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LINEARIZERS  101

LINEARIZERS –DISTORTION REDUCTION IN HIGH POWER AMPLIFIERS

BY: DR. ALLEN KATZ

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OUTLINE

n WHY  LINEARIZEn LIMITATIONS/IDEAL  LIMITERn DECISION  TO  LINEARIZEn TYPES  OF  LINEARIZERSn PREDISTORTION  LINEARIZERSn SOME  RESULTSn MEMORY  EFFECTSn EXTENDING  THE  LIMITS  n CONCLUSIONS

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WHY  LINEARIZE

-­MUST  OPERATE  AMPLIFIER  AT  REDUCEDPOWER  LEVEL    (BACKOFF  FROM  SATURATION)

IR≥BW[1+log2(S/N)]

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ALLOWS  OPERATION  CLOSER  TO  SAT

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CHANGE  IN  PHASE  WITH  LEVEL

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DISTORTION  DUE  AM/AM  &  AM/PM

dn(gain)/d(Pin)n & dn(Ø)/d(Pin)n

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IDEAL  HPA  CHARACTERISTIC

WANT  CONSTANT  GAIN  AND  PHASESETS  A  LIMIT  ON  WHAT  CAN  ACHIEVE

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IDEAL  HPA  CHARACTERISTIC

WANT  CONSTANT  GAIN  AND  PHASEüSETS  A  LIMIT  ON  WHAT  CAN  ACHIEVE

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IDEAL  HPA  CHARACTERISTIC

WANT  CONSTANT  GAIN  AND  PHASEüSETS  A  LIMIT  ON  WHAT  CAN  ACHIEVE

WHY PAR IS SO IMPORTANT

ONCE ABOVE SAT, NOTHING YOU CAN DO

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PAR  LIMITS  IDEAL  PERFORMANCE  

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IDEAL  HPA  CHARACTERISTIC

CREST  FACTOR  REDUCTION  (CFR)

WHY PAR IS SO IMPORTANT

ONCE ABOVE SAT, NOTHING YOU CAN DO

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FIRST  RULE

YOU CAN’T LINEARIZE AN AMPLIFIERTHAT IS ALREADY LINEAR!

WANT  TO  OPTIMIZE  EFFICIENCY  AND  SATURATED  POWER,  NOT  LINEARITY

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DECISION  TO  LINEARIZEn PRIMARILY  ECONOMICn THE  MORE  NONLINEAR  A  DEVICE  IS,  THE  GREATER  THE  ADVANTAGE  OF  LINEARIZATION

n THE  MORE  LINEARITY  REQUIRED,  THE  GREATER  THE  ADVANTAGE  OF  LINEARIZATION    -­ FOR  MANY  APPLICATIONS  C/I  ~ 30  dB  ENOUGHn FOR  MANY  HPAs  [GaN]  CAN  ACHIEVE-­ 3  TO  6  dB  MORE  POWER-­ MORE  THAN  DOUBLING  OF  EFFICIENCY-­ REDUCED  SIZE  AND  WEIGHT

BY  LINEARIZATION

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IMPROVED  EFFICIENCY

TYPICAL  LINEARIZER  EFFICIENCY  IMPROVEMENT

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n LINEARIZERS  ALSO  OFFER  HPAs  REDUCED  THERMAL  LOAD

n SINCE  LIN  COST  IS  ~  FIXED,  THE  BIGGER  THE  HPA,  THE  MORE  ECONOMICAL  IT  BECOMES-­ LINEARIZING  A  10  kW PA  IS  MUCH  MORE  COSTEFFECTIVE  THAN  A  1  W PA

n LINEARIZATION  IS  ALSO  EASIER  TO  JUSTIFY  AT  HIGHER  FREQ  WHERE  POWER  IS  MORE  $$

DECISION  TO  LINEARIZE

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n THREE  COMMON  FORMS:1)  FEEDFORWARD2)  FEEDBACK3)  PREDISTORTION

n +  TECHNIQUES  TO  IMPROVE  EFFICIENCY  USING  NL  PAs

TYPES  OF  LINEARIZATION

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FEEDFORWARD

§ RELATIVELY  COMPLEX  &  LIMITED  EFFECIENCY                                                                                    § NOT  EFFECTIVE  FOR  OPBOs  <  6  dB§ MOST  USEFUL  FOR  VERY  HIGH  LINEARITY  APPS

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FEEDBACK

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FEEDBACK

<  ~  10  MHz  MAX(1  MHz)

