Abstract:
A buffer has an amplifier that receives an external signal, a reference voltage, and outputs an amplified signal. The amplified signal is responsive to the difference between the external signal and the reference voltage. An inverter receives the amplified signal and generates an inverted signal. A voltage supply circuit is configured to provide an adjusted power supply voltage to the inverter responsive to the reference voltage. A ground voltage supply circuit is configured to provide an adjusted ground voltage to the inverter responsive to the reference voltage.

Description:
RELATED APPLICATION  
         [0001]    This application claims the benefit of Korean Patent Application No. 2002-25625, filed May 9, 2002, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates to input/output buffers, and more particularly, to differential type input/output buffers and related methods.  
           [0004]    2. Description of the Related Art  
           [0005]    A semiconductor memory device typically includes various circuits. One such circuit: is an input/output buffer. FIG. 1 is a block diagram of a conventional differential type input/output buffer. Referring to FIG. 1, the conventional differential type input/output buffer comprises a differential amplification portion  111  and an inverting portion  121 .  
           [0006]    A reference voltage Vref and an external signal IN are applied to the differential amplification portion  111 . The external signal IN is converted into a complementary metal oxide semiconductor (CMOS) level voltage by the differential amplification portion  111 , inverted by the inverting portion  121 , and output as Vout.  
           [0007]    The reference voltage Vref applied to the differential amplification portion  111  may vary as a result of external factors, for example noise. As a result, the common mode of the differential amplification portion  111  can vary. If the common mode of the differential amplification portion  111  varies, a relatively large amount of skew  221  and  231  may occur in the output signal Vout of the inverting portion  111  as shown in FIG. 2.  
           [0008]    In other words, if the reference voltage Vref increases above a reference value, for example, 1.25 volts, the rising time of the output signal of the differential amplification portion  111  may become slow and the falling time of the output signal may become fast. Accordingly, the falling time of the output signal Vout of the inverting portion  121  may become slower ( 222 ) than the reference signal  211 , and the rising time of the output signal may become faster ( 221 ) than the reference signal  211 .  
           [0009]    On the other hand, if the reference voltage Vref decreases below the reference value, the rising time of the output signal of the differential amplification portion  111  may become fast and the falling time of the output signal may become slow. Accordingly, the falling time of the output signal Vout of the inverting portion  121  may become faster ( 232 ) than that of the reference signal  211 , and the rising time of the output signal may become slower ( 231 ) than that of the reference signal  211 . Accordingly, a large amount of skew may occur in the output signal Vout of the inverting portion  121  as shown in FIG. 2.  
           [0010]    As described above, a large amount of skew may occur in the output signal Vout of the inverting portion  121  as the reference voltage Vref applied to the amplifier  111  is changed. Semiconductor devices equipped with the input/output buffer can malfunction due to skew.  
         SUMMARY OF THE INVENTION  
         [0011]    According to embodiments of the present invention, a buffer may have an amplifier that receives an external signal, a reference voltage, and outputs an amplified signal. The amplified signal is responsive to the difference between the external signal and the reference voltage. An inverter receives the amplified signal and outputs an inverted signal. A voltage supply circuit is configured to provide an adjusted power supply voltage to the inverter responsive to the reference voltage. A ground voltage supply circuit, is configured to provide an adjusted ground voltage to the inverter responsive to the reference voltage.  
           [0012]    In this configuration, the power supply voltage and the ground voltage provided to the inverter can be adjusted responsive to the reference voltage so that if a reference voltage increases or decreases above or below the reference value, the resulting skew at the output signal of the input/output buffer may be decreased.  
           [0013]    In other embodiments according to the invention, a method for reducing skew in a buffer includes receiving a power supply voltage, a ground voltage, and external signal and a reference voltage. An output signal is output responsive to the difference between the external signal and the reference voltage. The output signal is inverted responsive to the reference voltage.  
           [0014]    According to certain embodiments of the invention, a differential type input/output buffer may have a differential amplification portion which receives an external signal and a reference voltage, amplifies the external signal and outputs the amplified signal. An inverting portion inverts the output of the differential amplification portion and outputs the inverted signal. A power source provides a power supply voltage to the inverting portion, and varies the quantity of electric charge provided to the inverting portion in response to the reference voltage. A ground voltage supply portion provides a ground voltage to the inverting portion, and varies the quantity of electric charge provided to the inverting portion in response to the reference voltage.  
