Lecture 07-Zero value time constant analysis

Lecture 07-Zero value time constant analysis - EE 214...

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Unformatted text preview: EE 214 Lecture 7 (HO#10) B. Murmann 1 Lecture 7 Design Example 2 (Continued) Zero-Value Time Constant Analysis Boris Murmann Stanford University murmann@stanford.edu Copyright 2004 by Boris Murmann EE 214 Lecture 7 (HO#10) B. Murmann 2 Overview Reading 7.3.0, 7.3.1 7.3.2 (Zero-Value Time Constant Analysis) 7.3.3 (Cascade Amplifier Frequency Response) Supplementary document "Bandwidth estimation techniques," by Tom Lee (optional, see website). Introduction Last lecture, we found that using a simple circuit model based on only intrinsic capacitance is not sufficient for accurate bandwidth prediction in our CS stage. However, having learned about the involved extrinsic capacitances and the "Miller effect," we are now in a position to improve our hand analysis and match the Spice result with good precision. After this refinement, we will take at look at the the "Zero-Value Time Constant Analysis" as an alternative to the Miller approximation. This method will turn out to be useful for a much broader class of circuits, beyond simple common source stages. EE 214 Lecture 7 (HO#10) B. Murmann 3 Design Example 2 Revisited Last lecture, we only considered C gs in our estimate Let's apply the above refined result to see how accurately we can match Spice Need estimates of C gd and C gb C gs +C gb + v gs- R i v i (1+g m R L )C gd ( ) [ ] gd L m gb gs i dB sC R g C C R f + + + 1 1 2 1 3 EE 214 Lecture 7 (HO#10) B. Murmann 4 Extrinsic Capacitance Estimates Estimating C gd is simple: fF . fF . m fF . W C gd 4 7 23 32 23 = = = How about C gb ? We can include C gb by using a modified transit frequency chart Previously, we used gs m T C g f 2 1 = Taking extrinsic capacitances into account, we can redefine gg m gd gb gs m T C g C C C g f 2 1 2 1 == + + = The chart on the following slide was generated with the Spice deck shown on slide 4, lecture 5, with the following modified line .probe ft = par('1/2/3.142*gmo(mn1)/(-cggbo (mn1))') EE 214 Lecture 7 (HO#10) B. Murmann 5 Modified Transit Frequency Chart 8 5 10 15 20 25 5 10 15 20 25 30 NMOS L=0.35um g m /I D [1/V] f T [GHz] Improved estimate using C gg Previous estimate 11.25 EE 214 Lecture 7 (HO#10) B. Murmann 6 Improved Bandwidth Estimate Using the transit frequency chart, we find...
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Lecture 07-Zero value time constant analysis - EE 214...

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