SPICEing Op Amp Stability

RE: SPICEing Op Amp Stability

James Kim — Thu, 26 Aug 2021 12:31:26 GMT

My suggestion for improving the stability and for the same frequency bandwidth of this circuit is shunting the R2 concrete contractor resistor with 27pF capacitor and don't use the input R3, C1 filter.

RE: SPICEing Op Amp Stability

Soufiane Bendaoud — Fri, 09 Nov 2012 04:08:01 GMT

Agreed Collin. I simply use a very large cap (Pspice doesn't care) in series with the inverting input and and a 100k feedback (think differentiator) and hit it with an AC. I can then see the dominant pole with any kind peaking.

RE: SPICEing Op Amp Stability

Stephen Power — Tue, 30 Oct 2012 10:53:49 GMT

The Harris app note is freely available on the Intersil website at www.intersil.com/.../an9536.pdf.

RE: SPICEing Op Amp Stability

Harald7170 — Fri, 26 Oct 2012 16:07:10 GMT

@ Clyde: try http://www.pdfunlock.com/

RE: SPICEing Op Amp Stability

Collin Wells — Tue, 25 Sep 2012 23:23:30 GMT

@ Otto:

Hello, I see your concern with the RC filter on the input and agree that if you drive the input signal through the RC filter before it reached the non-inverting input then you would not effectively test the stability of the circuit because the RC filter will dampen the step input. However, if you place the step input directly on the non-inverting input as shown in the second schematic in the blog then the input will see the actual step input and the overshoot of the output will provide a very accurate representation of the circuit phase-margin once you correlate the overshoot to phase-margin using the final image in the blog. This is very effective in SPICE if you're interested in quickly determining if your circuit may have stability issues. In the real world on the bench there will be some non-idealities due to the non-zero ramp time of your function generator (and perhaps even a bit of overshoot out of the function generator) but this test will still provide the easiest method to test the stability of your OPA design.

RE: SPICEing Op Amp Stability

Torsten Dreisbach — Mon, 24 Sep 2012 21:49:27 GMT

If I see that right, all this measures the input to output transfer. I do not see the inner loop, more like a black box.

The feedback loop is closed by the divider R1/R2. When SPICE is doing an AC analysis, it applies short circuits to inductors and open circuit to capacitors. So inserting a very large inductor in between the OpAmp resistive divider and R2 would still result in a good operating point (the ESR of the inductor should be a few Ohms). Now I have a node - Inductor/R2 - which I can connect an AC source to with a very large capacitor value. This source would be blanked out at operating point calculation due to the capacitor.

Now you can set the AC value of that source to '1' - a theoretical calculation will take place, which does not know saturation. With '1' at the AC source, the Opamp output node is the inner feedback loop. (Vtest=0!)

This is how I would do such a simulation to have an information about stability - So far I did that with OpAmp/MOSFET current sources. My experience is that many sources looked critical but stable, but in reality they were not stable... I had to apply a compensation and with much margin in the loop the simulation did a good job.

What you think aboit that?

RE: SPICEing Op Amp Stability

Jim Luschen — Mon, 24 Sep 2012 19:53:03 GMT

I just accessed the files Time Sobering placed in his profile, I had no problems at all downloading them. The Harris Ap Note looks quite good. Thanks Tim!

RE: SPICEing Op Amp Stability

Tim Sobering — Mon, 24 Sep 2012 18:20:15 GMT

Clyde...I put the files in my profile. I'm assuming you can access them...never done that before.

Tim

RE: SPICEing Op Amp Stability

Edmunds Sinevics — Mon, 24 Sep 2012 17:42:10 GMT

My suggestion for improving the stability and for the same frequency bandwidth of this circuit is shunting the R2 resistor with 27pF capacitor and don't use the input R3, C1 filter.

RE: SPICEing Op Amp Stability

Clyde Manning — Mon, 24 Sep 2012 16:51:32 GMT

I have downloaded both referenced app notes and bot come as as (secured) pdf files which will not let me print or save them.

Any Iideas?

RE: SPICEing Op Amp Stability

Tim Sobering — Mon, 24 Sep 2012 12:43:50 GMT

Thanks Simon. I've emailed out quite a few so there seems to be some interest. This raises a great point...old databooks and app notes are still golden. The newest generation of students grew up without these books, and I think it has an effect. Remember flipping through a databook to learn what else a manufacturer had to offer? Or looking at the many pages of example circuits? The web is wonderful for fast access to datasheets (remember faxback services :) ), but it can also obscure information if you don't know what to ask for.

RE: SPICEing Op Amp Stability

Simon Bagley — Mon, 24 Sep 2012 10:20:26 GMT

For anyone interested, I've found the Harris application note here:

www.datasheetarchive.com/AN9536-datasheet.html

RE: SPICEing Op Amp Stability

Otto Hunt — Sun, 23 Sep 2012 23:08:00 GMT

I would argue that this is not a true test of circuit stability because the + op amp input will never see an ideal step. The RC input filter has a time constant of 20 ns. In fact, the only time op amps ever see a ideal step inputs are in SPICE simulations.

RE: SPICEing Op Amp Stability

Tim Sobering — Sun, 23 Sep 2012 18:02:32 GMT

There is a really great app note on how to fake out Pspice to accurately give you Open Loop Gain, Beta, Noise Gain (1/Beta), and Loop Gain (AvB) -- "PSPICE Performs Op Amp Open Loop Stability Analysis (HA5112)" Harris AN9536.1. This makes stability analysis easier and you can get the location of the poles and zeros, which helps with compensation. I can't find it on the web but let me know and I can email it to you.