Unit-I (Feedback amplifiers) Features of feedback amplifiers. Presentation by: S.Karthie, Lecturer/ECE SSN College of Engineering

Transcription

1 Uni-I Feedback ampliiers Feaures eedback ampliiers Presenain by: S.Karhie, Lecurer/ECE SSN Cllege Engineering

2 OBJECTIVES T make he sudens undersand he eec negaive eedback n he llwing ampliier characerisics: Gain Disrin Nise i/p & /p impedance Frequency respnse & Bandwidh

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4 Feedback mpliier Circuis Feedback is reurn par he upu he inpu r a circui/sysem Feedback is very useul in Cnrl Thery and Sysems and is well researched mpliier circui can have negaive eedback and psiive eedback. Negaive eedback reurns par he upu ppse he inpu, whereas in psiive eedback he eedback signal aids he inpu signal. Bh negaive eedback and psiive eedback are used in ampliier circuis Negaive eedback can reduce he gain he ampliier, bu i has many advanages, such as sabilizain gain, reducin nnlinear disrin and nise, cnrl inpu and upu impedances, and eensin bandwidh

5 General srucure he eedback ampliier This is a signal-lw diagram, and he quaniies represen eiher vlage r curren signals

6 : : he he pen β : eedback s β : lp, β clsed gain lp i lp gain ceicien Gain rmula gain <, he hen he negaive ampliier ampliier eedback I β >>, hen / β. Thus, he clsed-lp gain wuld be much mre sable and is nearly independen changes pen-lp gain β I β >>, s s,s i s 0. β Thus, in a negaive eedback ampliier, he upu akes he value drive he ampliier inpu alms 0 his is summing pin cnsrains.

7 mpliier negaive eedback: T reduce nnlinear disrin

8 I a pre-ampliier wih gain 000 is placed bere he nnlinear ne s ha he whle ampliier is used wih negaive eedback, β >> and he gain r whle ampliier becmes: 9.99 r 0 < 9.98 r -0 < < 0 < 0 which grealy reduce he nnlinear disrin. This is achieved hrugh cmpensary disrin he inpu signal

9 mpliier negaive eedback: nise reducin SNR nise s nise s nise s β β β nise s nise s SNR I an ampliier assumed be nise ree r very lw nise is placed bere he nisy ampliier, hen he Signal--Nise SNR rai is grealy enhanced by a acr equal he preceding ampliier gain

10 Feaures negaive eedback ampliier Sabiliy: Due emperaure, aging and replacemen eecs, he ampliier gain becmes unsable. The racinal change in ampliicain wih eedback divided by he racinal change wihu eedback is called he sensiiviy he ranser gain. d β d sensiiviy I β >>, hen ~ /β Gain depends enirely n he eedback newrk The main reasn r he insabiliy is due he acive devices invlved, i he eedback newrk cnains nly sable passive elemens, he imprvemen in sabiliy is epeced.

11 Feedback plgies s menined abve, he ur eedback plgies can be deined as: cnnecin eedback newrk inpu erminal cnnecin eedback newrk upu erminal i.e, Series-shun inpu F: Vlage, upu F: Vlage Series-series inpu F: Vlage, upu F: Curren Shun-series inpu F: Curren, upu F: Curren Shun-shun inpu F: Curren, upu F: Vlage

12 The ur basic eedback plgies: a vlage-miing vlage-sampling series shun plgy; b curren-miing curren-sampling shun series plgy; c vlage-miing curren-sampling series series plgy; d curren-miing vlage-sampling shun shun plgy.

13 In general Eec negaive eedback n gain In series vlage eedback, inpu signal is vlage and upu vlage is sampled, s i is naural mdel he ampliier as a vlage ampliier. In general, v,s v β vβ mpliier emplying series curren eedback is mdeled as a ranscnducance ampliier. Gm, his is mdeled by a ranscnducance ampliier, s Gm β Gmβ mpliier emplying parallel vlage eedback is mdeled as a ransresisance ampliier. Rm In general, his is mdeled by a ransresisance ampliier, s Rm β Rmβ mpliier emplying parallel curren eedback is mdeled as a curren ampliier. i In general, his is mdeled by a curren ampliier, s i β β i

14 Negaive eedback n inpu impedance Fr series eedback, he llwing mdel can be used r analysis inpu impedance he upu culd be eiher vlage r curren R i i Ri R β I he inpu impedance he pen-lp ampliier is Ri, hen he clsed-lp impedance is Ri Ri β, nice β >> r negaive eedback s, series eedback eiher curren r vlage increase he inpu impedance Similarly, he eec parallel eedback n inpu impedance can be analyzed using a similar mdel, he clsed-lp inpu impedance wuld hen be Ri Ri / β s, parallel eedback decrease he inpu impedance

15 Eec eedback n he inpu resisance ampliier Feedback miing remember Inpu resisance series increases shun decreases In eedback ampliiers, series miing ends increase he inpu resisance and shun miing ends decrease he inpu resisance.

16 Negaive eedback n upu impedance Fr vlage eedback, i culd be eiher series r parallel eedback, he clsed-lp impedance is R R / β s, vlage eedback decrease he upu impedance Similarly, r curren eedback eiher series r parallel eedback, he clsed-lp impedance is R R β s, curren eedback increase he upu impedance

17 Eec eedback n he upu resisance ampliier Feedback sampling remember vlage curren Hw? upu resisance decreases increases Le us hink vlage sampling. I R L increases s ha V increases, he eec eeding his vlage back he inpu in negaive manner will cause V increase less han i here were n eedback. Hence, he upu vlage remains cnsan as R L changes, which means R << R L. Fr curren sampling. In negaive eedback which samples he upu curren will hld his curren cnsan. Hence, an upu curren surce is creaed, which means R >> R L.

18 Eec negaive eedback n gain and bandwidh V v v i V v β v Bandwidh B v v v Bandwidh B β v >> v β Bu gain drps a higher and lwer requencies, herere β >> n lnger hlds. Gain bandwidh is same r a eedback and basic ampliier.

19 I is impran ne ha even β is cnsan gain depends n requency. Le us Cnsider a single ple ampliier. ; / / / / / / 0 H H H H H H H j j j j j β β β β L L H H β Where is midband gain wihu eedback and H is he high 3-dB requency. is gain wih eedback. L is lw 3-dB requency.

20 Summary s a summary, negaive eedback ends sabilize and linearize gain, which are desired eecs. Fr a cerain ype ampliier, negaive eedback ends prduce an ideal ampliier ha ype. Fr eample, series vlage eedback increases inpu impedance, reduces upu impedance, which ges clser an ideal vlage ampliier. S, negaive eedback shuld be used in ampliiers circuis.