Tài liệu Circuits & Electronics P10 doc

6.002 CIRCUITS AND ELECTRONICS Amplifiers -- Small Signal Model 6.002 Fall 2000 Lecture 10 1 Review  MOSFET amp S V L R DS i v O v I  Saturation discipline — operate MOSFET only in saturation region Large signal analysis 1. Find v O vs v I under saturation discipline. 2. Valid v I , v O ranges under saturation discipline. Reading: Small signal model -- Chapter 8 6.002 Fall 2000 Lecture 10 2 Large Signal Review 1 v O vs v I v O = V S − K ( v I −1 ) 2 R L 2 valid for v I ≥ V T and v O ≥ v I –V T (same as i DS ≤ K v O 2 ) 2 6.002 Fall 2000 Lecture 10 3 Large Signal Review 2 Valid operating ranges v V 5V corresponding v I −V T interesting v I −V T region for v O v I −V T S O O v = O v > O v < 1V v I V T 1V 2V “interesting” region for v I . Saturation discipline satisfied. 6.002 Fall 2000 Lecture 10 4 But… S V O v O v = I v 5V 1V v I −V T v I v O Demo V T 1V 2V Amplifies alright, but distorts v I v O t Amp is nonlinear … / 6.002 Fall 2000 Lecture 10 5 Small Signal Model ~ 5V V S ~ 1V Hmmm … ( ) L T I S O R VvK Vv 2 2 − − = Amp all right, but nonlinear! I v O v T V V 1 V 2~ Insight: ( ) O I V,V Focus on this line segment So what about our linear amplifier ??? But, observe v I vs v O about some point (V I , V O ) … looks quite linear ! 6.002 Fall 2000 Lecture 10 6 Trick o v i v I V O V ( ) OI VV , O v∆ looks linear ∆v I  Operate amp at V I , V O Æ DC “bias” (good choice: midpoint of input operating range)  Superimpose small signal on top of V I  Response to small signal seems to be approximately linear 6.002 Fall 2000 Lecture 10 7 Trick o v i v I V O V ( ) OI VV , O v∆ looks linear ∆v I  Operate amp at V I , V O Æ DC “bias” (good choice: midpoint of input operating range)  Superimpose small signal on top of V I  Response to small signal seems to be approximately linear Let’s look at this in more detail — I III from a circuit viewpoint graphically next II mathematically week 6.002 Fall 2000 Lecture 10 8 I Graphically We use a DC bias V I to “boost” interesting input signal above V T , and in fact, well above V T . interesting input signal + – + – S V L R v O ∆v I V I Offset voltage or bias 6.002 Fall 2000 Lecture 10 9 Graphically interesting v O ∆v I S V L R + – + – input signal V I S V O v O V operating point O I VV , I V T V O vv = 0 I −V T v I Good choice for operating point: midpoint of input operating range 6.002 Fall 2000 Lecture 10 10 . 6.002 CIRCUITS AND ELECTRONICS Amplifiers -- Small Signal Model 6.002 Fall 2000 Lecture. corresponding v I −V T interesting v I −V T region for v O v I −V T S O O v = O v > O v < 1V v I V T 1V 2V “interesting” region for v I . Saturation discipline