ho12.l09_zvtc

# ho12.l09_zvtc - Lecture 9 Dominant Pole Approximation...

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1 Lecture 9 Dominant Pole Approximation Zero-Value Time Constant Analysis R. Dutton, B. Murmann R. Dutton, B. Murmann 1 EE114 (HO #12) Stanford University 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 R. Dutton, B. Murmann 2 this method, along with its mathematical underpinnings EE114 (HO #12)

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2 Analysis Revisited (1) 2 1 1 C C C C C C R R RC R g R C C R C C g C s R g ) s ( v m gd m o + + + + + + + + = R. Dutton, B. Murmann 3 EE114 (HO #12) ( ) ( ) [ ] ( ) 2 2 1 0 1 1 s b s b z s a s s ) s ( v v gd gs db gd db gs i gd i m i gd gs gd db i + + = 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 0 20 ] Dominant pole Non-dominant pole(s) and ero(s R. Dutton, B. Murmann 4 EE114 (HO #12) 10 6 10 7 10 8 10 9 10 10 -60 -40 -20 f [Hz] |H(f)| [dB] Intrinsic cap only Intrinsic + extrinsic caps pole(s) and zero(s)
3 Dominant Pole Approximation (1) If our goal is to estimate the -3dB frequency only, we can discard the zero and write 1 s a 2 2 1 0 2 2 1 0 1 1 s b s b a s b s b z ) s ( v ) s ( v v v i o + + + + = 0 0 0 a a a ) s ( v Next, use the fact that |p 2 |>>|p 1 | , where p 2 is the non-dominant pole and p 1 is the dominant pole that sets the -3dB frequency R. Dutton, B. Murmann 5 EE114 (HO #12) 2 1 2 1 2 1 2 2 1 2 1 1 1 1 1 p p s p s p p s p s p s p s p s ) s ( v v v v i o + + =

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ho12.l09_zvtc - Lecture 9 Dominant Pole Approximation...

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