Abstract:
A digital signal power amplification apparatus with multiple digital amplification cells connected in series, each amplification cell processing a separate bit of the digital signal. The apparatus additively combines the output from each amplifier into a single amplified signal without the use of separate signal combining circuitry. The apparatus has high linearity, high efficiency, high bandwidth and high power.

Description:
FIELD OF THE INVENTION 
     The present invention is directed generally toward signal amplification; specifically amplification of a digital signal. 
     BACKGROUND OF THE INVENTION 
     The challenge of radio frequency (RF) power amplifiers is to combine high efficiency, high bandwidth, high linearity, and high power. One potential architecture solution is the power digital-analog-converter (PowerDAC). The PowerDAC concept utilizes a digital data stream directly applied to the control of switching-power-amplifiers in a manner analogous to that of more conventional DAC circuits. In frequency ranges of multiple decades (VHF and UHF), and in a dynamic range fourteen bits or greater, conventional power amplifier technology and combiner techniques are inadequate. 
     The class-D power amplifier can operate efficiently through multiple decades of bandwidth. The class-D amplifier is a switching amplifier; therefore the voltage amplitude of its output is determined directly by its power supply voltage. Class-D amplifiers can suffer from high output resistance. OSISE is an optically-coupled, isolated, gate-drive circuit permitting fabrication of broadband class-D power amplifiers with high-side, floating-source switches, OSISE circuits are a significant development because they allow fabrication of broad-band totem pole, class-D half-bridges. Totem-pole architecture Class-D amplifiers are capable of producing RF power without output baluns or transformers. 
     Separate PowerDAC circuits may amplify individual bit streams. However, it is difficult to combine the output from separate PowerDACs. Separate signals require separate combining circuitry, which may require filtering and de-coupling. Combining analog conversions of digital signals is ineffective unless the amplification voltage is precise for each progressively less significant bit. Where PowerDAC circuits are separate, there may be no direct relation between the amplification voltages for each bit. 
     Consequently, it would be advantageous if an apparatus existed that is suitable for amplifying a digital signal and converting the digital signal to an analog signal without the use of separate combining circuitry. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a novel apparatus for amplifying a digital signal and converting the digital signal to an analog signal without the use of separate combining circuitry. The apparatus combines high efficiency, high bandwidth, high linearity and high power. 
     An apparatus according to the present invention may have a plurality of bit amplifying cells. Each bit amplifying cell amplifies a single bit of the digital signal. Each amplifying cell receives a digital control signal, as well as a precisely-determined supply voltage, and produces as its output, two separate output voltages, with a differential voltage one half the differential voltage of that cell&#39;s supply voltage. Each amplifying cell is connected in series to a subsequent amplifying cell so that the output voltages of one amplifying cell become the supply voltages of the next amplifying cell. 
     Each bit amplifying cell may have capacitors arranged in series between the upper and lower inputs of its supply voltage. The common node of the capacitors may have a common node reference voltage one-half the average of that cell&#39;s differential supply voltage. The capacitors de-couple the bit amplifying cells. 
     Each bit amplifying cell may have two sets of transistors, each set of transistors arranged in a manner similar to a class-D amplifier. One set of transistors may have that cell&#39;s positive supply voltage and the common node reference voltage as amplification voltages. The other set of transistors may have the common node reference voltage and that cell&#39;s negative supply voltage as amplification voltages. The output of each set of transistors may become the differential supply voltages for the next bit amplifying cell. 
     The apparatus may also contain an amplifier termination cell connected in series to the chain of bit amplifying cells. The termination cell may correspond to the Least significant bit (LSB) of the system. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1  shows a diagram of a circuit according to one embodiment of the present invention; 
         FIG. 2  shows a diagram of a single bit amplifying circuit according to one embodiment of the present invention; 
         FIG. 3  shows a diagram of a amplifier termination circuit according to one embodiment of the present invention; 
         FIG. 4  shows a table of possible node voltages for a second bit amplifying cell based on possible bit states of a first and second bits; 
         FIG. 5  shows a table of possible node voltages for a third bit amplifying cell based on possible bit states of a first, second and third bit; 
         FIG. 6  shows a table of possible analog output voltages for an amplifier termination cell according to the present invention; 
         FIG. 7  shows a flowchart of another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. The scope of the invention is limited only by the claims; numerous alternatives, modifications and equivalents are encompassed. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description. 
     Referring to  FIG. 1 , one embodiment of a power digital-analog-converter (DAC)  100  according to the present invention may comprise one or more bit amplifying cells  102 ,  158 ,  160  connected in series. The Power DAC  100  may have an amplifier termination cell  104  connected in series to the series of bit amplifying cells  102 ,  158 ,  160 . Each bit amplifying cell  102 ,  158 ,  160  amplifies a single bit in a digital signal. Each bit may be represented by an appropriate rectangular waveform. A single bit amplifying cell is shown in  FIG. 2 , 
     Generalized Bit Amplifying Cell 
     Referring to  FIG. 2 , a single bit amplifying cell  200  is shown. The bit amplifying cell  200  may have a high bit transistor  206 , a low bit transistor  212 , a high bit complimentary transistor  210 , and a low bit complimentary transistor  208 . The bit amplifying cell  200  may also have a high voltage control capacitor  214  and a low voltage control capacitor  216  connected to each other in series. 
