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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Comparison between Middlebrook analysis and Spectr https://designers-guide.org/forum/YaBB.pl?num=1142633082 Message started by ee484 on Mar 17th, 2006, 2:04pm |
Title: Comparison between Middlebrook analysis and Spectr Post by ee484 on Mar 17th, 2006, 2:04pm Dear all, I have a question about why the loop gain resluts from Middle brook analysis and Spectre embeded are different. Both seem to agree each other well up to 1GHz (in my case), but at higher frequencies they look different. Any comments are welcome!!! How other analog experts do analyze the loop gain? Do they just do the time-domain (such as a step response) analysis? or do they just find closed loop response and infer the phase margin information from it?? I am attaching zip file showing simulation setup and results. Thanks in advance! ee484 :-/ |
Title: Re: Comparison between Middlebrook analysis and Sp Post by sheldon on Mar 19th, 2006, 7:24am ee484, Quoting from "Striving for Small-Signal Stability", M. Tian, et al., Circuits and Devices Magazine, Jan. 2001, page 31-41 The return-loop model used in Middlebrook’s approach implicitly assumes that signals flow through the feedback loop unilaterally. This is a reasonable assumption for most low-frequency applications. The issue is that at high frequencies the loading and the reverse return ratio may effect the results. If you look at section 3.4.1.4, Feedback Parameters of Real Circuits, in Ken's book, it discusses using four parameter characterization of blocks[z-parameter, g-parameter, ...] to isolate the contribution to the loop gain from the op-amp from the contribution to the loop gain due to loading, ... This approach correctly accounts for high frequency effects and should produce results consistent with the results from the approach implemented in the Spectre's stability analysis. Best Regards, Sheldon |
Title: Re: Comparison between Middlebrook analysis and Sp Post by ee484 on Mar 19th, 2006, 8:08am I see... I am a beginner is this area and guess many many things to learn. (I am also a new to this site) So, the bottom line is Middlebrook's method works well in low-frequencies. However, in high frequencies, other loading effect with bilateral effect should be accounted. Thank you for your explanation PS: Just wondering..if I simulate the circuit with loading effect, then bilater effect also gets counted? I guess the answer is no. Then, if Middlebrook's method fails because it doesn't account for bilateral effect. Just counting loading effect will solve this problem? |
Title: Re: Comparison between Middlebrook analysis and Sp Post by sheldon on Mar 19th, 2006, 4:09pm ee484, First just to be clear, the loading effect refers to parasitic capacitances "loading" the feedback network and modifying the return ratio as a function of frequency. The classic example is the board capacitance at the inverting input of the op-amp and the gain setting resistor in the feedback network. At high frequencies the stray capacitance shorts the gain setting resistor, creating a zero in the return ratio at high frequencies. Don't know if it is still in print but the Analog Devices High Speed Seminar used to have a detailed discussion of this issue. If you want to eliminate this effect then use a the ideal component method of setting gain described in Ken's book. For example, by replacing resistor feedback with a voltage-controlled, voltage-source to set the return ratio creates a unilateral feedback path. So if you use an ideal component to feedback the output, then you can create a unitlateral return ratio. In this case, the Middlebrook results should correlate better with Spectre's stability analysis. Best Case, Art Schaldenbrand |
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