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12th March 2008, 10:42 AM  #1 
diyAudio Member
Join Date: Jan 2008

Which point is right? for stability simulation!!!
Hello All DiyAudio member
I have one question of the openloop gain simulation/Test in the previous discussing, I know how to simulate openloop stability, it should add an AC source just before a highimpedance 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 highimpedance 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 highimpedance 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 highimpedance 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 
diyAudio Member

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.
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12th March 2008, 05:05 PM  #3 
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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 SmallSignal Stability", IEEE Circuits and Devices Magazine, vol. 17, no. 1, pp. 3141, January 2001. " also the Intusoft Spice website has articles, examples of loop gain measurement 
13th March 2008, 03:37 AM  #4 
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!!! 
13th March 2008, 09:16 AM  #5  
diyAudio Member

Re: Which point is right? for stability simulation!!!
Quote:
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 lowerimpedance. Q.E.D. The "loop probe" method that has now been mentioned again, in this new thread, gives a much better approximation of the openloop response while still running the closedloop system. But its approximation still might not be valid above xx MHz, IIRC.
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13th March 2008, 10:20 AM  #6 
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 breakpoint 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 <<Ye is necessary for the current driving case to give a correct result. Generally, for an actual network, it may not be possible to find a break point that satisfies either of these extreme conditions. Thanks Gootee and JCX and DiyAudio member help me to solve this problem, I think I find the answer!!!!! Thanks you very much!! 
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