Hi,Guys,I have some problem to understand why stb and ac do not match.When I run stb analyis in a loop,the phase starts from -180 and it increses to a positive value and gain curve is normal! When  I run ac analysis ,it seems like that phase and gain curve are normal  .Could someone tell me which one  simulation  is right?

Thanks any help in advacne

Besides the fact that your drawing is very hard to read (what is the part at the right side from C=1F ?), I assume that your ac analysis is run open loop, correct? In this case, the result shows that the dc feedback is positive (phase zero for s=0). A circuit with positive dc feedback cannot work because of an unstable bias point. Thus, something is wrong.

Xianweng, in your circuit diagrams the signal source is shown.
Question: Are you aware that it must be switched-off (Vin=0) during loop gain analysis?
As already mentioned, at dc you need negative feedback (phase=-180 deg).
But also the results of STB analysis are weird. The gain drops with rising frequency - and the phase function rises?
This is NOT correct.
Thus: Check the bias point for normal operation and don`t forget to ground the opamp input during open-loop analysis.

wave wrote on Nov 7^th, 2012, 2:00pm:


No, I don`t think so. As far as I know, the only difference is that the STB analysis also includes bi-directional signal travelling.
Thus, both should give identical results for uni-directional applications (as in our case).

wave wrote on Nov 7^th, 2012, 2:00pm:

You might want to take a look at http://www.designers-guide.org/Forum/YaBB.pl?num=1294178255 (and maybe http://www.designers-guide.org/Forum/YaBB.pl?num=1124688329).

buddypoor wrote on Nov 7^th, 2012, 7:17am:

1.the right side capacitor is 1F
2.the right side mos is pmos ,so it is negative feedback

Ken Kundert wrote on Nov 8^th, 2012, 12:24am:

I know the AC analysis approach does not give accurate reults ,but it could not give a wrong answer. And stb is more accurate.The ac analysis approach I used is describe in the book:designing_analog_chips.I attached the picture that the book used

xianweng wrote on Nov 8^th, 2012, 6:16am:


This resistor does not enable a correct operating point. Instead you must use a big inductor allowing dc feedback.

Ken Kundert wrote on Nov 8^th, 2012, 9:23pm:


OK agreed, I should have been more specific.
As mentioned somewhat later in my former posting - I consider a loop gain simulation as "correct" under the following pre-conditions:
*restauration of the correct dc bias point
*proper loading at the breakpoint (assuming uni-directional signal propagation).

I think, these conditions are fulfilled by Middlebrook´s double-injection technique.
However, for many opamp applications I have learned that an opening at the opamp output and injection of a series ac signal (without correction using a current source in an second step) gives results, which are exact enough.

Let me define "exact enough":
Knowing that

* each amplifier simulation model never can reflect real conditions
* in reality each passive part has tolerances
* each hardware realization suffers from parasitic influences

I think, the procedure to find the loop gain for a circuit with feedback may allow for a small systematic error - if this error remains within the limits set by the aforementioned uncertainties (other error sources).
As an engineer I follow the rule: A design as well as a measurement procedure must not be as exact as possible but as exact as necessary
(but I know that in some cases it is not easy to specify what really is "necessary").  

I hope now you understand why I ask for justification of your statement "ac analysis approach ...should never be used".
Thank you and regards.
B.