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stb break loop location (Read 2044 times)

newic

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Posts: 138

stb break loop location
May 19^th, 2015, 12:23am

hi

for the STB analysis in the circuit below, I have defined the 3 location that we can break the loop (break1, break2 & break3). All these gave me the same gain/phase plot. However when i use AC method (non-STB), it give different results in break1 & break3. The break2 shows the same result as in the STB. The STB can break in any locations as long as the loop is break?
can someone explain why?

the ac method is the one to break the loop by big R-C or L-C.

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newic Senior Member Offline Posts: 138	Re: stb break loop location Reply #1 - May 19^th, 2015, 12:25am picture
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newic Senior Member Offline Posts: 138	Re: stb break loop location Reply #2 - May 19^th, 2015, 12:25am picture
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newic Senior Member Offline Posts: 138	Re: stb break loop location Reply #3 - May 19^th, 2015, 12:26am picture
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Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley	Re: stb break loop location Reply #4 - May 19^th, 2015, 7:35am The "AC method" as you call it is wrong, don't use it. -Ken
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newic

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Re: stb break loop location
Reply #5 - May 19^th, 2015, 5:56pm

what shall i called for those using non-STB method?

one more question, according to Jacob Baker famous analog book, it uses simple RC to break the loop. Furthermore it inject ac at the non-feedback path. The result should be similar to STB method if really big RC is used and include the loading after the breaking point right. any comment?

i think most ppl using this method due to his book since it is so famous.
any comment?

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Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley	Re: stb break loop location Reply #6 - May 20^th, 2015, 8:54pm It is not accurate. Do not use it. -Ken
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ywguo

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Shanghai, PRC

Re: stb break loop location
Reply #7 - Jul 4^th, 2015, 7:55pm

Hi newic,

Probably in your case the inverting input of the amplifier has minor effect on the loop gain. So gain and phase response of AC analysis is similar with that of STB analysis. However, even the inverting input amplifier may affect the bandwidth for some wide-band application. Do NOT use AC analysis instead of STB.

Yawei

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newic Senior Member Offline Posts: 138	Re: stb break loop location Reply #8 - Jul 27^th, 2015, 12:43am how to simulate stb in hspice?
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Frank Wiedmann Community Fellow Offline Posts: 677 Munich, Germany	Re: stb break loop location Reply #9 - Jul 28^th, 2015, 1:35am Search the manual for .LSTB (Loop Stability Analysis).
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