THS4031: Stability Analysis for circuit used THS4031 on PSpice circuit simulation

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Good morning Yamamoto-san,

I will let the product group respond on the Middlebrook methodology, but here is a simpler analysis that is normally quite effective for VFA op amps, 

I was talking to Dr. Sergio Franco while I was doing this article and he had just written a really good article on this vs the Rosenstark method that  I put in the references - these are very thorough approaches that should yield exact results - however, since the TINA models are only typical with wide spread around that, exact analysis results are not to be taken literally to physical results - so approx. results are often quite suitable, 

Anyway, couple of comments first, 

1. Be sure to include your load, that can strongly influence phase margin

2. We normally only look at phase margin, in op amps it rarely (never) happens that a good phase margin design will have poor gain margin. 

Ok, 1st, let's check the model Aol curve, this is a 1999 model with no apparent update, The specified load of 150ohm (figure 11 in datasheet) pulls this back from an open load to a 105Mhz gain bandwidth product, I get 115Mhz open load, 

And then this is the LG approach I use, those large LC simply isolate the core op amp to a good DC point before the AC run starts, be sure to include the input C at the summing junction - that becomes important for large R values in feedback, frankly - that simple 1.5pF in the specs seems too low and simple - normally looking for a Cdiff and Ccm value. 

This nominal phase margin of 67deg looks very good, from some other work I was doing back when writing this stability article, I was able to map phase margin to bandwidth extension over LG xover F. At 67deg, that is about 1.57X. Doing that estimates a closed loop small signal from this 33.73MHz LG=0dB xover to a 53MHz closed loop SSBW. Doing that simulation gives a very close match - getting 54Mhz in the closed loop sim. 

I am using Figure 4 in this article to make this estimate, 

https://www.planetanalog.com/stability-issues-for-high-speed-amplifiers-introductory-background-and-improved-analysis-insight-5/#

Let me clarify what I mean by approximate. This curve comes from an exact analysis of a 2nd order LG gain - so if the LG is in fact 2nd order, it is exact - since none are, it is approximate - but often very close. 

Hello,

A lot of useful info provided here by Michael. In addition to this, I've provided below the no-load analysis (although load should be added) to use for comparison purposes to the middlebrook method that was initially provided. A phase margin of 73.25 degrees was found. 

THS4031_stability.TSC

Best,

Hasan Babiker

Hello,

Thank you for your useful comments.
I will execute circuit simulation including load circuit at next morning.

My circuit simulation environment is OrCAD PSpice.
Therefore, I created simulation circuit with OrCAD PSpice based on
the contents of figure 10 on the reference material JAJA097.
(This is the circuit diagram posted by Yamamoto-san)

Would you please comment if there are any mistakes in the simulation circuit
with OrCAD PSpice that I created?

Figure 10 on the reference material JAJA097 have been added below as reference.

MESH

Yes Hasan, I was wondering why I did not get 73deg? I had missed that added 29.86 resistor in the gain path slightly decreasing the noise gain, if I make these changes (also right V+ R and Rload = 150ohm as Aol plot shows), yea - right at about 74deg now. This matches the original simulation using the MIddlebrook approach way at the top, so that would suggest that is working well. Going back to that figure 4, a 74deg phase margin is a closed loop BW extension of 1.38 Fxover, or about an F-3dB of 1.38*38MHz = 52.4Mhz. 

Hello, Michael

Thank you for helping me.

It was very helpful for me to have the verification performed with another circuit simulation model.

Regards,

MESH

Hello, Hasan

Thank you for your reply.

Your reply and simulation result are very helpful for me.

Regards,

MESH