Feedback Principle :-

Transcription

1 Feedback Prncple : Feedback amplfer s that n whch a part f the utput f the basc amplfer s returned back t the nput termnal and mxed up wth the nternal nput sgnal. The sub netwrks f feedback amplfer are: Blck dagram f a feedback netwrk (a) SAMPLNG NETWOK: Ths netwrk samples ut a part f utput n rder t send t t the nput. t may cnsst f resstr, cndenser, nductr and als electrnc cmpnents. (b) FEEDBACK NETWOK: Feedback netwrk s used t get a desred magntude f utput and als desred phase t send t t the nput sutably. (c) MXE NETWOK: The functn f ths netwrk s t mx up the feedback entty wth external nput sutably. Pstve feedback : f the feed back entty mxed up wth external sgnal s n same phase then the feedback s called pstve feedback feedback. Negatve feedback: f the feedback entty mxed up wth the external sgnal s n ppste phase then feedback s called negatve feedback. 1

2 Gan wth feedback: Let, S, X = external nput sgnal. X O = utput sgnal. Xf = feedback entty. X = sum f nput sgnal and feedback entty. X =X ± X f (1) X f β = = fractn f utput feedback t the nput. XO X A = = gan f basc amplfer. X A f = X X = vltage gan wth feedback. Dvdng equatn (1) by X O we get, X' X X f = ± X X X 1 1 = ± β A A 1 1 = m β Af A 1 1m β = Af A A Af = 1m β Fr, A Pstve feedback; Af =. 1 Aβ A Negatve feedback; Af = 1 Aβ f 2

3 Fur tptges f feedback amplfer: (a) Vltage amplfer wth vltage seres feedback : Here the feedback netwrk s cnnected n seres wth the nput sgnal. (b) Current amplfer wth current shunt feedback: Here feedback termnal s cnnected n parallel t the external nput sgnal. (c) Transcnductance amplfer wth current seres feedback: Here the feedback netwrk s cnnected n seres wth the (samplng netwrk) utput sgnal. (d) Transresstance amplfer wth vltage shunt feedback : Here the feedback netwrk s cnnected n parallel wth the samplng netwrk (utput sgnal). 3

4 (a) Vltage amplfer: Fr a vltage amplfer the utput vltage (V O ) s prprtnal t the nput vltage (V ). V O V V O = A V V Where A V s called the vltage gan. Thevenn s equvalent crcut f a tw prt netwrk whch represents a vltage amplfer s gven n fgure belw; Here, V S = sgnal vltage S = surce resstance = nput resstance s V V Vs n the absence f lad.e. pen ckt. utput vltage 1 V. = AvV = Av Vs = AvVs s 1 s As ncreases V O als n creases. V = VS s Nw f ; V O = A V V S. S nput resstance f an deal vltage amplfer shuld be very hgh. n the presence f lad resstance the utput f the amplfer, V = V L = V L as 0; V V s V Vs V L V' S a gd vltage amplfer shuld have very lw utput resstance. 4

5 (b) Current amplfer: n an deal current amplfer the utput current s prprtnal t the nput current. O O = A Where, A s called the current gan. Open ckt. utput current, = A. = A s s s s s f decreases then O ncreases. When 0; O = A S the max m utput s = s s current. S nput resstance f a gd current amplfer shuld very lw. f the ut termnal s cnnected t sme lad L then the current thught the L s gven by, L = 2 1 L s s L As O ncreases the L ncreases and attans the maxmum value L = O when O S a gd current amplfer shuld very hgh utput resstance. 5

6 (c) Transcnductance amplfer: An deal transcnductance amplfer prduces an utput current whch s prprtnal t the nput vltage. O V O =g m V Here g m s called transcnductance. Nw n the absence f any lad, O = g m V = g m.v S s 1 = g m V S. S 1 s V Vs V = Vs s As ncreases O als ncreases and attans the maxmum value O g m V S when. S fr an deal transcnductance amplfer the nput resstance shuld be very hgh. n the presence f lad the current thrugh the lad s L = = O L 1 O 1 L s Vs V L ' L As O ncreases L als ncreases and reaches ts maxmum L O when O. S fr an deal transcnductance amplfer the utput resstance shuld be very hgh. 6

7 (d) Transresstance amplfer: An deal transresstance amplfer supples an utput vltage whch hs prprtnal t the nput current. V O Where s called transresstance. V O = n the absence f any lad the utput vltage, V O =. = s s s s s s = s s V V O ncreases as decreases and V O s maxmum when 0. S fr an deal Transresstance amplfer nput resstance shuld be very lw. Cnsder a lad L s cnnected t the utput termnal. utput vltage, V L = L L O V O s s L VL As fr an deal ncreases and reaches the maxmum value V L V O when O 0. S fr an deal Transresstance amplfer the utput resstance shuld be very lw. 7

8 Nature f and O fr dfferent feedback tplges (deal case): Feedback tplgy O Vltage amplfer 0 Current amplfer 0 Transcnductance amplfer Transresstance amplfer 0 0 8

