Electronic – How to test op amp stability

operational-amplifieroscillationstress-testingtesttesting

I've got a circuit I'm developing using an op amp to drive a power MOSFET for an electronic load:

enter image description here

I'd like to test it for stability, but I don't have enough experience to know what perturbations are most likely to show any instabilities.

I have the following ideas:

  • Inject a square wave (offset such that low value is >= 0V) into the non-inverting input while monitoring the voltage on the sense resistor (load current waveform). This would simulate sudden changes in the control voltage (1V/A) likely to be supplied later by a DAC.

  • Apply stepped input voltage to IN+, simulating connection of power supply under test.

Are these sensible ideas that are likely to uncover any op-amp related instabilities?

Also, are there LTSpice simulation exercises that might be worth trying?

UPDATE:

  • I revised the schematic to be more suitable for simulation. I found and used models for the specific parts I'm using and removed all compensation elements for a baseline simulation.

New schematic

  • I ran an analysis feeding a DC-offset square wave into the control pin (non-inverting input in this circuit). The result was dead stable. There's a teeeny-weeeny 1mV overshoot on the rise if you zoom way in, otherwise it absolutely mirrors the input (except it represents current flow of course). I even reduced the rise time to 1ns to see if I could get it to ring, but no luck 🙂

plot

  • I ran an .AC analysis like @Kevin White suggested, and found a 62 degree phase margin in the open loop gain.

  • I built it up as @mkeith suggested, and unfortunately it oscillates like crazy on the breadboard 🙂 I was able to make some progress stabilizing it a bit until I accidentally blew out my MOSFET. It's down to the store tomorrow to get a new one so I can carry on from there 🙂

Best Answer

Your methods are fine. What you are looking for in the step response is overshoot or undershoot. What you'd like to see is something approaching critical damping for fast response and good phase margin. Graph borrowed from this SMPS app note.

enter image description here

Your circuit has a strong potential to oscillate- I would definitely add a compensation loop around the amplifier.