           [0015]    In other embodiments, a differential type input/output buffer may have a differential amplification portion which receives an external signal and a reference voltage, amplifies the external signal, and outputs the amplified signal. A pull-up portion receives the reference voltage and a power supply voltage, and outputs an output signal having the power supply voltage level in response to the reference voltage when the output signal of the differential amplification portion is logic low. A pull-down portion receives the reference voltage and a ground voltage, and outputs an output signal of the ground voltage level in response to the reference voltage when the output signal of the differential amplification portion is logic high.  
           [0016]    In further embodiments, a differential type input/output buffer may have a differential amplification portion which receives an external signal and a reference voltage, amplifies the external signal, and outputs the amplified signal. An inverting portion inverts the output of the differential amplification portion and outputs the inverted signal. A power supply voltage supply portion receives the reference voltage, and delays the time for an output signal of the inverting portion to be transited from logic low to logic high if the reference voltage increases above a reference value when an output of the differential amplification portion is transited from logic high to logic low, and increases the time for an output signal of the inverting portion to be transited from logic low to logic high if the reference voltage decreases below the reference value.  
           [0017]    In other embodiments, a differential type input/output buffer may have a differential amplification portion which receives an external signal and a reference voltage, amplifies the external signal, and outputs the amplified signal. An inverting portion inverts the output of the differential amplification portion and outputs the inverted signal. A power supply voltage supply portion receives the reference voltage, and increases the time for an output signal of the inverting portion to be transited from logic high to logic low if the reference voltage increases above a reference value when an output of the differential amplification portion is transited from logic low to logic high, and delays the time for an output signal of the inverting portion to be transited from logic high to logic low if the reference voltage decreases below the reference value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram of a conventional differential type input/output buffer;  
         [0019]    [0019]FIG. 2 is a graph of an output signal of an inverting portion and the resulting skew when the reference voltage shown in FIG. 1 is varied;  
         [0020]    [0020]FIG. 3 is a block diagram of a buffer according to embodiments of the present invention;  
         [0021]    [0021]FIG. 4 is a circuit diagram of input/output buffers according to embodiments of the present invention;  
         [0022]    [0022]FIG. 5 is a graph illustrating the state where the reference voltage shown in FIGS. 3 and 4 is varied;  
         [0023]    [0023]FIG. 6 is a graph of the output signal of a differential amplification portion when the reference voltage shown in FIGS. 3 and 4 is varied;  
         [0024]    [0024]FIG. 7 is graph of the output signal of the buffer shown in FIGS. 3 and 4; and  
         [0025]    [0025]FIG. 8 is a block diagram of a buffer according to further embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which various embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It will be understood that when an element is referred to as being “coupled” or “connected” to another element, it can be directly coupled or connected to the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly coupled” or “directly connected” to another elemtn, there are no intervening elements present. Like reference numerals in the drawings denote like members.  
         [0027]    [0027]FIG. 3 is a block diagram of a differential type input/output buffer according to embodiments the present invention. Referring to FIG. 3, the differential type input/output buffer  301  includes a differential amplification portion  311 , an inverting portion  321 , a power supply voltage supply portion  331  and a ground voltage supply portion  341 .  
         [0028]    An external signal IN and a reference voltage Vref are input to the differential amplification portion  311 , and the differential amplification portion  311  amplifies the external signal IN and outputs VOUT 1 . That is, if the external signal IN is higher than the reference voltage Vref, the differential amplification portion  311  outputs a ground voltage Vss level signal VOUT 1 , and if the external signal IN is lower than the reference voltage Vref, the differential amplification portion  311  outputs a power supply voltage Vdd level signal VOUT 1 . For example, if the input/output buffer  301  is used as an input buffer, the external signal IN can have a voltage of a transistor transistor logic level (TTL) or a stub series terminated logic (SSTL) level. The power supply voltage Vdd may be a CMOS level voltage. Therefore, in some embodiments, an external signal IN having the TTL or SSTL level is input into the differential amplification portion  311  and is converted into a CMOS level signal and output VOUT 1 .  
