System and method for linearizing power amplifiers

Abstract

A power encoder includes a pulse width modulator for modulating a signal according to a set of thresholds to produce a pulse width modulated (PWM) signal and a switch mode power amplifier for amplifying the PWM signal by switching states of switching devices according to amplitudes of the PWM signal. At least one or combination of a distribution of values of the voltage thresholds in the set and a distribution of values of a current generated by different switching devices are non-uniform. The set of voltage thresholds includes at least two positive voltage thresholds.

We claim: 
     
       1. A power encoder, comprising:
 a pulse width modulator for modulating a signal according to a set of thresholds including at least five voltage thresholds to produce a pulse width modulated (PWM) signal having at least five levels; and 
 a switch mode power amplifier for amplifying the PWM signal by switching states of switching devices according to amplitudes of the PWM signal, wherein at least one or combination of a distribution of values of the voltage thresholds in the set and a distribution of values of a current generated by different switching devices are non-uniform, wherein in the non-uniform distribution of values of the voltage thresholds a distance between values of a first pair of neighboring voltage thresholds does not equal a distance between values of a second pair of neighboring voltage thresholds; 
 wherein the set of threshold includes a first voltage thresholds V 1  for a first level of the PWM signal and a second voltage threshold V 2  for a second level of the PWM signal, wherein a ratio av=V 1 /V 2  is between 0.3 and 0.4 inclusively; and 
 wherein the power amplifier includes a first switching device for generating a current with a first value I 1 , such that the power amplifier produces I 1  current for the first level of the PWM signal, 
 wherein the power amplifier includes a second switching device for generating a current with a second value I 2 , such that the power amplifier produces I 1 +I 2  current for the second level of the PWM signal, wherein a ratio ai=I 1 /(I 1 +I 2 ) is between 0.2 and 0.4 inclusively. 
 
     
     
       2. The power encoder of  claim 1 , wherein the set of thresholds includes a first voltage threshold and a second voltage threshold, wherein a ratio of the first voltage threshold and the second voltage threshold is between 0.3 and 0.4. 
     
     
       3. The power encoder of  claim 2 , wherein the ratio is 0.35. 
     
     
       4. The power encoder of  claim 1 , wherein the switching devices of the power amplifier include transistors having different dimensions to produce different currents. 
     
     
       5. The power encoder of  claim 1 , wherein the PWM signal has (2n+1) levels, n is a positive natural number, wherein the set of thresholds includes n non-zero values of voltage thresholds, and wherein the power amplifier includes 2n switching devices with non-uniform total current capability for the n non-zero levels of the PWM signal. 
     
     
       6. The power encoder of  claim 1 , wherein the PWM signal has (2n+1) levels, n is a positive natural number, wherein the set of thresholds includes n non-zero values V n  of the voltage thresholds, and wherein the power amplifier includes 2n switching devices for the n non-zero levels of the PWM signal to produce I n  current for each level, wherein a ratio a v =V n-1 /V n  and a ratio a i =I n-1 /(I n-1 +I n ) is between 0.2 and 0.4 inclusively. 
     
     
       7. The power encoder of  claim 1 , further comprising:
 an amplitude-phase splitter for splitting an input signal into an envelope signal and a phase modulated signal; 
 a pre-distortion unit for distorting the envelope signal using a look-up table (LUT) to produce a distorted envelope signal, wherein the look-up table stores a non-linear mapping of a transformation function; 
 a digital converter for combining the distorted envelope signal with the phase modulated signal to produce a distorted input signal, wherein the signal is the distorted input signal, such that the modulator modulates the distorted input signal according to a transformation function with the set of thresholds to produce the PWM signal, wherein a relationship between the distorted input signal and the PWM signal is non-linear. 
 
     
     
       8. The power encoder of  claim 7 , wherein the modulator is a radio frequency (RF) modulator, and the digital converter up-converts the distorted envelope signal and the phase modulated signal with an RF carrier signal, such that the PWM signal is an RF pulse train. 
     
     
       9. The power encoder of  claim 7 , wherein the modulator is an intermediate frequency (IF) modulator, and the digital converter up-converts the distorted envelope signal and the phase modulated signal with an IF carrier signal, such that the PWM signal is an IF pulse train. 
     
     
       10. The power encoder of  claim 9 , further comprising:
 a second digital up-converter for converting the IF pulse train to a radio frequency (RF) pulse train and for submitting the RF pulse train to the switch mode power amplifier. 
 
     
     
       11. The power encoder of  claim 7 , wherein a value of each threshold is based on a probability density function (PDF) of a portion of the input signal. 
     
     
       12. The power encoder of  claim 11 , further comprising
 a memory for storing a frame of the distorted input signal; 
 a processor for determining the PDF from data in the frame, for integrating the PDF to generate a curve of a cumulative distribution function (CDF), and for selecting values of each fixed threshold based on the curve. 
 
     
     
       13. The power encoder of  claim 12 , wherein the processor determines, for each frame of the distorted input signal, the set of fixed thresholds, the transformation function, and the LUT. 
     
     
       14. A method for power encoding, comprising:
 modulating a signal according to a set of thresholds including at least two positive voltage thresholds to produce a pulse width modulated (PWM) signal having at least five levels; and 
 amplifying the PWM signal by switching states of switching devices according to amplitudes of the PWM signal, wherein at least one or combination of a distribution of values of the voltage thresholds in the set and a distribution of values of a current generated by different switching devices are non-uniform, wherein in the non-uniform distribution of values of the voltage thresholds a distance between values of a first pair of neighboring voltage thresholds does not equal a distance between values of a second pair of neighboring voltage thresholds; 
 wherein the set of threshold includes a first voltage thresholds V 1  for a first level of the PWM signal and a second voltage threshold V 2  for a second level of the PWM signal, wherein a ratio av=V 1 /V 2  is between 0.3 and 0.4 inclusively; and 
 wherein the amplifying is performed with a power amplifier including a first switching device for generating a current with a first value I 1 , such that the power amplifier produces I 1  current for the first level of the PWM signal, and 
 second switching device for generating a current with a second value I 2 , such that the power amplifier produces I 1 +I 2  current for the second level of the PWM signal, wherein a ratio ai=I 1 /(I 1 +I 2 ) is between 0.2 and 0.4 inclusively. 
 
     
     
       15. The method of  claim 14 , wherein the set of thresholds includes a first voltage threshold and a second voltage threshold, wherein a ratio of the first voltage threshold and the second voltage threshold is between 0.3 and 0.4. 
     
     
       16. The method of  claim 15 , wherein the ratio is 0.35. 
     
     
       17. The method of  claim 15 , wherein the amplifying is performed with transistors having different dimensions to produce different currents. 
     
     
       18. The method of  claim 14 , wherein the PWM signal is an intermediate frequency (IF) pulse train, further comprising:
 converting the IF pulse train to a radio frequency (RF) pulse train for the amplifying.