     The high voltage control capacitor may connect to a high input node  218  and an intermediate node  222 . The to voltage control capacitor  216  may connect to the intermediate node  222  and a low input node  220 . The high voltage control capacitor  214  and low voltage control capacitor  216  work to de-couple the bit amplifying cell  200  from any previous bit amplifying cells in the operating range of a circuit containing the bit amplifying cell  200 . A circuit containing the bit amplifying cell,  200  may operate in the radio frequency range or any frequency range wherein the switching speed of the transistors is adequate. The high voltage control capacitor  214  and low voltage control capacitor  216  in each bit amplifying cell  200  perform switched-capacitor power conversion to create a floating voltage source at the intermediate node  222 , of an amplitude half that of the supply preceding it. 
     The source of the high bit transistor  206  may connect to the high input node  218  and the drain may connect to a high output node  224 . The source of the high bit complimentary transistor  210  may connect to the intermediate node  222  and the drain may connect to the high output node  224 . The source of the low bit complimentary transistor  208  may connect to the low input node  220  and the drain may connect to a low output node  226 . The source of the low bit transistor  212  may connect to the intermediate node  222  and the drain may connect to the low output node  226 . 
     A bit may drive the high bit transistor  206  and the low bit transistor  212  through isolated drive circuitry connected to the gate of each transistor  206 ,  212 . A complimentary first bit may drive the high bit complimentary transistor  210  and the low bit complimentary transistor  208  through isolated drive circuitry connected to the gate of each transistor  208 ,  210 . 
     Two bit amplifying cells  200  may be connected in series by connecting the high output node  224  of a primary bit amplifying cell to the high input node  218  of a secondary bit amplifying cell, and connecting the low output node  226  of the primary bit amplifying cell to the low input node  220  of the secondary bit amplifying 
     First Bit Amplifying Cell 
     Referring to  FIG. 1  and  FIG. 2 ,  FIG. 1  shows a first bit amplifying cell  102 , a second bit amplifying cell  158 , a third bit amplifying cell  160  and an amplifier termination cell  104  all connected in series. Each bit amplifying cell  102 ,  158 ,  160  is an implementation of the generalized bit amplifying cell  200 , amplifying successively Less significant bits in a digital signal. 
     The high input node  218  of the first bit amplifying cell  102  may connect to an amplifying voltage source  156 . The amplifying voltage source  156  supplies a voltage that represents the maximum available signal amplification voltage (V max ). The low input node  220  of the first bit amplifying cell  102  may be tied to ground or some other minimum amplification voltage (V min ). In this disclosure, V min  is assumed to be zero for simplicity; however it will be apparent to one skilled in the art that V min  need not be zero. In this configuration, the differential voltage between the high input node  218  and the low input node  220  of the first bit amplifying cell  102  is V max . The first bit amplifying cell  102  may have an intermediate node  222  defined by the common node of a high first bit voltage control capacitor  114  and a low first bit voltage control capacitor  116  connected in series between the high input node  218  and the low input node  220  of the first bit amplifying cell  102 . The differential voltage between the high input node  218  and the intermediate node  222  of the first bit amplifying cell  102  is ½ V max . 
     A rectangular wave of sufficient amplitude to drive a transistor into a high state represents a first bit in a digital signal. The first bit connects to the gate of the high first bit transistor  106  and the gate of the low first bit transistor  112  through appropriate isolated drive circuitry. A complimentary rectangular wave of sufficient amplitude represents a complimentary first bit. The complimentary first bit is the logical opposite of the first bit; whenever the first bit is in a high state, the complimentary first bit is in a low state, and whenever the first bit is in a low state, the complimentary first bit is in a high state. The complimentary first bit connects to the gate of the high first bit complimentary transistor  110  and the gate of the low first bit complementary transistor  108  through appropriate isolated drive circuitry. 
     Whenever the first bit is in a high voltage state the high first bit transistor  106  and the low first bit transistor  112  are driven into a high state. Furthermore, whenever the first bit is in a high voltage state, the complimentary first bit is in a low voltage state. The complimentary first bit being in a low voltage state, the high first bit complimentary transistor  110  and the low first bit complimentary transistor  108  are driven into a low state. 
     In this embodiment, when the first bit is in a high voltage state, current may flow through the high first bit transistor  106  and the low first bit transistor  112 . Current may not flow through the high first bit complimentary transistor  110  or the low first bit complimentary transistor  108 . Therefore, the high output node  224  would be at the same voltage as the high input node  218 , or V max . Meanwhile, the low output node  226  would be at the same voltage as the intermediate node  222 , or ½ V max . 
     On the other hand, if the first bit is in a low voltage state the high first bit transistor  106  and the low first bit transistor  112  are driven into a to state. When the first bit is in a low voltage state, the complimentary first bit is in a high voltage state. When the complimentary first bit is in a high voltage state, the high first bit complimentary transistor  110  and the low first bit complimentary transistor  108  are driven into a high state. 
     In this embodiment, when the first bit is in a low voltage state, current may flow through the high first bit complimentary transistor  110  and the low first bit complimentary transistor  108 . Current may not flow through the high first bit transistor  106  or the low first bit transistor  112 . Therefore, the high output node  224  would be at the same voltage as the intermediate node  222 , or ½ V max . Meanwhile, the low output node  226  would be at the same voltage as the low input node  220 , or V min . 
     The first bit amplifying cell  102  may generate two possible sets of output voltages based on a first bit of digital input. When the first bit is high, the high output node  224  is at voltage V max  while the low output node  226  is at voltage ½ V max . When the first bit is low, high output node  224  is at voltage ½ V max  while the low output node  226  is at voltage V min . In both cases, the differential voltage between the high output node  224  and the low output node  226  is ½ V max . 