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WAYS  TO  IMPROVE  EFF  WITH  NL  PAs

n MANY  WAYS  TO  ACCOMPLISH.n CLASSICAL  “KHAN  METHOD”  DEMODS  ENVELOPE  &  LIMITS  SIGNAL.  THEN  REMODULATES  AT  OUTPUT  PA

n LINC  SYSTEMS  OBTAINS  LINEAR  AMPLIFICATION  BY  COMBINING  TWO  NON-­LINEAR  PAs.

n LOAD  MODULATION  AND  OUTPHASING                                    (DOHERTY – MOST  SUCCESSFUL  EXAMPLE)      

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PREDISTORTION

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DIGITAL  PDL  DOMINANT

(CAN’T  EASILY  COMBINEMULTIPLE  SIGNALS)~ALL  USE  ADAPTIVE  APP.

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DIGITAL  VS.  ANALOG  PDL•  DIGITAL  PDL– IF  DIGITAL  MODULATOR  AVAILABLE  AND  SUFFICIENTLY  NARROW  BAND  – NO  BRAINER!•  ANALOG  PDL– AS  BW  INCREASES  ANALOG  GAINS  THE  ADVANTAGE  AS  DIGITAL’S  COST  AND  POWER  OVERHEAD  INCREASE– HAS  ADVANTAGE  FOR  SIGNAL  BW  >  ~ 25  MHz– SELECTION  DEPENDS  ON  BENEFIT  PROVIDED– FOR  MANY  APPS  PERFORMANCE  SIMILAR– FOR  MULTI  GHz/MULTI  OCTAVE  OPERATION  ANALOG  IS  THE  ONLY  PRACTICAL  OPTION  (DIGITAL  CANNOT  EASILY  ADJ  NL  WITH  FREQ)

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SOME  RESULTS  – GaN  HPAs

n FOR  C/I  =  30  dB,  ALMOST  2.5  dB  MORE  POWER  

n AND  AN  INCREASE  IN  EFFICIENCY  >  60%

n FOR  C/I  =  30  dB,                                              >  7  dB  MORE  POWER  

n AND  AN  INCREASE  IN  EFFICIENCY  >  200%

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SOME  RESULTS  – GaN  HPAs

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ME  ARE  CHANGES  IN  A  HPA’S NONLINEARITY  DUE  TO  PAST  HISTORY  OF  THE  INPUT  SIGNAL

Vo  =  f(Vin,  time)SOURCES  OF  ME

-­ Frequency  ME-­ Drain/collector  ME-­ Gate/base  ME-­ Device  related  ME-­ Thermal  ME

MEMEORY  EFFECTS  (ME)

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GAIN  VS.  INPUT  POWER  IS  AFFECTED  BY  FREQUENCY

PHASE  VS.  INPUT  POWER  IS  AFFECTED  BY  FREQUENCY

n Standard  DPD  look-­up  tables  have  the  same    correction  for  every  frequency

n Real  PA  nonlinearities   do  change  with  frequency

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TWO  KINDS  OF  BANDWIDTH1)  STATIC  BANDWIDTH  -­ ABILITY  OF  LIN  MAG/PHASE  TRANSFER  RESP  TO

EQUALIZE  AMP  AT  ALL  FREQ  OF  INTEREST

2)  DYNAMIC  BANDWIDTH  -­ ABILITY  OF  LIN  MAG/PHASE  TRANSFER  RESP  TO  FOLLOW  ENVELOPE  OF  SIGNALS

-­ MEAS  WITH  2  CLOSE  SPACED  TONES  AT  ALL  FREQ  OF  INTEREST

-­ MEAS  WITH  2-­TONE  SIGNAL  IN  WHICH  THESPACING  OF  THE  TONES  IS  INCREASED

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MAJOR  CAUSE  OF  DEGRADATION  -­-­

ENVELOPE  FREQUENCY Fe =  FΔ/2

TRANSFER  CHARACTERISTICS  CHANGE  WITH  Fe

INABILITY  OF  AMPLIFIERS  TO  FOLLOW  RAPIDLY  CHANGING  ENVELOPE

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IMDS  CAUSED  BY  THE  PA  NONLINEARITY  SUBTRACT  FROM  THE  RIPPLE  INDUCED  IMDS

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A  LOW  IMPEDANCE  NETWORK  AT  ENVELOPE  FREQUENCIES  ACROSS  THE  DRAIN  AND  EFFECTIVE  POWER  SUPPLY  DECOUPLING  CAN  MINIMIZE  ME

