of the tail currents are equal, the voltage at the base of emitter follower transistor  331  [VDD−I T1 *R 302 , or VDD−2*I T1 *R 302 ] is higher than the voltage at the base of emitter follower transistor  329  [VDD−I T1 *R 304 , or VDD−2*I T1 *R 304 ], as the value of resistor  302  is less than half the value of resistor  304 , and YEFZ  386  is at a higher voltage than YEFZ  384 . The same condition occurs in the opposite condition where input  332  is asserted low relative to input  334  and input  336  is asserted high relative to input  338 . 
         [0038]    At this point, it should be noted that an OR gate can be similarly created by appropriately selecting values for resistor  302 , resistor  304  and the tail current as before, but to satisfy the conditions of an OR gate similar to that discussed above in relation to  FIG. 3 . 
         [0039]    Based on the disclosure provided herein, one of ordinary skill in the art will appreciate a variety of advantages that may be had through implementing logic using the architecture of the present invention. For example, using the present architecture, stacked devices are eliminated which enabled operation from lower supply voltages and the resulting power reductions. In addition, a purely differential logic family may be developed, as opposed to a single ended emitter coupled logic family that trades off noise margins. Yet further, more complex logic functions may be implemented at a given supply voltage. For example, a four or more input device may be implemented which may not be possible using the same supply voltage level in a stacked architecture. Yet further, the input common-mode range is increased because the transistors are not stacked. As such, a designer need only maintain saturation in the tail devices. Each differential pair exhibits increased common-mode range compared to having any cascoded devices between the differential pair and a resistive load. Yet further, all input common-modes are decoupled from one another, thus there is no need to level shift outputs up or down to accommodate downstream logic. In addition, the typical speed for a given level of current consumption can be greater since fewer level shifters equals less loading. Also, for some cases of fully differential ECL designs, emitter followers cannot be used which dramatically slows down operation. One or more of the aforementioned advantages, or other unlisted advantages may be had in accordance with one or more embodiments of the present invention. 
         [0040]    While only a two input AND/OR gate and a three input AND/OR gate have been presented herein, based on the disclosure provided herein, one of ordinary skill in the art will recognize a variety of other gate types that may be implemented consistent with the architecture disclosed herein, and in accordance with various embodiments of the present invention. As an example, the architecture set forth herein may also be used for, but is not limited to, creating differential NAND and NOR gates. 
         [0041]    In conclusion, the present invention provides novel systems, devices, methods for implementing and using parallel emitter coupled logic. While detailed descriptions of one or more embodiments of the invention have been given above, various alternatives, modifications, and equivalents will be apparent to those skilled in the art without varying from the spirit of the invention. Therefore, the above description should not be taken as limiting the scope of the invention, which is defined by the appended claims.

Technology Category: h