Output Stages and Power Amplifiers

Transcription

1 Output Stages and Power Aplfers Output stages wth a low output resstance can delver the output voltage to the load wthout loss of gan. arge sgnal aplfer are used to drve a CT, a loud speaker, a servootor, etc. power ap. = large sgnal ap Power ap.can delver large voltage or current or a large aount of power. Theral consderatons are portant for power ap. 3 - Electroncs(3), 03

2 arge-sgnal Aplfers C s the quescent collector current (DC sgnal). c s the nstantaneous varaton fro C. (Sall sgnal) c and c are the rs values of v c and c. s the peak of snusodal current swng. CC C ax C B B C 0 Q B B3 BB n n C 0 ax CC v c t 3 - Electroncs(3), 03

3 arge-sgnal Aplfers (Cont.) c c ax ax n n Assung perfect lnearty,.e. no nonlnear dstorton) Output power P c c ( c ax n )( 8 ax n ) 3-3 Electroncs(3), 03

4 Haronc Dstorton Nonlnear Dynac transfer characterstc s not a straght lne. Sall sgnal arge sgnal Both have nonlnearty probles. nd haronc dstorton nonlnear relatonshp between b and c s assued. 3-4 Electroncs(3), 03

5 Haronc Dstorton(Cont.) b c c C G b b G cost b G b cost G G b cost G B0 B cost B B C c C 0 where G & G are constant. cos t b G cos t B cost B b b cos t cos t a. DC level s changed. b. nd haronc dstorton s ntroduced. 3-5 Electroncs(3), 03

6 Haronc Dstorton (Cont.) B nd haronc dstorton D B nter-odulaton For cosω t cosω c contans,,,,, nd haronc dstorton Hgher order haronc dstorton =G +G +G + 3 c b b 3 b c c D =, D 3 =, B B where D n represents the nth haronc dstorton HD n nter-odulaton = +B +B cos ωt+b cos ωt+b cos3 ωt+ B b b b B t 3-6 Electroncs(3), 03

7 3-7 Electroncs(3), 03 Total Haronc Dstorton(THD) Power delvered at the fundaental frequency s Total power output s B P... D D D THD P... D D... B B B P

8 Aplfer Classfcaton Class A Output ~ X nput X Class B Output ~ nput BE 3-8 Electroncs(3), 03

9 Aplfer Classfcaton (Cont.) Class AB Output BE ~ nput Class C Output BE ~ nput 3-9 Electroncs(3), 03

10 Power Converson Effcency portant under. lted source of power. (e.g. n a satellte). axu power-dsspatng consderaton. Converson effcency sgnal power delvered to load dc power suppled to output crcut Class A aplfer wth c and v c output v C c c η C snω t snω t CC c C 00% 50 CC C % 00% 3-0 Electroncs(3), 03

11 Power Converson Effcency (Cont.) Sall sgnal,.e.(. 0 ) are sall. statc power consupton CC C even no exctaton Maxu sgnal,.e. = C, =0.5 CC 5% Class A operaton s a poor choce for power aplfcaton. C 3 - Electroncs(3), 03

12 Class B OPAMP Class B Push-Pull Aplfer Q CC C Dynac transfer characterstc O ~ Q CC CC C 0 0 B B B B ω t B v ~ C Exctaton ω t 3 - Electroncs(3), 03

13 Class B Push-Pull Aplfer (Cont.) Effcency of Class B push-pull Output power DC power (suppled by CC ) dc P Effcency η For π π P o CC Po π 00% P 4 η CC Two transstors for push-pull CC ax 00% 78.5% No statc power consupton under zero exctaton π 4 00% 3-3 Electroncs(3), 03

14 Class B Push-Pull Aplfer (Cont.) Average power dsspated n the class B stages. CC P P P A (Two transstors push-pull) Dfferentatng Eq.(A) wth respect to and equatng the dervatve to zero gves the that results n axu P D D For π o CC π P CC D(ax) P π o,ax P D P D(ax) CC CC power output η 50% η 78.5% π CC /π CC CC 3-4 Electroncs(3), 03

15 Dstorton n the Class B Push-Pull Stage CC Q O Q Haronc dstorton CC For atched devces Q & Q and are dentcal except shfted n phase by 80 ( ω t) C C B B 0 B cos ω t B ( ω t π) 0 B cos ω t B cos ω t B cos3 ω t cos ω t B cos3 ω t (B cos ω t B cos3 ω t ) Electroncs(3), 03