9 Effect f negetve feedback n nput and utput resstances f amplfers: (a) Vltage seres feedback : (Vltage amplfer) () nput resstance wth negatve feedback : V=Vs V' AvV' L V f Vf f f' f the negatve feedback sgnal s returned t the nput n seres wth the appled vltage, t gves rse t an ncrease n nput resstance. t s because f the fact that feedback vltage V f ppses the nput vltage V S (sgnal vltage) as they are n ppste phase. As a result the nput current s less then that whch wuld be f V f s absent. And hence the nput resstance appears t ncrease. Applyng KVL at the equvalent nput ckt. We can wrte, V f V S = V f = βv O (1) [Q β = ] V Nw frm utput crcut we can wrte, AV V O = v L Av V L = = = AVL (2)[ Q V = ] L L A Where, A VL = v L (3) L Nw usng the value f V O frm (2) n (1) we get, V S = βa VL V s = (1 β AVL) f = (1 β AVL ) f = nput resstance n presence f feedback. Hence we see that the nput resstance ncreases by a factr (1βA VL ) due t negatve vltage seres feedback. 9

10 () Output resstance wth negatve feedback: The effete f negatve seres vltage feedback s t decreases the utput resstance f an amplfer. Nw we cnsder tw cases: n the frst case here remve L : T fnd ut utput resstance wthut L we remve external sgnal Vs s that V = V f. We mpse a vltage V acrss the utput termnal and then calculate the current delvered by V. as shwn n ckt. Belw.n ths case, V f = (4) V' AvV' V Vf f Nw frm the utput ckt, V AvV = V AvVf = V Av βv = [ V f = β V ] V = 1 β Av Of = 1 β Av Of O. n the 2 nd case we cnsder the presence f L : Nw we cnsder the presence f lad resstance L acrss utput termnal. n ths case L becmes parallel t Of and the effectve utput resstance s gven by, f = f L 10

11 V' AvV' L V Vf f f' f = f L f. L f = f L. L f = 1 β Av L 1 β A v = L L Lβ Av L ( L) = 1 β A L v L f = 1 β AVL where = L = L L AV and A L VL = L 11

12 (b) Current shunt feedback: (current amplfer) () nput resstance wth negatve feedback: s f s f V A Negatve feedback, n whch feedback sgnal s returned t the nput by the methd f current shunt feedback, gves rse t decrease n nput resstance. Frm the nput ckt applyng KCL we have, s f = => f s = f = β (5) [ β = ] Nw frm utput termnal the current thugh L s A = = AL (6) = L A Where, A L = (7) L Usng equatn (6) n (5) we get, s = AL = (1 A L) (8) nput resstance wthut feedback, V = And nput resstance wth feedback, V f = = s ( 1 β AL) f = ( 1 β AL ) < f Current shunt negatve feedback causes a decrease n the nput resstance. L V 12

13 FEEDBACK PNCPLES & AMPLFES 1 (a) Draw the equvalent crcut fr vltage amplfer. (b) fr the deal vltage amplfer what are the values f what s the dmensn f gan. 2 (a) Draw the equvalent crcut fr current amplfer. (b) fr the deal current amplfer what are the values f and and what s the dmensn f gan. 3 (a) Draw the equvalent crcut fr Transcnductance amplfer. (b) fr the deal Transcnductance amplfer what are the values f what s the dmensn f gan. 4 (a) Draw the equvalent crcut fr Transresstance amplfer. (b) fr the deal Transresstance amplfer what are the values f.. and and what s the dmensn f gan. 5. wrte dwn the basc characterstcs f a feedback amplfer. 6. what are the dfferent netwrks n feedback amplfer? What are the functns f each netwrk? 7. (a) what are the fur pssble tplges f a feedback amplfer? (b) dentfy utput sgnal X and the feedback sgnal f.. X each tplges. Defne the feedback factr β. 8. fnd s the relatnshp between transfer gan wth feedback and wth ut feedback. 9. what are the advantages f negatve feedback ver pstve feedback? 10. Lst fve characterstcs f an amplfer whch are mdfed by negatve feedback State hw and change n the case f negatve feedback vltage amplfers,current amplfer, transcnductanu amplfer and Transresstance amplfer. 12. Fnd ut the factrs mdfyng and n the case f (1) vltage seres feedback and (2) current shunt feedback. 13. Explan hw an amplfer utput s made stable by negatve feedback. 14. What s autmatc gan cntrl? Hw des t wrk? 15. Defne senstvty fnd ut ts expressn. 16. Defne Desenstvty fnd ut ts expressn Dscuss the effect f negatve feedback n bandwdth f an amplfer. 18. Fr scllatr we requre pstve feedback and fr stablty we requre negatve feedback why? 19. What s the effect f negatve feedback n the nn lnear dstrtn f an amplfer Mentn fur uses f pstve feedback? Why s t called regeneratve feedback? 13