         [0029]    The reference signal  611  as shown in FIG. 6 is a signal output from the differential amplification portion  311  when the reference voltage Vref is equal to the reference value  511  as shown in FIG. 5. If the reference voltage Vref increases above the reference value Va, for example, 1.25 volts, the differential amplification portion  311  may output an output signal VOUT 1  ( 631  in FIG. 6) faster than a reference signal ( 611  in FIG. 6). That is, the transition from logic high to logic low of the output signal VOUT 1  ( 631  in FIG. 6) may occur sooner than the reference output signal  611 , and the slope is greater. If the reference voltage Vref decreases below the reference value Va (in FIG. 5), the differential amplification portion  311  may output an output signal VOUT 1  ( 621  in FIG. 6) slower than the reference signal ( 611  in FIG. 6). That is, the transition from logic high to logic low of the output signal VOUT 1  ( 621  in FIG. 6) may occur later than the reference signal  611 .  
         [0030]    The inverting portion  321  inverts the output of the differential amplification portion  311  and outputs it as an output signal VOUT 2  of the input/output buffer  301 . The power source  331  supplies the power supply voltage Vdd to the inverting portion  321 . The power source  331  changes the quantity of electric charge of the power supply voltage Vdd supplied to the inverting portion  321  in response to the reference voltage Vref. That is, if the reference voltage Vref increases above the reference value (Va in FIG. 5), the quantity of electric charge of the power supply voltage Vdd provided to the inverting portion  321  decreases. On the other hand, if the reference voltage Vref decreases below the reference value (Va in FIG. 5), the quantity of electric charge of the power supply voltage Vdd supplied to the inverting portion  321  increases.  
         [0031]    The ground voltage supply portion  341  provides a ground voltage Vss to the inverting portion  321 . The ground voltage supply portion  341  changes the quantity of electric charge of the ground voltage Vss provided to the inverting portion  321  in response to the reference voltage Vref. That is, if the reference voltage Vref increases above the reference value (Va in FIG. 5), the quantity of electric charge of the ground voltage Vss provided to the inverting portion  321  increases. On the other hand, if the reference voltage Vref decreases below the reference value (Va in FIG. 5), the quantity of electric charge of the ground voltage Vss provided to the inverting portion  321  decreases. In other words, if the reference voltage Vref increases above the reference value (Va in FIG. 5), the amount of the current flowing to the ground terminal increases. If the reference voltage Vref decreases below the reference value (Va in FIG. 5), the quantity of electric charge flowing to the ground terminal decreases.  
         [0032]    [0032]FIG. 4 is a circuit diagram of the input/output buffer  301  illustrated in FIG. 3.  
         [0033]    The differential amplification portion  311  includes NMOS transistors NM 1  and NM 2 , a current mirror  411 , and a PMOS transistor PM 1 . A reference voltage Vref and an external signal IN are input to NMOS transistors NM 1  and NM 2 , respectively. When a control signal P 1  input to a gate of the PMOS transistor PM 1  is logic low, the PMOS transistor PM 1  becomes active and provides a power supply voltage Vdd to the current mirror  411 . The PMOS transistor PM 1  may be omitted from the differential amplification portion  311 . The differential amplification portion  311  may also include a current source (not shown) connected between the node N 1  and the ground voltage Vss. The current mirror  411  includes PMOS transistors PM 2  and PM 3 .  
         [0034]    The inverting portion  321  includes a PMOS transistor PM 4  and an NMOS transistor NM 3 . An output signal VOUT 1  of the differential amplification portion  311 , which is input to a node N 2 , is inverted and output-as an output signal VOUT 2  of the input/output buffer  301  from a node N 3 .  
         [0035]    The power supply voltage supply portion  331  includes PMOS transistors PM 5  and PM 6 . The reference voltage Vref is applied to the gate of the PMOS transistor PM 5 , and the gate of the PMOS transistor PM 6  is grounded. The PMOS transistor PM 6  is maintained to be active. Accordingly, if the reference voltage Vref increases above the reference value (Va in FIG. 5), the gate-source voltage Vgs of the PMOS transistor PM 5  decreases and the quantity of electric charge provided to the inverting portion  321  decreases. On the other hand, if the reference voltage Vref decreases below the reference value (Va in FIG. 5), the gate-source voltage Vgs of the PMOS transistor PM 5  increases and the quantity of electric charge provided to the inverting portion  321  increases.  