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n GAIN  AND  PHASE  (TD)  MUST  BE  MAINTAINED  OVER  FREQUENCY  – CAN  CORRECT  WITH  EQUALIZER  

n GAIN  RIPPLE  A  MAJOR  PROBLEM  – LIMITS  CORRECTION  

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WB  LIMITATIONS

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n LINEARIZER  DESIGNED  FOR  4  TO  18  GHz  GaN  MMIC  SSPA

n USED  2  ACTIVE  FET  NON-­LINEAR  GENERATORS,  COMBINED  USING  180°HYBRID  MAGIC  TEE  BALUNS

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WB  GaN  LINEARIZER

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1  dB  CP  MOVED  >  6  dB  CLOSER  TO  SAT  FROM  6  TO  16  GHz  PHASE  SHIFT  REDUCED  FROM  >  30°TO  <  10  °OVER  BAND  

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WB  GaN  LINEARIZER

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2-­TONE  CARRIER  TO  INTERMOD  (C/I)  IS  A  COMMON  MEASURE  OF  

DISTORTION  REDUCTION  

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WB  GaN  LINEARIZER

At 6, 10, & 16 GHz:C/I INCREASE OF 5-11 dB FOR OPBOs OF 5–8 dB

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FOR  A  C/I  OF  25  dB,  LINEARIZATION                          PROVIDES  ~  5  dB  MORE  POWER

MOVED  P1dB  7  dB  CLOSER  TO  SAT  &  ∆PHASE <5°

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n LINEARIZERS  ESSENTIAL  FOR  BW-­EFFICIENT,  HIGH  DATA  RATE  COMMUNICATIONS

n INCREASE  HPA’s  EFF  POWER  AND  EFFICIENCYn DIGITAL  PDL  DOMINANT  WHEN  HPA  IS  USED  WITH  A  DIGITAL  MODULATOR  AND  SIGNAL  BW NARROW  ENOUGH

n ANALOG  PDL  GAINS  THE  ADVANTAGE  FOR  WIDEBAND  APPLICATIONS

n UNDERSTANDING  LIMITATIONS  ENABLES  BETTER  HPA  DESIGNS

n LINEARIZERS  FOR  MULTI  GHz/OCTAVE AND  TO  >  100  GHz AVAILABLE

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n A.  Katz,  “Linearization:   Reducing  Distortion  in  Power  Amplifiers,”  IEEE  Microwave  Magazine,  pp.  37-­49,  December  2001.

n M.  Franco,  A.  Guida,  A.  Katz,  and  P.  Herczfeld,   “Intermodulation  Distortion  Products  in  Radio  Frequency  Power  Amplifiers  with  Digital  Predistortion  Linearization,”  MTT-­S  International  Microwave  Symposium  Digest,  San  Francisco,  CA,  June  11-­16,  2006.

n A.  Katz,  B.  Eggleston  and  D.  McGee,  “A  Linear  GaN  UHF  SSPA  with  Record  High  Efficiency,”  MTT-­S  International  Microwave  Symposium  Digest,  Boston,  MA,  ,  pp.  769  – 772,  June  7-­12,  2009.

n A.  Katz,  M.  Kubak and  G.  DeSalvo,  “A  6  to  16  GHz  Linearized  GaN  Power  Amplifier,”  MTT-­S  International  Microwave  Symposium  Digest,  San  Francisco,  CA,  pp.  1364-­1367,  June  11-­16,  2006.

n M.  Franco,  and  et  al,  “Minimization  of  Bias-­Induced  Memory  Effects  in  UHF  Radio  Frequency  High  Power  Amplifiers  with  Broadband  Signals,”  2007  IEEE  Radio  and  Wireless  Sym.,  Long  Beach,  CA,  pp.  369-­372,  Jan.  9-­11,  2007

n A.  Katz,  R.  Gray  and  R.  Dorval,   “Truly  wideband  linearization,”   IEEE  Microwave  Magazine,  Vol.  10,  Issue  7,  Part  Supplement,  pp.  20-­27,  December   2009.

n A.  Katz,  and  et  al,  “Wide/Multi-­band  Linearization  of  TWTAs  Using  Predistortion,”  IEEE  Trans.  on  Electron  Devices,  Vol.  56,  pp.  959-­964,  May,  2009.

n A.  Katz,  M.  Chiappetta and  R.  Dorval,  “Predistortion  Linearization   to  100  GHz,”  Radio  and  Wireless  Symposium,  PAWR  Topical  Conference  Proceedings,  Austin,  TX,  Jan.  20-­23,  2013.