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09_Lecutre

# 09_Lecutre - Lecture 9 EE114 Lecture 9 Dominant Pole...

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Lecture 9 EE114 R. Dutton, B. Murmann 1 EE114 (HO #13) Lecture 9 Dominant Pole Approximation Zero-Value Time Constant Analysis R. Dutton, B. Murmann Stanford University R. Dutton, B. Murmann 2 Motivation Last lecture we saw that the Miller approximation is a very useful tool that allows us to estimate the -3dB bandwidth of our CS stage quickly and intuitively Wouldn’t it be nice to have a similar technique of this kind that works for a broader class of circuits? The zero-value time constant (ZVTC) method is a tool that meets this demand We will continue to use the CS amplifier example to illustrate this method, along with its mathematical underpinnings EE114 (HO #13)

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Lecture 9 EE114 R. Dutton, B. Murmann 3 Analysis Revisited (1) EE114 (HO #13) C gs g m v gs + v gs - + v o - R v i /R i C db C gd R i 1 2 ( ) ( ) [ ] ( ) 2 2 1 0 2 1 1 1 1 s b s b z s a C C C C C C R R s RC R g R C C R C C s g C s R g ) s ( v ) s ( v v gd gs db gd db gs i gd i m i gd gs gd db m gd m i o + + ! " # \$ % & = + + + + + + + + ! ! " # \$ \$ % & = R. Dutton, B. Murmann 4 Analysis Revisited (2) We know that The transfer function of our circuit has a dominant pole that sets the -3dB bandwidth The non-dominant pole and zero have little influence on the -3dB bandwidth of the circuit Can we somehow use this fact to simplify the analysis? Without circuit-specific “tricks” like the Miller approximation EE114 (HO #13) 10 6 10 7 10 8 10 9 10 10 -60 -40 -20 0 20 f [Hz] | H ( f ) | [ d B ] 200 ) ] [ d e g ] Intrinsic cap only Intrinsic + extrinsic caps Dominant pole Non-dominant pole(s) and zero(s)
Lecture 9 EE114 R. Dutton, B. Murmann 5 Dominant Pole Approximation (1) If our goal is to estimate the -3dB frequency only, we can discard the zero and write EE114 (HO #13) 2 2 1 0 2 2 1 0 1 1 1 s b s b a s b s b z s a ) s ( v ) s ( v v v i o + + ! + + " # \$ % & ( = 2 1 2 1 0 2 1 2 2 1 0 2 1 0 1 1 1 1 p p s p s a p p s p s p s a p s p s a ) s ( v ) s ( v v v v i o + !

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