         [0036]    An increasing Vref results in an increasing resistance through transistor PM 5  and a reducing current from Vdd to inverter  321 . A decreasing Vref results in a decreasing resistance through transistor PM 5  and an increasing current from Vdd to inverter  321 .  
         [0037]    The ground voltage supply portion  341  comprises NMOS transistors NM 4  and NM 5 . A reference voltage Vref is applied to the gate of the NMOS transistor NM 4 , and a power supply voltage Vdd is applied to the gate of the NMOS transistor NM 5 . The NMOS transistor NM 5  is maintained to be active. Accordingly, if the reference voltage Vref increases above the reference value (Va in FIG. 5), the gate-source voltage Vgs of the NMOS transistor NM 4  increases, and the quantity of electric charge flowing from the inverting portion  321  to the ground terminal increases. On the other hand, if the reference voltage Vref decreases below the reference value (Va in FIG. 5), the gate-source voltage Vgs of the NMOS transistor NM 4  decreases and the quantity of electric charge flowing from the inverting portion  321  to the ground terminal decreases. FIG. 5 shows the reference voltabe Vref ( 521 ) increasing above the reference value Va and the reference voltage Vref ( 531 ) decreasing below the reference value Va. Referring to FIG. 5, the reference value is not constant but varies.  
         [0038]    An increasing Vref results in a decreasing resistance through transistor NM 4  and an increasing current from inverter  321  to ground. A decreasing Vref results in an increasing resistance through transistor NM 4  and a decreasing current from inverter  321  to ground.  
         [0039]    [0039]FIG. 7 shows the output signal of the input/output buffer  301  shown in FIGS. 3 and 4. The reference signal  711  refers to the signal VOUT 2  output from the input/output buffer  301  when the reference voltage Vref is equal to the reference value (Va in FIG. 5). When the reference voltage Vref increases above the reference value (Va in FIG. 5), the output signal  721  of the input/output buffer  301  has reduced skew relative to the reference signal  711 . When the reference voltage Vref decreases below the reference value (Va in FIG. 5), the output signal  731  of the input/output buffer  301  likewise has reduced skew relative to the reference signal  711 . Thus, skew may be decreased compared to the prior art and compared to the output of the amplification portion ( 311  FIGS. 3 and 4). For example, when the reference voltage Vref is varied, the skew of the output signal Vout of the conventional input/output buffer  101  can range from −107 to +77. However, the skew of an output signal from a buffer according to embodiments of the present invention such as VOUT 2  of the input/output buffer  301  may be reduced to the range of −21 to +22. That is, according to certain embodiments of the present invention, the skew may be improved by 75%.  
         [0040]    The operation of the input/output buffer  301  shown in FIGS. 3 and 4 will now be described with reference to FIG. 7.  
         [0041]    First, a case where a reference voltage Vref is higher than a reference value (Va in FIG. 5) will be described. If the external signal IN is lower than the reference voltage Vref, the output signal VOUT 1  of the differential amplification portion  311  is logic high and the output signal VOUT 2  of the inverting portion  321  is logic low.  
         [0042]    In this state, if the external signal IN increases above the reference voltage Vref, the output signal VOUT 1  of the differential amplification portion  311  transitions from logic high to logic low. However, the reference voltage Vref is higher than the reference value (Va in FIG. 5). Thus, the output signal VOUT 1  of the differential amplification portion  311  may transition relatively quickly ( 631 ) as shown in FIG. 6. As the output signal VOUT 1  of the differential amplification portion  311  transitions to logic low, the PMOS transistor PM 4  of the inverting portion  321  becomes active and the output signal VOUT 2  of the inverting portion  321  is transferred from logic low to logic high.  
