Abstract:
Apparatus and methods greatly improve AC and, optionally, DC power supply rejection in a switching amplifier. The method broadly includes the steps of modulating the amplifier output with a compensatory signal necessary to maintain a minimum difference between an otherwise unmodulated ancillary reference switching amplifier and a static or dynamic reference.

Description:
REFERENCE TO RELATED APPLICATION 
   This application claims priority to U.S. Provisional Patent Application Ser. No. 60/369,411, filed Apr. 2, 2002, the entire content of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to switching amplifiers and, in particular, to a power supply rejection technique therefore. 
   BACKGROUND OF THE INVENTION 
   The measured ability of any amplifier to isolate incoming power supply noise and disturbances from the load has obvious sonic and economic benefits. To meet this objective, analog amplifiers employ many techniques, such as heavily-filtered bias supplies and differential drive. 
   Switching amplifiers, particularly those with saturated output switching devices connected directly to the incoming power supply, have proven more intractable to improvements in this area; in that the supply is switched directly to the load. A need exists for a technique through which power supply rejection of incoming disturbances can be improved in switching amplifiers. 
   SUMMARY OF THE INVENTION 
   The present invention provides apparatus and methods for greatly improving AC and, optionally, DC power supply rejection in a switching amplifier. The method broadly includes modulating the amplifier output with a compensatory signal necessary to maintain a minimum difference between an otherwise unmodulated ancillary reference switching amplifier and a static or dynamic reference. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a preferred embodiment of the present invention employing analog feedback; and 
       FIG. 2  shows a preferred embodiment of the present invention employing digital feedback. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a preferred embodiment of the present invention employing analog feedback. In this circuit, a voltage controlled oscillator  101  supplies a pulse train  103  to both a fixed-period one-shot  102  and a pulse-width modulator  111 , the frequency of which is directly proportional to incoming control voltage  119 . The output of one shot  102  directly drives switching device  105 . 
   When activated, switching device  105  presents zero volts to resistor  106 ; resistor  104  presents V+ volts to resistor  106  when switching device  105  is inactivated. Resistor  106 , in conjunction with capacitor  107 , filters the resultant pulse-width-modulated voltage  120  of resistor  104  and switching device  105  for input to the non-inverting input of error amplifier  109 . Note that the filtered pulse-width-modulated voltage  120  is directly proportional to supply voltage V+ and inversely proportional to the pulse train  103  output from VCO  101 . Resultantly, voltage  120  is inversely proportional to control voltage  119 . 
   Error amplifier  109 , the output of which controls voltage-controlled oscillator  101  via control voltage  119 , compares feedback voltage  120  to reference voltage  108 . This foregoing feedback loop closure results in a trigger pulse train  103  frequency necessary to maintain the average product of V+ and one shot  102  period at reference voltage  108 . This condition can be seen to be stable with all V+ variances both slower than the response time of the resistor  106  and capacitor  107  filter, and within system compliance. 
   Pulse-width modulator  111  receives modulation input  110  and trigger pulse train  103  as input, processing the information into control pulse width trains  112  and  113  to control switching devices  114  and  115 , respectively. It is assumed that pulse-width modulator  111  is comprised primarily of down-counters clocked by the incoming pulse stream  103  and initiated by sample data imbedded in data stream  110 , in a manner well-known in the art. It is further assumed that the down-counter form of pulse-width modulation will result in decreased control pulse widths to switching device  114  (with resultant decreased V+ pulse output) with increasing frequency of pulse stream  103 . 
   Inductor  116 , in conjunction with capacitor  117 , filters pulse widths obtained from switching devices  114  and  115  into an average modulated voltage across load  118 . Specific pulse-width modulation techniques abound in the art and are not important to this discussion, within the constraint that modulator  111  produces eventual output across load  119  that is inversely proportional to the frequency of trigger pulse train  103 , and preferably directly proportional to modulating input  110  as well. 
   By the discussion above, it can be seen that the eventual output expressed across load  118  is the product of V+, modulating input  110  (subject to linearity of pulse-width modulator  111 ), and the period of trigger pulse train  103 . In that the frequency of trigger pulse train  103  has been shown to be that required to maintain an average pulse width/V+ product  120  approaching that of reference voltage  108 , V+ variances can be seen to be negated by the frequency of trigger pulse train  103 . In that the same V+ is used for both feedback voltage  120  and the eventual output to load  118 , it can therefore be seen that the frequency control of trigger pulse train  103  as well cancels V+ variances at load  118 , within system compliance. 
   Note that use of a fixed absolute voltage for reference  108  will result in fixed-power amplifier operation (similar to that of feedback-controlled analog amplifiers known in the art), and that use of a dynamic reference  108  that is a function of the incoming power supply V+ will result in amplification with a variable output power proportional to the incoming power supply V+. With either dynamic or static reference  108 , the technique disclosed herein has been shown to successfully reject even large power supply v+ perturbations extremely well. 
   Referring now to  FIG. 2 , multiplier  202  multiplies constant  201  by a corrective term  213  to provide the controlling input for pulse width modulator  203 . Pulse-width modulator  203  yields a pulse width which is inverted by inverter  204  and applied as control to switching device  205 . Switching device  205 , resistors  206  and  207 , capacitor  208 , reference  210 , and error amplifier  209  all provide equivalent function to their counterparts switching device  105 , resistors  104  and  106 , capacitor  107 , reference  108 , and error amplifier  109 , all of  FIG. 1 . Integrator  212  increases or decreases the correction term  213 , under control of error amplifier  209 . 
   Corrective term  213  is applied as well to incoming data stream  223 , through the action of multiplier  214 , whose output is applied as control of pulse width modulator  215 . Control signals  216  and  217 , switching devices  218  and  219 , inductor  220 , capacitor  221 , and load  222  all provide equivalent function to their counterparts control signals  112  and  113 , switching devices  114  and  115 , inductor  116 , capacitor  117 , and load  118 , respectively, all of  FIG. 1 . Although the eventual output applied to load  222  is directly proportional to corrective term  213 , the eventual output applied to load  118  of  FIG. 1  is inversely proportional to control voltage  119 . Equivalent functionality exists between the embodiments represented in  FIGS. 1 and 2 . 
   While pulse-width modulation, frequency modulation, and digital multiplicative scaling are shown herein, equally efficacious alternative corrective approaches and other modulation techniques are as well anticipated, as are various output stage topologies, including impedance transformation amplifiers, wherein output switching devices do not necessarily connect directly to the incoming power supply.