OPA205: The phase of the open-loop gain

For all op amps the phase shift near DC is 180°.  This is accounting for the inversion, so the open-loop test signal is applied to the inverting input.  At low frequency the op amp has a single pole (sometimes called the "dominant pole").  The dominant pole introduces a 90º phase shift.  The phase shift for the dominant pole will at a frequency one decade below the pole and finish one decade after the pole for a total phase shift of 90º.  Near the unity gain bandwidth (where AOL = 0dB), most op amps will have additional poles and zeros.  The additional poles and zeros will make further changes in the phase.  In this example, and in most op amps the phase begins to drop below 90º near the unity gain frequency. This Stability app note covers a simple model of an op amp in section 7.  The effect of the dominant and secondary poles and zeros on phase is covered in more detail.  If you review many op amp data sheets you will see a plot with a similar phase to what is shown here.  Note that the graph here makes the phase look like it starts at 200º.  This is really a rounding error and the phase actually starts at 180°.

 Best regards, Art