         [0043]    In such a case, because the reference voltage Vref is higher than the reference value (Va in FIG. 5), the gate-source voltage Vgs of the PMOS transistor PM 5  decreases and the quantity of electric charge output from the power source  331  decreases. Accordingly, the time for the output signal VOUT 2  of the inverting portion  321  to transition from logic low to logic high is delayed and the resulting output signal  721  may be close to the reference signal  711 .  
         [0044]    In this state, if the external signal IN decreases below the reference voltage Vref, the output signal VOUT 1  of the differential amplification portion  311  transitions from logic low to logic high. However, the reference voltage Vref is higher than the reference value (Va in FIG. 5). Thus, the output signal VOUT 1  of the differential amplification portion  311  transistions relatively slowly ( 632 ) as shown in FIG. 6. As the output signal VOUT 1  of the differential amplification portion  311  transitions to logic high, the NMOS transistor NM 3  of the inverting portion  321  becomes active and the output signal VOUT 2  of the input/output buffer  301  transitions from logic high to logic low.  
         [0045]    Because the reference voltage Vref is higher than the reference value (Va in FIG. 5), the gate-source voltage Vgs of the NMOS transistor NM 4  increases and the quantity of electric charge output from the inverting portion  321  to the ground terminal increases. Accordingly, the time for the output signal VOUT 2  of the inverting portion  321  to transition from logic high to logic low becomes fast and the output signal  722  closes to the reference signal  711 .  
         [0046]    Second, a case where a reference voltage Vref decreases below a reference value (Va in FIG. 5) will be described. If the external signal IN is lower than the reference voltage Vref, the output signal VOUT 2  of the differential amplification portion  321  is logic high and the output signal VOUT 2  of the inverting portion  321  is logic low.  
         [0047]    In this state, if the external signal IN increases above the reference voltage Vref, the output signal VOUT 1  of the differential amplification portion  311  transitions from logic high to logic low. However, since the reference voltage Vref is lower than the reference value (Va in FIG. 5), the output signal VOUT 1  of the differential amplification portion  311  may transition relatively slowly ( 621 ) as shown in FIG. 6. As the output signal VOUT 1  of the differential amplification portion  311  transitions to logic low, the PMOS transistor PM 5  of the inverting portion  321  becomes active and the output signal VOUT 2  of the inverting portion  321  transitions from logic low to logic high.  
         [0048]    Because the reference voltage Vref is lower than the reference value (Va in FIG. 5), the gate-source voltage Vgs of the PMOS transistor PM 5  increases and the quantity of electric charge output from the power supply voltage supply portion  331  increases. Accordingly, the time for the output signal VOUT 2  of the inverting portion  321  to be transited from logic low to logic high becomes fast and the resulting output signal  731  may have reduced skew relative to the reference signal  711 .  
         [0049]    In this state, if the external signal IN decreases below the reference voltage Vref, the output signal VOUT 1  of the differential amplification portion  311  transitions from logic low to logic high. However, the reference voltage Vref is lower than the reference value (Va in FIG. 5). Thus, the output signal VOUT 1  of the differential amplification portion  311  is transferred relatively quickly ( 622 ) as shown in FIG. 6. As the output signal VOUT 1  of the differential amplification portion  311  transitions to logic high, the NMOS transistor NM 3  of the inverting portion  321  becomes active and the output signal VOUT 2  of the input/output buffer  301  transitions from logic high to logic low.  
         [0050]    Because the reference voltage Vref is lower than the reference value (Va in FIG. 5), the gate-source voltage Vgs of the NMOS transistor NM 4  decreases and the quantity of electric charge output from the inverting portion  321  to the ground terminal decreases. Accordingly, the time for the output signal VOUT 2  of the inverting portion  321  to transition from logic high to logic low is delayed and the output signal  732  may have reduced skew relative to the reference signal  711 .  
         [0051]    As described above, despite variation in the reference voltage Vref, the skew of the output signal VOUT 2  of the input/output buffer  301  may be reduced as shown in FIG. 7.  
         [0052]    [0052]FIG. 8 is a circuit diagram of a differential type input/output buffer according to further embodiments of the present invention. Referring to FIG. 8, the differential type input/output buffer  801  includes a differential amplification portion  811 , a pull-up portion  821 , and a pull-down portion  831 .  
