Which point is right? for stability simulation!!! - diyAudio
 Which point is right? for stability simulation!!!
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 12th March 2008, 10:42 AM #1 mclarenpingu   diyAudio Member   Join Date: Jan 2008 Which point is right? for stability simulation!!! Hello All DiyAudio member I have one question of the open-loop gain simulation/Test in the previous discussing, I know how to simulate open-loop stability, it should add an AC source just before a high-impedance input, like OPAMP negative input side but this has one point that make me feel stranger why not add the AC source just before the B circuit (whole feedback loop) an amplifier feedback circuit (B circuit) always has divider resistor, error amp (OPAMP negative side) if we add the AC source just before the high-impedance input, it seem we ignore the divide resistor and capacitor why not add the AC source before the divide resistor, and don't break the "B" circuit This confuse me very much, I have simulate those two situation, if I put AC source before the divide resistor (not the high impedance node), also can call I put the AC source between the output and feedback side the result is very strange, like a mess but if I put AC source just before a high-impedance input, the result is perfect Could someone teach me what the root cause is?? in the simulation, that say "add an AC source just before a high-impedance input" is right but what the reason is?? thanks a lot, this will be a great help for me have a nice day!! Best Regard!!!
 12th March 2008, 11:47 AM #2 peranders   Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden You should have less than 180 degrees phase shift at 0 dB gain but you should strive have not more than 135 degrees and even better 90 degrees at 0 dB. __________________ /Per-Anders (my first name) or P-A as my friends call me PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.
 12th March 2008, 05:05 PM #3 jcx   diyAudio Member   Join Date: Feb 2003 Location: .. Read up on Tain and Middlebrook, join the LtSpice users group on Yahoo LtSpice example file "LoopGainProbe2": "This example is based on posts contributed by Frank Wiedmann to the independent users' group at http://groups.yahoo.com/group/LTspice [1] Michael Tian, V. Visvanathan, Jeffrey Hantgan, and Kenneth Kundert, "Striving for Small-Signal Stability", IEEE Circuits and Devices Magazine, vol. 17, no. 1, pp. 31-41, January 2001. " also the Intusoft Spice website has articles, examples of loop gain measurement
 13th March 2008, 03:37 AM #4 mclarenpingu   diyAudio Member   Join Date: Jan 2008 Hello Thanks your great help I have found the paper in http://www.kenkundert.com/pubs.html It seem great, if I have any what one I have learned I will share with everybody thanks a lot have a nice day!!!
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Re: Which point is right? for stability simulation!!!

Quote:
 Originally posted by mclarenpingu Hello All DiyAudio member I have one question of the open-loop gain simulation/Test in the previous discussing, I know how to simulate open-loop stability, it should add an AC source just before a high-impedance input, like OPAMP negative input side but this has one point that make me feel stranger why not add the AC source just before the B circuit (whole feedback loop) an amplifier feedback circuit (B circuit) always has divider resistor, error amp (OPAMP negative side) if we add the AC source just before the high-impedance input, it seem we ignore the divide resistor and capacitor why not add the AC source before the divide resistor, and don't break the "B" circuit This confuse me very much, I have simulate those two situation, if I put AC source before the divide resistor (not the high impedance node), also can call I put the AC source between the output and feedback side the result is very strange, like a mess but if I put AC source just before a high-impedance input, the result is perfect Could someone teach me what the root cause is?? in the simulation, that say "add an AC source just before a high-impedance input" is right but what the reason is?? thanks a lot, this will be a great help for me have a nice day!! Best Regard!!!
Remember that inserting the AC source is only a way to **approximate** actually breaking the loop, to try to examine the OPEN-loop response (even though the loop is actually still closed), as is needed for this type of stability analysis.

The approximation is 'better' if the AC source is inserted just before a very high impedance point in the loop. The approximation is 'worse' if the AC source is inserted before a lower-impedance. Q.E.D.

The "loop probe" method that has now been mentioned again, in this new thread, gives a much better approximation of the open-loop response while still running the closed-loop system. But its approximation still might not be valid above xx MHz, IIRC.
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 13th March 2008, 10:20 AM #6 mclarenpingu   diyAudio Member   Join Date: Jan 2008 Thanks a lot!!!! according the paper of Tian he said To analyze the stability of the feedback loop, it does not matter whether the designer views voltage or current as the signal of interest; both produce the same answer, as long as the dc impedances are not disturbed when the loop is broken. In practice, this requires careful selection of the break-point location. For the voltage driving case, the break point should be located where the impedance Yf looking backward from the break point is sufficiently smaller than the impedance Ye looking forward from the break point; the opposite condition,Yf <

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