     Second it Amplifying Cell 
     The first bit amplifying cell  102  connects to the second bit amplifying cell  158  in series such that the high output node  224  of the first bit amplifying cell  102  shares a common node, and therefore a common voltage with the high input node  218  of the second bit amplifying cell  158 . Furthermore, the low output node  226  of the first bit amplifying cell  102  shares a common node, and therefore a common voltage with the low input node  220  of the second bit amplifying cell  158 . As with the first bit amplifying cell  102 , the second bit amplifying cell  158  may have an intermediate node  222  defined by the common node of a high second bit voltage control capacitor  126  and a low second bit voltage control capacitor  128  connected in series between the high input node  218  and the low input node  220  of the second bit amplifying cell  158 . The intermediate node  222  of the second bit amplifying cell may be at a voltage half-way between the voltage at the high input node  218  of the second bit amplifying cell  158  and the voltage at the low input node  220  of the second bit amplifying cell  158 . Unlike the voltage at the intermediate node  222  of first bit amplifying cell  120 , the voltage at the intermediate node  222  of the second bit amplifying cell  158  depends on the state of the first bit. If the first bit is high, the high input node  218  of the second bit amplifying cell  158  would be at V max  while the low input node  220  would be at ½ V max . In that case the intermediate node  222  of the second bit amplifying cell  158  would be at ¾ V max . Conversely, if the first bit is low, the high input node  218  of the second bit amplifying cell  158  would be at ½ V max  white the low input node  220  would be at V min . In that case the intermediate node  222  of the second bit amplifying cell  158  would be at ½ V max . In either case, the differential voltage between the intermediate node  222  and the high input node  218  of the second amplifying cell  158  is one-half the differential voltage between the intermediate node  222  and the high input node  218  of the first bit amplifying cell  102 . Likewise, the differential voltage between the intermediate node  222  and the low input node  220  of the second amplifying cell  158  is one-half the differential voltage between the intermediate node  222  and the low input node  220  of the first bit amplifying cell  102 . 
     A rectangular wave of sufficient amplitude to drive a transistor into a high state represents a second bit in the digital signal. The second bit connects to the gate of the high second bit transistor  118  and the gate of the low second bit transistor  124  through appropriate isolated drive circuitry. A complimentary rectangular wave of sufficient amplitude represents a complimentary second bit. The complimentary second bit is the Logical opposite of the second bit; whenever the second bit is in a high state, the complimentary second bit is in a low state, and whenever the second bit is in a low state, the complimentary second bit is in a high state. The complimentary second bit connects to the gate of the high second bit complimentary transistor  122  and the gate of the low second bit complementary transistor  120  through appropriate isolated drive circuitry. 
     Whenever the second bit is in a high voltage state the high second bit transistor  118  and the low second bit transistor  124  are driven into a high state. Furthermore, whenever the second bit is in a high voltage state, the complimentary second bit is in a low voltage state. The complimentary second bit being in a low voltage state, the high second bit complimentary transistor  122  and the low second bit complimentary transistor  120  are driven into a low state. 
     In this embodiment, as with the first bit amplifying cell  102 , when the second bit is in a high voltage state, current may flow through the high second bit transistor  118  and the to second bit transistor  124 . Current may not flow through the high second bit complimentary transistor  122  or the low second bit complimentary transistor  120 . Therefore, the high output node  224  would be at the same voltage as the high input node  218  while the low output node  226  would be at the same voltage as the intermediate node  222 . Unlike the first bit amplifying cell  102 , the voltage at the high output node  224  and low output node  226  of the second bit amplifying cell  158  depend on the state of the first bit as well as the second bit because the first bit determines the voltage at the high input node  218  and low input node  220  of the second bit amplifying cell  158 . 
     If the second bit is in a low voltage state the high second bit transistor  118  and the low second bit transistor  124  are driven into a low state. When the second bit is in a low voltage state, the complimentary second bit is in a high voltage state. When the complimentary second bit is in a high voltage state, the high second bit complimentary transistor  122  and the low second bit complimentary transistor  120  are driven into a high state. 
     In this embodiment, again as with the first bit amplifying cell  102 , when the second bit is in a low voltage state, current may flow through the high second bit complimentary transistor  122  and the low second bit complimentary transistor  120 . Current may not flow through the high second bit transistor  118  or the low second bit transistor  124 . Therefore, the high output node  224  would be at the same voltage as the intermediate node  222  while the low output node  226  would be at the same voltage as the low input node  220 . Again, the voltage at the high output node  224  and low output node  226  of the second bit amplifying cell  158  depend on the state of the first bit as well as the second bit because the first bit determines the voltage at the high input node  218  and low input node  220  of the second bit amplifying cell  158 . 
     The second bit amplifying cell  158  may generate four possible sets of output voltages based on a second bit of digital input, and the state of a prior first bit amplifying cell  102 . The table below ( FIG. 4 ) shows possible states for the high output node  224  and the low output node  226  of the second bit amplifying cell  158 . 
                                                             First Low           First High   First High    First Low    Second            Second High   Second Low   Second High   Low                   High Output Node   V max     ¾ V max     ½ V max     ¼ V max         Low Output Node   ¾ V max     ½ V max     ¼ V max     V min                      
Third Bit Amplifying Cell
 
