As shown in Figure 2-9, the slope of the open-loop gain curve changes from 6 dB per octave to 12 dB per octave at fc2. The phase lag also increases further by another 45 degrees. This phase lag does not cause any problem when fc2 is higher than the unity gain cross frequency (fT). However, even when fc2 is lower than fT, care should be taken when using op-amps as unity gain amplifiers such as voltage followers. (If the datasheet for an op-amp states that it can be used with unity gain, it has a second pole at a frequency higher than fT.)
In order to avoid abnormal oscillation, an op-amp should be used in the frequency range (fc to fc2) in which the open-loop gain decreases at a rate of 6 dB per octave. Note, however, that, at a frequency close to fc2, an op-amp is affected by a second pole, causing a power loss and a phase delay. To avoid its effects completely, the closed-loop bandwidth (fCL) should be less than one-fifth of fc2.
The above is a discussion on the oscillation of an op-amp itself.
It is also necessary to ensure that external circuitry is also free from oscillation (e.g., phase delay).
For example, this consideration applies to an application in which a capacitive load is driven by an op-amp. Oscillation occurs if the cut-off frequency due to the capacitive load is within the range in which the loop gain is greater than 1. In order to prevent oscillation, it is necessary, for example, to add a resistor in series with a capacitor. Even when an op-amp is not connected with a load, care should be taken as to wire or other capacitance. Minimize the length of the wire from the op-amp output to the subsequent device and that of the feedback loop.