Feedback and Oscillatrs Lessn #4 Impedances Sectin 9.35 65
Types f ffeedback Type f ffeedback k(the utput tentity fed dback): Vltage Feedback s. Current Feedback β s. β Hw it is achieed (the means t fed back the utput t the input): Series (input ltage) Feedback s. Parallel (input current) Feedback i s f s. i f 66
Types f ffeedback (Cntinued) Fur Cmbinatins Series Vltage: where amplifier input and utput are ltages and, therefre, the gain parameter is a ltage gain, A and the feedback is a ltage, β,which is prprtinal t the utput ltage Series Current : where amplifier nput is a ltage and its utput is a current and, therefre, the gain parameter is a transcnductance, G m, and the feedback is a ltage β, which is prprtinal t the utput current Parallel Vltage: where amplifier input is a current and its utput is a ltage and, therefre, the gain parameter is a transresistance gain, m and the feedback is a current, β,which is prprtinal t the utput ltage Parallel current: where amplifier input and utput are currents and, therefre, the gain parameter is a current gain, A i and the feedback is a current, β,which is prprtinal t the utput current. 67
Types f Feedback Circuits s i f AA A β Series Vltage i s f β AG m m Series Current A f m i AA f s β β Parallel Vltage Parallel Current 68
Feedback kelatins fr the 4 Types Since we deried the the feedback gain independent f whether the a current r ltage is fed back, then fr each type: A Series Vltage: A f A β s β i G m Series Current: G mf G s mβ Parallel Vltage: m mf i s mβ Parallel Current i A i A if i A β s i 69
Input Impedance s if s i f s i f x A i i i x A i i s i βa i if s Series βx i βx i ( βa) x A i Lad s if i f s βx i f x A β A i f βx i β x Parallel s i s if f x A ( β A) i ( β A) s i i Aβ 70
Output Impedance x s 0 x f β test A c itest test i0 x s 0 A sc x f β i test test test β i test f test i test Vltage test i test A c f test i testt Current test (i test A sc ) 0 x s x f x s 0 β test x s β test x s x f 0 β β βi test test (i test A sc βi test ) test i test A c β test f test i test 0 ( β A c ) test f 0 ( βa sc ) i test 7
Identifying i Negatie Feedback Determine the type f feedback: Vltage s Current Determine hw the feedback is applied: Series r Parallel Determine if the feedback is in ppsitin t hw the input is applied. See if the feedback is applied t the inerting (subtractie) input f the amplifier. 72
Identifying Negatie Feedback See the clumn n page 57 Output In ltage feedback, the input terminals f the feedback netwrk are in parallel with the lad In current feedback, the input terminals f the feedback netwrk are in series with the lad Input If the feedback signal anishes fr a pen circuit lad, then current feedback If the feedback signal anishes fr a shrt circuit lad, then ltage feedback 73
Identifying i Negatie Feedback (Cntinued) s s f i 2 L s s i f f L f β 2 i f f 2 2 2 β f 74
Identifying i Negatie Feedback (Cntinued) i f f i s L i s L f i f i f f i f β β f i f f ; since 0 i 75
Design f Feedback Amplifiers Dt Determine what httype f ffeedback kis required and the alue f β Select a circuit design Calculate the apprpriate p alues f the circuit elements (i.e., resistrs in the feedback netwrk) 76
Design Example Design a feedback circuit which prides a ltage equal t 0 times the input surce. Assume that the surce has a resistance f 2kΩ and the differential amplifier has an pen circuit gain f 0 4 with an input resistance f 5 kω and an utput resistance f 00 Ω 00 s 2k f 2 5k β 0. 0 0 β L 50 4 A i Aβ 0 0. 2 β. A f A A A f 2 9 Chse 2 4 2 > 000 O t reduce due t lading any lss f gain 77
Hmewrk Impedances Prblems: 9.22, 2736 Practical Netwrks Prblems: 9.38 78