Hi, I used tina to simulate OPAMP stability.
As you can see above, when i simulated without R1, phase goes to positive. I wonder why this happened.
Also, When i added R1, definitely this system got stable, but If i understood phase margin correctly, phase margin got lower than when there is no R1.
I wonder why there two thing happened.
Thanks,
Best regards,
Yunsik
Yunsik,
A large load capacitor, CL, at the output of OPA333 interacts with the internal open-loop output impedance forming a pole, which decreases the phase margin. If the pole occurs within the effective bandwidth of the circuit configuration, it will lead to oscillation. Adding a series output resistor outside the feedback loop, Riso, cancels the pole and thus increases the phase margin, stabilizing the circuit. There is several ways to analyze the circuit for stability - one method looks at the rate of closure between the open-loop gain, AOL, and 1/beta (close-loop gain) - circuit is stable if the rate of closure is 20dB -see left column of AC graphs below. Also, you may look at the SMALL-signal overshoot (10mV) where overshoot of 25% or less assure the circuit stability - see right column of transient plots. I have run several simulations for 5nF load with various Riso resistors and it takes Riso resistor of 700ohm or greater to assure stability of your circuit - to be on the safe side, I would recommend to use 1kohm to drive 5nF load.
Below please find the TI Precision Labs link that explains the different methods of stabilizing the circuit. I also attached the circuit schematic you may use for your own simulation.
