ho29.l26_ota4

ho29.l26_ota4 - OTA Feedback Circuits Feedback Circuits...

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1 OTA Feedback Circuits (Part IV) R. Dutton, B. Murmann Stanford University R. Dutton, B. Murmann 1 EE114 (HO #29) (Basic) Two-Stage OTA High gain ~ 0.5(g m r o ) 2 V DD Large output range Possible extensions – Cascodes in stage 1 • Gain becomes ~0.5(g m r o ) 3 (!) – Current mirror in stage 1 M1a V xm V o M3 V BN V BP M2 M4 V xp M1b R. Dutton, B. Murmann 2 EE114 (HO #29)
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2 AC Model with Capacitive Feedback R 1 R 2 v o C Ltot C 1 v i (-) v x C s C f C L C x G m1 g m2 2 1 1 m m g G = 3 1 1 2 o o r r R = 4 2 2 o o r r R = x s f f C C C C + + = β R. Dutton, B. Murmann 3 EE114 (HO #29) 1 1 2 gd gs x C C C + = ( ) 4 4 2 1 gd db db f L Ltot C C C C C C + + + β + = (ignoring C gd2 for the time being) 3 1 3 1 2 1 gd gd db db gs C C C C C C + + + + = Loop Gain R 1 R 2 v o C Ltot C 1 v i (-) v x C s C f C L C x G m1 g m2 R. Dutton, B. Murmann 4 EE114 (HO #29) () () () s a s a s a p s p s R g R G s T v v v m m β = β = β = 2 1 2 1 2 2 1 1 1 1 1 1 1 1 C R p = 2 2 2 1 C R p =
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3 Bode Plot of Loop Gain () ω j Mag s a β s a 2 If ω p1 and ω p2 are close to each other, the loop will have a very small phase margin 1 p ° 0 2 p j Phase s a 1 s a R. Dutton, B. Murmann 5 EE114 (HO #29) ° 90 ° 180 Introducing a Dominant Pole The problem is solved if we somehow manage j Mag s a ( ) s a 2 to make ω p1 << ω p2 –O r ω p2 << ω p1 The loop then behaves close to first order system around crossover frequency – Phase margin 1 p ° 0 2 p [] j T Phase s a 1 c R. Dutton, B. Murmann 6 EE114 (HO #29) – Phase margin follows from expression on slide 14 of lecture 25 ° 90 ° 180
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4 Creating a Dominant Pole Numerical example: mS g G m m 1 2 1 = = Ω = = k R R 100 2 1 pF C 1 2 = ° = 72 PM 5 0 . = β MHz . C R f p 6 1 2 1 2 2 2 = = π kHz f f p c 530 3 2 = = Hz R G R G f f m m c p 106 2 2 1 1 1 = = nF R f C p 15 2 1 1 1 1 = = Two issues Very low which means low closed loop bandwidth R. Dutton, B. Murmann 7 EE114 (HO #29) – Very low f c
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This note was uploaded on 11/09/2009 for the course EE 114 at Stanford.

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ho29.l26_ota4 - OTA Feedback Circuits Feedback Circuits...

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