Abstract:
A method and attendant circuitry reduces the number of regulatory and switching devices in a multi-reference switching amplifier. In the preferred embodiment, multiple independently-modulated effective references are summed at a load through use of both linear and switched control of switching devices.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Patent Application Ser. No. 60/406,207, filed Aug. 27, 2002, the entire content of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   This invention relates generally to multi-reference switching amplifiers and, in particular, to a simplified output topology associated with such amplifiers. 
   BACKGROUND OF THE INVENTION 
   Multi-reference switching amplifiers of the type shown, for example, in PCT application PCT/US99/26691, entitled “Multi-Reference High Accuracy Switching Apparatus,” yield significantly higher instantaneous resolution than standard switching amplifiers. The cost for this performance improvement, however, resides in an additional regulatory device and one or two switching devices (for non-bridged or bridged configurations, respectively) per reference added. 
   Particularly in cost-sensitive applications, there remains a need for a simplified output topology that retains the function and resolution inherent in multi-reference switching amplifiers. 
   SUMMARY OF THE INVENTION 
   The present invention resides in a method and attendant circuitry for reducing the number of regulatory and switching devices in a multi-reference switching amplifier. In the preferred embodiment, multiple independently-modulated effective references are summed at a load through use of both linear and switched control of switching devices. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 1 , switching devices  125 ,  126 ,  127 , and  128  form a bridged output known in the art as an “H” bridge. Inductors  129  and  130 , in conjunction with capacitor  131 , filter switching alias products from the load  132 . Note that in this case only four output switching devices are used. 
   Data separator  101  isolates coarse data  102  and fine data  103  from incoming data stream  100 . These data streams  102  and  103  are presented as inputs to pulsewidth modulators  104  and  105 , which proportionally convert said coarse data  102  and fine data  103  into modulated coarse pulse stream  133  and fine pulse stream  134 , respectively. If the sign  106  of the incoming data stream  100  is high, as indicated by data separator  101 , switching device  125  is modulated by the coarse pulsewidth stream  133 , through AND gate  107 . While the sign  106  is high, transmission gate  109  is activated, forcing the control input of switching device  126  to follow the complement of coarse pulse width stream  133 , as inverted by inverter  121 . Resistor  119  serves to limit output current of differential amplifier  111 . 
   Conversely, if the indicated sign  106  of the incoming data stream  110  is low; switching devices  127  and  128  are modulated by the coarse pulsewidth stream  133  (through AND gate  108 ) and its complement (through transmission gate  110  and inverter  121 ), respectively. Resistor  120  serves to limit output current of differential amplifier  112 . Coarse modulation in this fashion operates exactly as shown in the multi-reference application referenced above. 
   A second reference voltage, proportional to the power supply voltage V+, is formed by the resistor divider  123 / 124 , and input to differential amplifiers  111  and  112 . When not disturbed by transmission gate  109 , switching device  125 , or diode  115 , differential amplifier  111  outputs a voltage to cause the output of switching device  126  to equal the reference voltage formed by resistors  123  and  124 . When the indicated sign  106  is low, NOR gate  113  turns on diode  115  with the inverse (from inverter  135 ) of the fine pulsewidth stream from pulsewidth modulator  105 , forcing switching device  126  to turn on, through the resultant output increase of differential amplifier  111 . This results in switching at the output of switching device  126  between ground and the reference voltage formed by resistors  123  and  124 , inversely modulated by fine-resolution  103  provided to pulse width modulator  105 . 
   When not disturbed by transmission gate  110 , switching device  127 , or diode  116 , differential amplifier  112  outputs a voltage to cause the output of switching device  128  to equal the reference voltage formed by resistors  123  and  124 . When the indicated sign  106  is high, NOR gate  114  turns on diode  116  with the inverse (from inverter  135 ) of the fine pulse width stream from pulse width modulator  105 , forcing switching device  128  to turn on, through the resultant output increase of differential amplifier  112 . This results in switching at the output of switching device  128  between ground and the reference voltage formed by resistors  123  and  124 , inversely modulated by fine-resolution  103  provided to pulse width modulator  105 . 
   In the discussion above, coarse-resolution data  102  is used to modulate V+ on one side of load  132 , while fine-resolution data  103  is used to modulate the reference voltage formed by resistors  123  and  124  on the other side of load  132 , under control of data sign  106 . Although summation at the load of multiple references, modulated by appropriate resolutions, directly follows the technique disclosed in the multi-reference application referenced above, note that this is accomplished by the present invention with significantly fewer output switching devices.