16 Dstorton n the Class B Push-Pull Stage (Cont.) Even-order haronc dstortons have been elnated. f the -s of Q &Q are not dentcal, then evenorder haronc dstortons are expected. Crossover dstorton Wll be dscussed later. 3-6 Electroncs(3), 03

17 Output Stages deal output stages Supply external load current ow output pedance arge output swng CC EE deally Coonly used copleentary etter follower Each transstor s on for only half the te. 3-7 Electroncs(3), 03

18 Crossover dstorton Output Stages (Cont.) + CC o Q Q o Q (cut off) CC - CE(sat) - BE(on) BE(on) Q (sat) Q (cut off) o t Q (sat) - CC + CE(sat) - CC 3-8 Electroncs(3), 03

19 Output Stages (Cont.) Elnaton of crossover dstorton CC O bas CC - BE - CE(sat) D Q - BE(ON) D O Q - CC CE(sat) CC 3-9 Electroncs(3), 03

20 Elnaton of Crossover Dstorton of 74-Type OPAMP Output stage CC CC Q 3A Q 4 Q 9 6 o 40k Q 8 7 o Q 0 Q 3 EE EE 3-0 Electroncs(3), 03

21 Elnaton of Crossover Dstorton of 74-Type OPAMP (Cont.) Crcut Dagra CC 5 Q Q 9 Q 8 Q 3B Q 3A Q 9 Q 5 Q k n Q Q Q3 4 Q n C C 30pF 0 40k Q 8 Q 3 Q 7 Q 0 7 Q 6 Q EE Q k Q 7 Q5 Q6 k 3 50k k Q 9 50k Q Q 4 50k 3 - Electroncs(3), 03

22 Classcal CMOS Class AB Output Stage Q N and Q P are source followers Q and Q are dode-connected elnate crossover dstorton Quescent current Q D D GG GG BAS BAS slarfor assuek kn' k p GS n Q GSN ' N W GS tn ' W SG tp SG andq 0.5 k ' W k ' W n p 0.5 k ' W k ' W n p W k ' W k ' W k ' W p and n p SGP P, tn tn tp tp BAS Q n BAS v n Q p + DD W n W 3 - Electroncs(3), 03 + GG v o Q - Q P - SS Q p Q N BAS

23 estrcted Output Swng of Classcal Class AB vo v ax DD O Oax where v slarly, v On v O ON where O OP BAS : : O DD SS of O Q BAS P O vgsn BAS tp tn v v ON : n. voltage across current source of Q when t s supplyng O N OP ax : n. voltage across the transstor supplyng v when snkng the ax. + DD BAS v Q + GG v o Q - Q P (not Q N - SS shown) One drawback of classcalcmos classab output the restrctedoutput swngrange stages 3-3 Electroncs(3), 03

24 CMOS Class AB Utlzng Coon-Sources Allowable range of v O voax DD vop von SS von output swng range s ncreased Hgh output resstance out n Negatve seres-shunt feedback reduce the out of the aplfer vo v f the loop gan s large out // v out p μ μ + DD Q P DP DN Q N DD Q P Q N - SS v o v o - SS OUT 3-4 Electroncs(3), 03

25 out educton by Seres-Shunt Feedback v The top half crcut - + μ DD Q P outp v o of v the A crcut - + μ Q P v o the β crcut f O - A of outp out gp ( rop // ), o A g outp // of outn o p // r op ( r op // r // op r op // gp // g p g n ) g g p of p g 3-5 Electroncs(3), 03 n

26 oltage Transfer Characterstc n the quescent state v 0 and vo 0 DP Q k p ' W O p DN Q kn' W O n assue Q and Q are atched.e. k Q p ' W k ' W P k p O n N n k DD - SGP - SS + GSN DD + SGP - Q P μ μ + + GSN - Q Q Q N - SS Electroncs(3), 03

27 3-7 Electroncs(3), 03 O Q p Q O O Q O Q O O DN DP O O O Q DN O O Q O O O O O DP g g g v v v v v v v v v v v v k v v k ) ( ) ( Gan error snce Gan error slarly, oltage Transfer Characterstc (cont.) When v s appled Q P Q N v o DD - SS - + μ - + μ DP DN + - SGP ( DD SGP ) +μ(v o -v ) + - GSN (- SS + GSN ) +μ(v o v ) v

