tailieunhanh - Circuits & Electronics P21

Op Amps Positive Feedback Fall 2000 Lecture 21 1 .Negative vs Positive Feedback Consider this circuit — negative feedback vIN R1 R2 – R1 vIN + + R – + vOUT = − 2 vIN – R1 and this — positive feedback + – R2 vIN + – R1 sis e aly pag an t ee nex s on + R2 “vOUT = − R vIN ” – 1 What’s the difference? Consider what happens when there is a pertubation Positive feedback drives op amp into saturation: vOUT → ±VS Lecture Fall 2000 21 2 .Static Analysis of Positive Feedback Ckt v IN + – R1 + – v + R2 v − R2 vOUT v IN + – R1 + – A(v + − v −. | CIRCUITS AND ELECTRONICS Op Amps Positive Feedback Fall 2000 Lecture 21 1 Negative vs Positive Feedback Consider this circuit negative feedback R2 VIN and this positive feedback OUT What s the difference VlN r2 A R Consider what happens when there is a pertubation. Positive feedback drives op amp into saturation VOUT VS Fall 2000 Lecture 21 2 Static Analysis of Positive Feedback Ckt ---W vlN AMr viN Ri AAA l-o v R2 v v- CvOUT vOUT v _ Av A vOUT R1 R2 viN P I -----R1 viN AR1 R1 R2 v OUT AR1vIN R1 R2 AvIN vOUT AR1 R1 R2 _ viNA 1 R1 R1 R2 _ 1 R1 vOUT R1 R2 IN 1 R2 R1 viN 1 2 _ Fall 2000 Lecture 21

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