         [0053]    The differential amplification portion  811  receives an external signal IN and a reference voltage Vref, amplifies the external signal IN and outputs it. That is, if the external signal IN is higher than the reference voltage Vref, the differential amplification portion  811  outputs a ground voltage level signal, and if the external signal IN is lower than the reference voltage Vref, the differential amplification portion  811  outputs a power supply voltage level signal. For example, if the input/output buffer  801  of the present invention is used as an input buffer, the external signal IN may have a voltage of a transistor transistor logic level or a stub series terminated logic level. The power supply voltage may be a CMOS level voltage. In such a case, the TTL or SSTL level external signal IN input to the differential amplification portion  311  can be converted into a CMOS level and output.  
         [0054]    If the reference voltage Vref increases above a reference value  511  as shown in FIG. 5, the differential amplification portion  811  outputs signals  631  and  622  faster than the reference signal  611  as shown in FIG. 6. If the reference voltage Vref decreases below the reference value  511 , the differential amplification portion  811  outputs signals  621  and  632  slower than the reference signal  611 . The differential amplification portion  811  includes a configuration of the differential amplification portion  311  in FIG. 4.  
         [0055]    The pull-up portion  821  receives an output signal of the differential amplification portion  811  and a reference voltage Vref. The pull-up portion  821  outputs a power supply voltage level signal Vdd as an output signal VOUT of the input/output buffer  801 , if the output signal is logic low.  
         [0056]    If the external signal IN becomes higher than the reference voltage Vref in the state where the reference voltage Vref is higher than the reference value (Va in FIG. 5), the output signal of the differential amplification portion  811  transitions from logic high to logic low slower than the reference signal  611 . The output signal VOUT of the pull-up portion  821  transitions from logic low to logic high slower than the reference signal  711 . Here, the pull-up portion  821  causes the output signal  731  to transition so that it may approach the reference signal  711  and so that it may have reduced skew as shown in FIG. 7.  
         [0057]    If the external signal IN becomes higher than the reference voltage Vref in the state where the reference voltage Vref is lower than the reference value (Va in FIG. 5), the output signal of the differential amplification portion  811  may transition from logic high to logic low faster than the reference signal  611 . The output signal VOUT of the pull-up portion  821  can transition from logic low to logic high faster than the reference signal  711 . Therefore, the pull-up portion  821  causes the output signal  721  to transition so that it may approach the reference signal  711  and so that it may have reduced skew as shown in FIG. 7. The pull-up portion  821  includes the power supply voltage supply portion  331  and the PMOS transistor PM 4  in FIG. 4.  
         [0058]    The pull-down portion  831  receives an output signal of the differential amplification portion  811  and a reference voltage Vref. The pull-up portion  831  outputs a ground voltage level signal Vss as an output signal VOUT of the input/output buffer  801 , if the output signal of the differential amplification portion  811  is logic high.  
         [0059]    If the external signal IN becomes lower than the reference voltage Vref in the state where the reference voltage Vref is lower than the reference value (Va in FIG. 5), the output signal of the differential amplification portion  811  may transition from logic low to logic high slower than the reference signal  611 . The output signal VOUT of the pull-down portion  831  can transition from logic high to logic low faster than the reference signal  711 . Therefore, the pull-down portion  831  causes the output signal  722  to transition so that it may approach the reference signal  711  and so that it may have reduced skew as shown in FIG. 7. The pull-down portion  831  includes the ground voltage supply portion  341  and the NMOS transistor NM 3  in FIG. 4.  
         [0060]    As described above, even though the reference voltage Vref may vary, the skew in the output signal of the input/output buffer  801  may be reduced as shown in FIG. 7.  
         [0061]    According to embodiments of the present invention, even though the reference voltage Vref increases/decreases above/below the reference value  511 , skew in the output signal of the input/output buffers  301  and  801  may be reduced as shown in FIG. 7. That is, according to certain embodiments of the invention, the skew may be reduced by 75% compared to the conventional input/output buffer  101 . If the skew is decreased, malfunctions in a semiconductor device equipped with the input/output buffers  301  and  801  may be reduced  
         [0062]    While this invention has been particularly shown and described with reference to preferred embodiments thereof, the preferred embodiments described above are merely illustrative and are not intended to limit the scope of the invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.