28 Error aplfers gan µ larger µ Desgn Trade-offs saller gan error and out saller µ Q s ore nsenstve to the nput offset voltages of the error aplfers Quescent current Q larger Q saller crossover dstorton, gan error, and out saller Q less power dsspaton 3-8 Electroncs(3), 03

29 Theral Desgn Consderatons C power aplfer Capable of delverng large power to external load. Exaple : A 5-W audo aplfer 0k 6 M C 3 0.uF 8 C 5uF C 0.uF 500μF speaker 8 The axu possble power converson effcency s about 75% (Class B or AB).e. for every 3-W output, W s dsspated wthn the aplfer. 3-9 Electroncs(3), 03

30 Theral Desgn Consderatons (Cont.) P D T TT J(ax) rreversble falure occurs where T J(ax) s the axu operatng juncton teperature of seconductor devce. Theral resstance q JC (between juncton and case) T T ΔT P θ J C unt: P D :Watt JC D q: 0 C/Watt JC 3-30 Electroncs(3), 03

31 Dsspaton Deratng Curve Exaple : N567 power transstor Maxu allowable dsspaton P D(ax) = 40W Dsspaton deratng curve θ JC C power deratng factor W P D,(ax),W Case teperature T C θ 0 JC 0.8W Exaple 3-7 and 3-8 of textbook T J ΔT JC ΔT CS C ΔT SA T A Chp (juncton) case heat snk 3-3 Electroncs(3), 03

32 Dsspaton Deratng Curve (Cont.) J : seconductor devce juncton C : case S : heat snk A : abent P D q jc q cs T J T jc T C T cs T S T J P D (θ JC θ CS θ SA ) T A q SA T sa T A 3-3 Electroncs(3), 03

33 Power FET Exaple : DMOS (Double-dffused MOS) Asyetrc source and dran. S and D ay not be nterchanged Conventonal Currently popular Source Gate Source Gate Dran P n Substrate n Source P n n Body n + n + p n + n - p - -substrate Current flow Dran 3-33 Electroncs(3), 03

34 araton on Class AB Confguraton Use of nput etter followers Hgh nput resstance CC Q 3 Quescent current n Q 3 and Q 4 s equal to that n Q and Q, f = and 3 = 4 0 Q CC CC c Q 3 4 Q 4 O CC 3 and 4 are sall and are ncluded to guard aganst the possblty of theral runaway due to teperature dfferences between the nput and output stage transstor Electroncs(3), 03

35 Copound Devces Darlngton confguraton. ncrease current gan. educe base current drve 3. Equvalent BE(eq.) = BE 4. Can be used for both NPN and PNP transstors Copound PNP confguraton. Used to prove PNP confguraton. Q s usually a lateral PNP havng low β ( 5 0) E E E E C C B B Q B B β ~ β β B Q B β ~ β β Q C C C C BE BE Q E BE E 3-35 Electroncs(3), 03

36 BE Multpler v Crcut ( Applcaton Exaple on DC evel Shftng CC 3 4 C C Q 3 Q 4 v o v o A O O For g BE 4 O BE ( 3 g BE BE BE ( ( 3 4 O & 4 ) ) ) ) g BE g ( g 4 4 )( g, A 4 ) 3-36 Electroncs(3), 03

37 BE Multpler (Cont.) Elnaton of crossover dstorton usng BE ultpler equvalent 3-37 Electroncs(3), 03

38 Class AB utlzng a Darlngton NPN and a Copound Bas s obtabed usng a BE ultpler BE ultpler s requred to provde 3 BE CC Q Q Q 5 O Q 3 Q 4 CC 3-38 Electroncs(3), 03

39 Class AB utlzng a Darlngton NPN and a Copound (Cont.) Short-crcut protecton BE5 C5 B C CC Q Q 3 Q 5 Q 4 E O E Q CC 3-39 Electroncs(3), 03

40 Class AB utlzng a Darlngton NPN and a Copound (Cont.) Theral shutdown T, Z BE Q absorbs bas current of OPAMP C CC Q Z Q CC 3-40 Electroncs(3), 03