     The second bit amplifying cell  158  connects to the third bit amplifying cell  160  in series such that the high output node  224  of the second bit amplifying cell  158  shares a common node, and therefore a common voltage with the high input node  218  of the third bit amplifying cell  160 . Furthermore, the low output node  226  of the second bit amplifying cell  158  shares a common node, and therefore a common voltage with the Low input node  220  of the third bit amplifying cell  160 . As with the first bit amplifying cell  102  and second bit amplifying cell  158 , the third bit amplifying cell  160  may have an intermediate node  222  defined by the common node of a high third bit voltage control capacitor  138  and a low third bit voltage control capacitor  140  connected in series between the high input node  218  and the low input node  220  of the third bit amplifying cell  160 . The intermediate node  222  of the third bit amplifying cell may be at a voltage half-way between the voltage at the high input node  218  of the third bit amplifying cell,  160  and the voltage at the to input node  220  of the third bit amplifying cell  160 . The voltage at the intermediate node  222  of the third bit amplifying cell  160  depends on the state of the first bit and the second bit. The voltage at the intermediate node  222  of the third amplifying cell as measured from the low input node  220  is half the differential voltage between the low input node  220  and the high input node  218  of the third amplifying cell  160  for each first and second bit state shown in the table above and in  FIG. 4 . 
     A rectangular wave of sufficient amplitude to drive a transistor into a high state represents a third bit in the digital signal. The third bit connects to the gate of the high third bit transistor  130  and the gate of the low third bit transistor  136  through appropriate isolated drive circuitry. A complimentary rectangular wave of sufficient amplitude represents a complimentary third bit. The complimentary third bit is the logical opposite of the third bit; whenever the third bit is in a high state, the complimentary third bit is in a low state, and whenever the third bit is in a to state, the complimentary third bit is in a high state. The complimentary third bit connects to the gate of the high third bit complimentary transistor  134  and the gate of the low third bit complementary transistor  132  through appropriate isolated drive circuitry. 
     Whenever the third bit is in a high voltage state the high third bit transistor  130  and the low third bit transistor  136  are driven into a high state. Furthermore, whenever the third bit is in a high voltage state, the complimentary third bit is in a to voltage state. The complimentary third bit being in a low voltage state, the high third bit complimentary transistor  134  and the low third bit complimentary transistor  132  are driven into a low state. 
     In this embodiment, as with the first and second bit amplifying cells  102 ,  158 , when the third bit is in a high voltage state, current may flow through the high third bit transistor  130  and the low third bit transistor  136 . Current may not flow through the high third bit complimentary transistor  134  or the low third bit complimentary transistor  132 . Therefore, the high output node  224  would be at the same voltage as the high input node  218  while the low output node  226  would be at the same voltage as the intermediate node  222 . The voltage at the high output node  224  and low output node  226  of the third bit amplifying cell  160  depend on the state of the first and second bits as well as the third bit because the first and second bits determines the voltage at the high input node  218  and low input node  220  of the third bit amplifying cell,  160 . 
     If the third bit is in a low voltage state the high third bit transistor  130  and the low third bit transistor  136  are driven into a low state. When the third bit is in a low voltage state, the complimentary third bit is in a high voltage state. When the complimentary third bit is in a high voltage state, the high third bit complimentary transistor  134  and the low third bit complimentary transistor  132  are driven into a high state. 
     In this embodiment, again as with the first and second bit amplifying cells  102 ,  158 , when the third bit is in a low voltage state, current may flow through the high third bit complimentary transistor  134  and the low third bit complimentary transistor  132 . Current may not flow through the high third bit transistor  130  or the low third bit transistor  136 . Therefore, the high output node  224  would be at the same voltage as the intermediate node  222  while the low output node  226  would be at the same voltage as the low input node  220 . Again, the voltage at the high output node  224  and low output node  226  of the third bit amplifying cell  160  depend on the state of the first and second bits as well as the third bit because the first and second bits determine the voltage at the high input node  218  and low input node  220  of the third bit amplifying cell  160 . 
     The third bit amplifying cell  160  may generate eight possible sets of output voltages based on a third bit of digital input, and the state of prior first and second bit amplifying cells  158 . The table below ( FIG. 5 ) shows possible states for the high output node  224  and the low output node  226  of the third bit amplifying cell  160 . 
     
