Linear analysis limitations

Transcription

1 RF power amplifier he final stage (output stage) allows delivering power needed by devies suh as speakers, antennas, et he output signal shows high dynami both in voltage and in urrent, so overing a great portion of output - plane Aording to their ondution angle (i.e. the fration of time period in whih amplifier ative devie is onduting urrent), amplifiers are operating in Class A, AB, B, C 1 Linear analysis limitations Linear ampliier: L linearly inreases versus N G L N ost Real devies are non-linear ORON For instane BJ: Q v 1 s * exp BE BEQ v v BE BE 3 BE 3 v 6 1

2 v BE Linear analysis limitations ^ os t Bias point variation non linear inrease of H1 harmonis generation intermodulations generation (multi-tone ase) n 1 m istortions ^ Q1 4 Q 4 Q Q 4 ^ ^ 1 ost 8 ^ ^ 3 3 os 3t os t C H1 H H3 3 istortions 1dB ompression point ynami Range (R): N range where gain is onstant R 1dB M (M=minimum detetable signal) 4

3 ntermodulations 3 rd order interept point nput: lose tones f 1, f Output: intermodulations; in partiular f1 f f1 f f 1 f1 5 ntermodulations: M3 and 3 out out_ = 3 in + G O3 out_ = in + G N out G G 3 in M FR f in = 3: 3 + G = 33 + G G = G

4 ntermodulations: M3 and 3 3 an be evaluated by measuring out_ and out_ at whatever in within the R out O3 out_ = in1 + G out1_ M3 out1_ out1_ = 3 in1 + G - 3 N out in1 3 in out_ - out1_ = M3 = - in1-3 3 = M3/ + in1 O3 = M3/ + out1 7 petral regrowth nput spetrum Harmoni and intermodulations produe spetral regrowth Output spetrum 8 4

5 Effiieny definitions ower onversion effiieny outrf inc t is a measure of effiieny in onversion of C power from power supply into RF power to load ower-added effiieny AE outrf inc inrf 1 1 G t takes into aount also input power (and therefore Gain) 9 Amplifier Effiieny Why it is so important 1 1 diss outrf ower dissipation % diss / outrf % 65 % 53.8 % 85 % 17.6 % Heat dissipation issue Maximum values for and (afe operating Area - OA) limit maximum outrf ower onsumption from batteries in mobile system 1 5

6 RF power amplifier requirements Effiieny --> we need to minimize power onsumption for a given power delivered to the load - Heat dissipation from transistors - Battery lifetime in mobile systems Linearity --> we need to redue spetral regrowth (interferene to adjaent hannels) and to minimize BER (related to amplitude and phase distortions) - Use of modulation tehniques with high spetral effiieny (amplitude and phase modulation) 11 Effiieny vs linearity trade-off Effiieny and linearity are onfliting requirements High effiieny is obtained by * maximizing signal power * exploiting devie trans-harateristi saturation Linearity loss ossible solution EFFCENCY --> high-effiieny RF A arhitetures LNEARY --> linearization tehniques 1 6

7 Classifiation of RF power amplifiers EFFCENCY Linear Amplifiers Class A Class AB Class B Class C LNEARY 13 Arhiteture of a power amplifier ingle-ended onfiguration 14 7

8 Arhiteture of a power amplifier ush-pull onfiguration 15 Linear power amplifiers CLA: t speifies the operation mode, the bias point related to both the devie threshold and the maximum peak of the input sine waveform 16 8

9 Linear power amplifiers CONUCON ANGLE (g): portion of the period (for sine waveform) in whih urrent is flowing in the transistor Classe A AB B C g <g< g< 17 Class A Amplifier 18 9

10 Class A Amplifier C o t Q ost t ost t ost t ost o C Q R g max max Q 19 Class A Amplifier R Load line opt max Bias point (Q) C Q max 1

11 Class A Amplifier inc Q R NO EENEN ON NU GNAL!!! outrf R 1 Effet of sat : sw sw sat 1 Class B Amplifier - push pull f BF is not used, ross-over distortions are present 11

12 Class B Amplifier - push pull o m n t sint o t Rot osint t sint C1 max m n o m R n Load line R opt m n max 3 Class B Amplifier - push pull C inc 1 1 t C dt outrf m n R

13 Class B Amplifier - push pull diss R R diss inc outrf Maximum dissipation 5 Linear amplifiers with lower ondution angle g Q Heat dissipation with no input signal is redued Effiieny is enhaned, as the urrent ondution time is lowered A BF is needed for harmoni power attenuation More input power is needed (lower Gain) <g< <g< lass C lass AB lass B: partiolar ase g= 6 13

14 Class C amplifier 7 Class C amplifier gg <, threshold voltage of JFE 8 14

15 Class C amplifier ynami equivalent iruit: At JFE drain, apaitor, transformer and load R L an be onsidered as a parallel resonator, tuned at the frequeny of input signal. 9 Class C amplifier M : input voltage orresponding to C = max : input voltage orresponding to C = nput dynami: = M - Q Condution angle: g=a 3 15

16 Class C amplifier Effiieny evaluation is done by onsidering both a linear behavior of drain urrent (approximated), and a quadrati one (exat) Linear behavior of rain urrent (approximation): Real behavior of rain urrent: t G ˆ os t GQ t G ˆ os t GQ 31 Linear ase: Class C amplifier Evaluation of (ω ) from Fourier series expansion: fundamental omponent of drain urrent tos tdt G os t ˆ os t Gˆ sin. GQ dt 3 16

17 Linear ase: Class C amplifier Ī evaluation: 1 G Gˆ dt sin os tdt ˆ os t GQ. Effiieny: max sin 4sin os 33 Quadrati ase: Class C amplifier Evaluation of (ω ) from Fourier series expansion: fundamental omponent of drain urrent os t tos tdt ˆ os t G G G os tdt os tdt ˆ os t GQ t Gˆ GQ GQ dt dt Gˆ sin os sin sin os sin os

18 Quadrati ase: Ī evaluation: Effiieny: Class C amplifier 1 G G dt tdt ˆ os t ˆ os G Gˆ Gˆ 1 1 os sin 4 max tdt os t GQ GQ os sin. dt 3 sin sin os sin os sin 4 GQ dt 35 18