[SOLVED] Op amp stability (buffer/non-inverting)

[grabbo]

Hello,

I would like to find out the stability of the circuit of an op amp ( OPA2340). I included phase/gain characteristics and the circuit. It's connected as a buffer and non inverting.

For non-inverting:

Gain=3.3. Feedback factor F=0.23. K=20log(0.23)= -12.77dB

Gain 0db crosses at around 6MHz. With K=0.23, it's shifted downward by -12.77dB to approximately 2Mhz. Phase is -100.

For buffer:
Gain=1 , K=0dB, Phase=-110.

Am i right in these assumptions? Those phases are phases margins or responses ?
Any suggestions?

 

[saro_k_82]

Yes. So you would have phase margin roughly about 80deg and 70deg respectively, but wait. These are excluding any loads that they are going to drive. The ac plot assumes that there is no load (or whatever is provided in the data sheet). You will have to include your load in to the frequency response for both the amps, but I dont think the 5Hz filter in-between to affect the PM in your case.

 

[grabbo]

Thank you for response! System measures heart beat 0.5Hz-5Hz. Is it too low to include it ,isn't it. it shouldn't change anything in phase margin.

Phase margin for noninverting is 180-100=80 and buffer's 180-110=70. Is it enough for op amps in my system be stable during the whole time? I remember margin above 45 and below 75 is acceptable but desired is 65.
Would you recommend to change something with the non inverting, to lower phase margin?

 

[saro_k_82]

Higher phase margins are desirable. The only concern with high phase margins is the sluggish response. If you worry about a few percent of settling time in your application it would make sense to find an optimum phase margin, but as your application sounds, more phase margin would be better for you. If the output load is not overly capacitive you would be fine