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                   
                   
                   
                 First High 
               
               
                   
                 First High 
                 First High 
                 First High 
                 Second  
               
               
                   
                 Second High 
                 Second High 
                 Second Low 
                 Low 
               
               
                   
                 Third High  
                 Third Low 
                 Third High 
                 Third Low 
               
               
                   
               
               
                 High Output Node 
                 V max   
                 ⅞ V max   
                 ¾ V max   
                 ⅝ V max   
               
               
                 Low Output Node 
                 ⅞ V max   
                 ¾ V max   
                 ⅜ V max   
                 ½ V max   
               
               
                   
               
               
                   
                   
                   
                   
                 First Low 
               
               
                   
                 First Low 
                 First Low 
                 First Low 
                 Second  
               
               
                   
                 Second High 
                 Second High 
                 Second Low 
                 Low 
               
               
                   
                 Third High 
                 Third Low 
                 Third High 
                 Third Low 
               
               
                   
               
               
                 High Output Node 
                 ½ V max   
                 ⅜ V max   
                 ¼ V max   
                 ⅛ V max   
               
               
                 Low Output Node 
                 ⅜ V max   
                 ¼ V max   
                 ⅛ V max   
                 V min   
               
               
                   
               
             
          
         
       
     
     Each bit amplifying cell  102 ,  158 ,  160  operates at a differential input voltage one-half the differential input voltage of the previous bit amplifying cell. That is to say each bit amplifying cell  102 ,  158 ,  160  operates in a voltage range one order of magnitude less on a binary scale than the previous bit amplifying cell. Therefore, each bit amplifying cell  102 ,  158 ,  160  modifies the output voltage of the series of bit amplifying cells by a factor appropriate to convert a binary signal to an analog signal without any additional signal combining circuitry. 
     Amplifier Termination Cell 
     The series of bit amplifying cells  102 ,  158 ,  160  connects to an amplifier termination cell  104 . The amplifier termination cell  104  resolves the voltage difference between the high output node  224  and the low output node  226  of the final bit amplifying cell, in this case the third bit amplifying cell  160 . The amplifier termination cell  104  also incorporates the final two least significant bits (LSB) in the digital signal. 
     Referring to  FIG. 3 , an amplifier termination cell  104  according to the present invention may have a fourth bit voltage control capacitor  150 , a fifth bit voltage control capacitor  152 , a fourth bit transistor  142 , a fourth bit complimentary transistor  144 , a fifth bit transistor  146  and a fifth bit complimentary transistor  148 . The fourth bit voltage control capacitor  150  may connect at one terminal to a high LSB input node  300  and at the other terminal to a low LSB input node  302 . The source of the fourth bit transistor  142  may connect to the high LSB input node  300  and the drain of the fourth bit transistor  142  may connect to a high LSB output node  304 . The source of the fourth bit complimentary transistor  144  may connect to the low LSB input node  302  while the drain of the fourth bit complimentary transistor  114  may connect to a low LSB output node  306 . The fifth bit voltage control capacitor  152  may connect at one terminal to the high LSB output node  304  and at the other terminal to the low LSB output node  306 . The source of the fifth bit transistor  146  may connect to the high LSB output node  304  while the drain of the fifth bit transistor may connect to the analog output  154 . The source of the fifth bit complimentary transistor  148  may connect to the low LSB output  306  while the drain of the fifth bit complimentary transistor  148  may connect to the analog output  154 . 
     The amplifier termination cell  104  resolves the two least significant bits in the digital signal into a single voltage with three potential values; +LSB, 0 or −LSB. The actual voltage value of LSB is dependent on the number of preceding bit amplifying cells  102 ,  158 ,  160  wherein each preceding bit amplifying cell reduces the voltage value of LSB by one-half and the amplifier termination cell  104  also reduces the voltage value of LSB by one-half, in the present embodiment, LSB would represent a voltage change of 1/16 V max  in the final analog output  154 . 
