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Simulators >> Circuit Simulators >> Weird stability analysis results in a bandgap circuit
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Message started by thechopper on Jul 14^th, 2009, 6:00am

Title: Weird stability analysis results in a bandgap circuit
Post by thechopper on Jul 14^th, 2009, 6:00am

All,

I'm analyzing the stability of a band-gap reference circuit in a BICMOS process. The gain of loop circuit is well known and
equal to the product of [GM(Q0)/gm(M0)] and [gm(M1)/gm(Q1)]; where GM(Q0) is the composite transconductance of Q0 and the 2K resistor.
When running the stability analysis with a probe located in A I get the expected low freq gain of ~ -8.2dB, as shown in the figure.
However when I move the probe to B the DC gain and the high frequency response both change drastically.
As far as I know the location of the probe for running stability analysis is not important as long as I place it in a single loop that
at least can include local feedback loops, like this is the case (beta helper of Q1 is another feedback loop).

Any hints or clues about what is going on? Anything I might be missing?
Your comments are more than welcome.

Regards
Tosei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by thechopper on Aug 2^nd, 2009, 11:11am

Nobody can help on this one?

Title: Re: Weird stability analysis results in a bandgap circuit
Post by wave on Aug 3^rd, 2009, 4:21pm

HdrChopper wrote on Aug 2^nd, 2009, 11:11am:

Nobody can help on this one?

I have a hard time believing they are the *same* loop.
However, I can't quite convince myself why.

None the less, if the simulator indicates they are different, that is very likely the reason.

Wave

Title: Re: Weird stability analysis results in a bandgap circuit
Post by thechopper on Aug 4^th, 2009, 5:49pm

wave wrote on Aug 3^rd, 2009, 4:21pm:

I have a hard time believing they are the *same* loop.
However, I can't quite convince myself why.

Hi Wave,

Thanks for your reply. Why aren´t you convinced the loop is not the same?
Both source A and B are in "series" with the signal.
In any case if you believe they are not the same, which one would you choose for analyzing stability for the band-gap? and why? (you already know which one is correct. What I cannot get is why the other one is not.)

Thanks
Tosei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by ywguo on Aug 5^th, 2009, 9:24pm

Hi Tosei,

My simulation result is different from yours. Both A and B proves the same loop gain magnitude and phase response. The low-frequency loop gain is -8.37dB.

By the way, it seems that the 100 kOhm resistor can be removed. That has nothing effect on the function.

Best Regards,
Yawei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by raja.cedt on Aug 6^th, 2009, 7:44am

hi,

@Hi Tosei,
i simulated the ckt what you have given in hspice ...it is giving more or less same results in both cases

@Yawei,
how come you got same without 100k, i feel because of this resistor npn transistors are getting biased, correct me if am wrong

Thanks,
Rajasekhar.

Title: Re: Weird stability analysis results in a bandgap circuit
Post by thechopper on Aug 6^th, 2009, 7:12pm

Hi Yawei / Rajasekhar,

Thanks both for your inputs. Based on what you got I should assume there must be something wrong with my simulation set up, I guess?

Rajasekhar, I understand both cases should give exactly the same result, which is what Yawei got and I was expecting. However you said "more or less" the same result. What is the difference you observed?

Thanks and Regards
Tosei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by raja.cedt on Aug 6^th, 2009, 9:17pm

hi tosei,
i did this simulation in hspise...so i used to put LC filter at the point where i want break...so what i feel is while doing this some loading we have to take care (generally i keep total ckt wherever i want to adjust loading )..but this will not give much difference for this ckt....i got minute difference in the magnitude and no difference in the phase plot. In fact this expected because at A both sides are high impedance nodes..but at B one side is low impedance ........Let me know if any issue and thanks a lot because i learned a lot through this discussion

thanks,
Rajasekhar.

Title: Re: Weird stability analysis results in a bandgap circuit
Post by ywguo on Aug 9^th, 2009, 1:58am

raja.cedt wrote on Aug 6^th, 2009, 9:17pm:

Hi Rajasekhar, you are right. So spectre has a convenient way, stb analysis, to check the loop gain/phase response without breaking the loop. It adopts the method named return ratio in fact. Please search IEEE or Paul Gray's textbook for return ratio.

Best regards,
Yawei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by thechopper on Aug 9^th, 2009, 3:50pm

Hi Rajasekhar,

Actually the metod Yawei pointed out is the one I used for analyzing the stability of this circuit. My point is that according to this methodology theory the port which is used for computing the return ratio could be placed at any point in the loop. No loading effects has to be considered since the loop is not broken (therefore the loading conditions are intact).
However I got different results when placed at A or B.

Regards
Tosei

Title: Re: Weird stability analysis results in a bandgap circuit
Post by Frank Wiedmann on Aug 10^th, 2009, 12:52am

ywguo wrote on Aug 9^th, 2009, 1:58am:

You can find the simulation method used by the stb analysis in http://www.kenkundert.com/docs/cd2001-01.pdf. Some more information about loop gain simulation is available on my webpage http://sites.google.com/site/frankwiedmann/loopgain.

Regarding the differences between Tosei's and Yawei's results: It might be helpful if you would post your netlists so that you can see if there are any differences between your simulation setups.

Title: Re: Weird stability analysis results in a bandgap circuit
Post by nobody on Aug 10^th, 2009, 3:55am

Frank proposes a good way to solve this problem.
I believe Yawei simulated that ckt using Spectre and it is
complicated to do loop gain simulation by using Hspice.

Title: Re: Weird stability analysis results in a bandgap circuit
Post by raja.cedt on Aug 10^th, 2009, 6:56am

hi,
i am using hspice in office and may be i can use middle broke method in LT spice...if any body using hspice can you explain how you to do stability analysis in h spice......for my office work also i am using like LC method only

Thanks,
Rajasekhar.