I testing an inverting op amp circuit in LTspice.
First I observed the graphic for the gain, and calculted the bandwidth of the circuit that is 11995Hz.
Then I did a simulation at that frequency, and I was very intrigued with the result.
I thought that the saturation limit was 15 V however when I put an input amplitude of 0.15 the output amplitude is 4.2V and the form of the signal is a sine, but as I increase the value of the input amplitude the output amplitude does not change but the form of the signal becomes a triangle.
I do not understand how the phenomenon occurs.
Best Answer
You've discovered the phenomenon called "slew rate". The small signal bandwidth is exactly that, for small signals.
When the amplitude gets larger, there's a large signal phenomenon called slew rate that limits the dv/dt of the output voltage.
The reason for this is the compensation cap that forms the dominant pole in the amplifier. There's a limited amount of current to charge and discharge the compensation capacitor, so when the input voltage dv/dt exceeds what the current source can supply to the compensation cap, the output slews at a rate governed by I=C*dv/dt. This makes a sine into a triangle at some value of frequency and amplitude.
The derivative of Asin(wt) is Awcos(wt) and the maximum of the derivative (max rate of change) is Aw so A*w must be less than the rated slew rate of the amplifier to avoid any slew rate distortion. [Edited to correct initial brain fade.]
Here's a link