     A rectangular wave of sufficient amplitude to drive a transistor into a high state represents a fourth bit in the digital signal. The fourth bit connects to the gate of the fourth bit transistor  142  through appropriate isolated drive circuitry. A complimentary rectangular wave of sufficient amplitude represents a complimentary fourth bit. The complimentary fourth bit is the logical opposite of the fourth bit; whenever the fourth bit is in a high state, the complimentary fourth bit is in a low state, and whenever the fourth bit is in a low state, the complimentary fourth bit is in a high state. The complimentary fourth bit connects to the gate of the fourth bit complimentary transistor  144  through appropriate isolated drive circuitry. 
     A rectangular wave of sufficient amplitude to drive a transistor into a high state represents a fifth bit in the digital signal. The fifth bit connects to the gate of the fifth bit transistor  146  through appropriate isolated drive circuitry. A complimentary rectangular wave of sufficient amplitude represents a complimentary fifth bit. The complimentary fifth bit is the logical opposite of the fifth bit; whenever the fifth bit is in a high state, the complimentary fifth bit is in a low state, and whenever the fifth bit is in a low state, the complimentary fifth bit is in a high state. The complimentary fifth bit connects to the gate of the fifth bit complimentary transistor  148  through appropriate isolated drive circuitry. 
     Whenever the fourth bit is in a high voltage state the fourth bit transistor  142  is driven into a high state. Furthermore, whenever the fourth bit is in a high voltage state, the complimentary fourth bit is in a low voltage state. The complimentary fourth bit being in a low voltage state, the fourth bit complimentary transistor  144  is driven into a low state. In this embodiment, the voltage at the high LSB output  304  would be equal to the voltage at the high LSB input  300 , and the voltage at the low LSB output  306  would be equal to the voltage at the high LSB output  304  minus a voltage drop equal to the voltage value of one LSB due to voltage stored in the fifth voltage control capacitor  152 . In this case, if the fifth bit is in a high voltage state, the fifth bit transistor  146  is driven high while the fifth bit complimentary transistor  148  is driven low. Current may flow through the fifth bit transistor  146 , and the voltage at the analog output will be equal to the voltage at the high LSB output  304 . If the fifth bit is in a low voltage state, the fifth bit transistor  146  is driven low while the fifth bit complimentary transistor  148  is driven high. Current may flow through the fifth bit complimentary transistor  148 , and the voltage at the analog output will be equal to the voltage at the low LSB output  306 . 
     Whenever the fourth bit is in a low voltage state the fourth bit transistor  142  is driven into a low state. Furthermore, whenever the fourth bit is in a low voltage state, the complimentary fourth bit is in a high voltage state. The complimentary fourth bit being in a high voltage state, the fourth bit complimentary transistor  144  is driven into a high state. In this embodiment, the voltage at the low LSB output  306  would be equal to the voltage at the low LSB input  302 , and the voltage at the high LSB output  304  would be equal to the voltage at the low LSB output  306  plus a voltage gain equal to the voltage value of one LSB due to voltage stored in the fifth voltage control capacitor  152 . In this case, if the fifth bit is in a high voltage state, the fifth bit transistor  146  is driven high while the fifth bit complimentary transistor  148  is driven low. Current may flow through the fifth bit transistor  146 , and the voltage at the analog output will be equal to the voltage at the high LSB output  304 . If the fifth bit is in a low voltage state, the fifth bit transistor  146  is driven low while the fifth bit complimentary transistor  148  is driven high. Current may flow through the fifth bit complimentary transistor  148 , and the voltage at the analog output will be equal to the voltage at the low LSB output  306 . 
     The amplifier termination cell  104  may generate three distinct output voltages for any differential voltage between the high LSB input  300  and the low LSB input  302  based on four possible bit states of the fourth and fifth bits. One output voltage may be equal the voltage at the high LSB input  300 , one voltage may be equal to the voltage at the low LSB input  302 , and one voltage may be equal to one-half the voltage as measure from the high LSB input  300  to the low LSB input  302 . The table below show possible states for the analog output  154  where V high  is the voltage at the high LSB input  300  and V low  is the voltage at the low LSB input  302 . The actual voltages at the high LSB input  300  and the low LSB input  302  are dependent on the number of preceding bit amplifying cells  102 ,  158 ,  160  connected in series to the amplifier termination cell  104 , and on the digital values of each bit driving each bit amplifying cell  102 ,  158 ,  160 . In any case, the analog output  154  may pass through a band pass filter to remove undesirable frequencies. 
                                                 Fourth High   Fourth High   Fourth Low   Fourth Low           Fifth High   Fifth Low   Fifth High   Fifth Low                   Analog Output   V high     ½ (V high  +    1½ (V high  +    V low                 V low )   V low )                    
Method of Using the Apparatus
 
