Hi,

I am simulating the AC response of a unity-gain feedback amplifier using Spectre stb analysis. It works fine for most corners. But for some corners such as slow-fast(t=-40,125; vdda=2.05), the loop phase suddenly change to 0 degree at DC (it's supposed to be 180 degree). That means positive feedback! It's so werid. Is it a problem of spectre or a problem of the circuit? The simulation includes post-layout extraction.

Thanks
Sapphire

This behavior usually is due to some asymmetry in a differential circuit. In your case, this asymmetry probably comes from the post-layout extraction. I have also seen this behavior during mismatch Monte Carlo analysis.

The attached pdf file shows a simple circuit that duplicates this behavior. It models an amplifier with some coupling from the output to the positive input. Depending on the amount of coupling, you get either 180 degrees or 0 degrees at low frequencies.

Like ssahl already mentioned, this behavior usually does not indicate a stability problem. To convince yourself (or others), you can use the Nyquist stability criterion (see http://www.designers-guide.org/Forum/YaBB.pl?num=1244840188 and http://www.designers-guide.org/Forum/YaBB.pl?num=1217822985).

In my example circuit, the strange behavior is due to the fact that there is a second loop in the circuit that does not include the stb probe Vprobe. If I redraw the circuit so that Vprobe is part of both loops (see the second circuit in the attached pdf file), the strange behavior disappears.

No, it's not a bug in stb analysis. It's an artifact due to the fact that there are other loops in the circuit that do not include the stb probe. If you take my example circuits and do a detailed analysis with Spectre (or on paper), you will see how this can happen. Please see http://www.designers-guide.org/Forum/YaBB.pl?num=1201763532 for the limitations of stb analysis with respect to multiple loops.

Here's how you can analyze my example circuit: Because the impedance looking forward from Vprobe (into the controlling pin of Gota) is infinite, the loop gain is equal to the voltage loop gain in this case. The voltage loop gain is the ratio V(outp)/V(inn) when Vprobe is the only active independent source in the circuit. So, you can set Vin to 0V and Vprobe (DC and AC amplitudes) to 1V and do dc and ac analyses. If you annotate the DC node voltages to the circuit, you will quickly see why you get different positive and negative values for the DC loop gain when you vary the gain of the controlled source Ep.

If you do Nyquist plots of the loop gain data shown (which were generated by stb analysis), they will all indicate that the circuit is stable.

This is the second graph

Hi Frank,
      Thank you for the links and the information. I better understand this now. After looking into this now I have another question, The nyquist plot with the gain of 1.01m encircles the +1 point in the anticlockwise direction, this says that the loop gain has a Right half plane Pole but from the derivation of the loop gain expression:

-Vo/Vi = RGm/[(1+GRGm)+sRC]

it is clear that there is no RHP pole.  So that means the creation of the RHP Pole is an artifact of applying the stb analysis in a multiple loop system without breaking all the loops. Is my interpretation correct?

Thanks,

hello milind,

1. Nyquist criterion says closed loop[b] RHP-open loop RHP=clockwise encirclement[/b], So if you know about open loop RHP, number of encirclement's then you could comment on stability. But here you don't know about your open loop then you can't determine stability.

2. You have ignored few parasitic's like cgd which creates RHP zero and many like this.  What is your circuits.


Thanks,
Raj.

raja.cedt wrote on Mar 14^th, 2013, 1:39pm:


Hi Raj,
        I am just analyzing and simulating the circuit given by Frank in the earlier post. That does not have any parasitics it is a simple single pole circuit. I derived its loop gain based on the stb setup shown in the pdf which produces the problem of 0 phase at DC.
  Since my nyquist plot is encircling +1 which is equivalent to encircling -1 if I correct for the sign by stb analysis, that means an anticlockwise encirclement of the -1 point which in turn means my contour in the s plane contains a pole. Since my contour encircles the Right Half s-Plane that leads me to conclude that I have a pole in the right half plane.

Milind