OPA2227-EP: Op amp stability simulation (loop gain and phase margin)

Alassane GORY

Part Number: OPA2227-EP
Other Parts Discussed in Thread: OPA2227, TINA-TI

Hi,

I'm contacting you regarding my doubts about the simulation of phase margin i did to determine op amp stability and i also want to discuss about the calculation method of this phase margin. I actually have two concerns.

I'm using LTspice to measure the OPA2227 stability using the "big inductor/big capacitor" method given by TI the following presentation : opamps stability phase margin presentation

Here is the simulated circuit :

My first concern is that I followed the TI method however, the expected result of the open loop gain simulation is not exactly what is given in the datasheet. Although it's very close to the datasheet graph but, as you can see below, there is a 180 degrees shift that in the phase that was not expected :

OPA2227 OPEN-LOOP GAIN/PHASE vs FREQUENCY

OPA2227 OPEN-LOOP GAIN/PHASE vs FREQUENCY SIMULATION

My second concern is about determining the phase margin. I see in the litterature that the phase margin represents how far the phase is shifting from -180 degrees and so, it should be equal to 180 plus phase shift (PhaseMargin = phase -(-180) ) at 0db loop gain but in the TI presentation link given above, it's said that the phase margin is equal to the phase shift at 0db loop gain. Could you please tell me where does this comes from?

Could you also help me to determine where is that 180 degrees phase shift coming from in my simulation?

Thank you in advance.

5 months ago

+1 Marek Lis 5 months ago

TI__Guru 60820 points

Alassane, please see my answers below:

The phase might be shown as Phase Shift (see in datasheet) or Phase Margin (shown in your simulation) where Phase Margin = 180deg - Phase Shift [deg]

You may ask how do I know that your simulation shows Phase Margin - I know it's phase margin because it shows 180deg at DC (at DC there is no shift in phase) - see below.

How do I know AOL graph in OPA2227 datasheet shows Phase Shift? I know it's phase shift because at DC it shows 0 degrees - see below.

Phase Margin (PM) is a measure how close the system LoopGain is to 180 degrees phase shift (PS). Graphically, this occurs at the intersection of AOL and 1/beta curves where on linear scale LoopGain = 1 while on logarithmic scale LoopGain=0dB - see below.

Running AC stability simulation of your circuit shows 65 degrees phase margin whereas recommended minimum phase margin is 45 degrees - see below.

Thus, your system is very stable. How do I know the simulation shows phase margin and not phase shift? I know it's phase margin because its phase shows 180 deg at DC (at DC there is no phase shift).

Running small-signal transient simulation shows a small-signal overshoot of just 8% (see below) whereas the maximum recommended overshoot is 25%. This confirms that the system is robust from the point of view of circuit stability.