Amplitude influence in the output signal in inverting op-amp

Question

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.

Answer 1

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.]

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