     Referring to  FIG. 7 , in another embodiment  700  of the present invention, an apparatus similar to that disclosed above amplifies individual bits from a digital stream, and converts the amplified bits into a single analog output. The apparatus performs  702  switched capacitor power conversion to create a floating voltage at the common node of two capacitors connected in series. The floating voltage may be one-half the voltage differential at the remaining nodes of the two capacitors. The apparatus amplifies  704  a first bit from the digital stream. In amplifying  704  the first bit, the apparatus establishes  706  two output voltages. The output voltages may vary in magnitude depending on the value of the first bit; however, the output voltages may always have a voltage differential one-half the voltage differential at the non-common nodes of the two capacitors. The apparatus may then perform  708  switched capacitor power conversion to create a floating voltage at the common node of two different capacitors connected in series. The floating voltage may be one-half the voltage differential of the two output voltages. The apparatus amplifies  710  a second bit from the digital stream. The apparatus produces a voltage based on the value of the first bit, the second bit, and a reference voltage. The apparatus may then resolve  712  two least significant bits into one least significant bit voltage that may have one of three possible values based on the values of the two least significant bits and some reference voltage. The apparatus may then resolve  714  the voltage based on the value of the first bit and the value of the second bit, and the least significant bit voltage to produce a single analog output voltage. 
     It is believed that the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.