LM833: What is the meaning of Q at OP-AMP ? Quality Factor or Resonance frequency?

Hi Boseung,

Q: In this case, what is the exactly meaning of Q? Resonance frequency or Quality Factor or another ? I'm a little bit confusing of meaning of Q

In a closed op amp feedback system, op amp's high frequency should roll off at a rate of -20dB/decade or -6dB/Octave. If you have this type of roll off characteristics, the circuit will have good phase margin, and hence an op amp will be stable in a closed loop. I see that you removed 1pf compensation capacitor  in parallel with R2 resistor. 

Q factor has another name as gain peaking or quality factor,. In the circuit above, Q of the circuit is >1 (You will still observe gain peaking with Q=1 in a closed op amp loop). In another words, the circuit has 2nd poles generated in the closed-loop. -20dB/decade is a single pole roll off in a closed loop op amp. When Q is greater than 1, an op amp will roll off at a rate -40dB/decade. 

At approx. 843 kHz region, the Vout/Vin will be higher than in gain ratio or approx. 22.95dB (the gain is approx. 20dB prior to the Q). Your closed loop transfer function is Vo/Vin = Aol/(1+Aol*beta). If you observe a Q in the bode plot, Aol*beta at a specific frequency (843khz in this case) will be close to -1, where the circuit does not have adequate phase margin in the feedback loop. 

When op amp peaking is observed, you have to compensate it with a zero to cancel the additional pole. In this case, the additional pole is occurred around 843kHz. By adding 1pf capacitor, the additional pole can be canceled, and op amp's high frequency will continue to roll off at -20dB/decade. 

We have series of training videos that can identify op amp stability issues in a closed loop. There are 7 video series on the topic, see one of the link below. 

https://training.ti.com/ti-precision-labs-op-amps-stability-introduction?context=1139747-1139745-14685-1138805-13848  

Best,

Raymond

Sincerely, thank you for your answer

Best,

Boseung.