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Design >> Analog Design >> Basic question about stb analysis in spectre
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Message started by XY-oriented on Jun 5^th, 2008, 3:46am

Title: Basic question about stb analysis in spectre
Post by XY-oriented on Jun 5^th, 2008, 3:46am

hi all,
I have designed a simple 2 stage opamp to control a bandgap reference ...
the op-amp taken alone apear to be stable when I do open loop ac anlysis, but when I put it in the bandgap circuit stability analysis with spectre tells me that the loop PM is about 7 degree...
so my question is : is the open loop stability criteria (PM > 45dgree )still applay in the closed loop case ? is my circuit stable ? :-/

Title: Re: Basic question about stb analysis in spectre
Post by thechopper on Jun 6^th, 2008, 8:36pm

If you want to know whether your opamp is stable or not you should simulate the open loop ac analysis with the actual loads and input impedances such opamp will be driving when closing the loop.
The stability analysis should always be performed on the open loop and not in the closed loop. Therefore stability criteria apply for open loops only, you cannot get PM and GM information if the loop is already closed.

Tosei

Title: Re: Basic question about stb analysis in spectre
Post by Ken Kundert on Jun 8^th, 2008, 11:20pm

Actually, you should never open the loop. Using the stb analysis on the closed loop is far superior to manually opening the loop as it mathematically opens the loop without changing the circuit. If you manually open the loop you change the circuit in ways that are very difficult to compensate for, and so you invariable get inaccurate results.

-Ken

Title: Re: Basic question about stb analysis in spectre
Post by buddypoor on Jun 10^th, 2008, 8:14am

To "complete the confusion" here is my summary:

Both, Tosei and Ken, are right - however, perhaps their contributions sound contradictionary, but they aren´t !
Phase as well as gain margins are defined only for the open loop system (of course, otherwise no input/output can be defined for loop gain). However, it is best and sometimes necessary to measure/simulate it in a closed loop configuration. For this purpose,Middlebrook has recommended a specific simulation topologie. For simple cases (when there is no severe load problem) you can simply place a series voltage source within the feedback path and let calculate Vout/Vin.