If the slew rate is low for the frequency used, the square wave will become a trapezoidal wave and the sine wave will become a triangular wave.

Voltage gain decreases at frequencies above the 3dB cutoff frequency of the open-loop gain. In this part, not only does the voltage gain factor vary from product to product, but if a signal with a wide bandwidth is input, the gradient of the amplification factor causes waveform distortion. Please apply negative feedback to the operational amplifier so that the gain is flat up to the operating band.

Also, be careful of oscillations at frequencies close to unity gain. There is an explanation in the op amp e-learning 2.3. Oscillation. Oscillation. please refer.

the op amp e-learning 2.3. Oscillation

- Slew rate (SR):

Even when the ideal rectangular waveform (a fast-rising signal) is applied to the input of an op-amp, its output does not provide the ideal rectangular waveform as shown in Fig. 3.

A change in output voltage per 1 μs is called a slew rate.

As shown in Fig. 3, in the case of an op-amp with a low slew rate, a rectangular input signal appears as a trapezoidal signal at the output, and a sinusoidal input appears as a triangular signal at the output.

- Unity Gain Cross Frequency (f
_{T}):

Frequency at which the open-loop gain becomes unity gain (0 dB)

As shown in Fig. 2, an op-amp with a higher cut-off frequency provides a greater bandwidth with the same closed-loop gain.

If you use an op amp with a low slew rate, the shape of the waveform will change and distortion will worsen.

Its maximum output frequency (f_{max}) can be calculated from the slew rate.For simplicity, we will first explain the case of using op-amp in unity gain.

The slew rate (SR) is expressed as:

SR = ΔV_{o} / Δt_{r} or ΔV_{o}/ Δt_{f}

Waveform distortion occurs when the maximum value of the output signal differentiated by time becomes higher than this SR.

Let the voltage of the input sine wave with an amplitude of A be Vin = Asinωt. Then, no waveform distortion occurs if the maximum change (differential value) in the amplitude of the input signal is less than the slew rate.

Since the differential value of the input signal is dVin / dt = Aωcosωt, its maximum value is Aω. Therefore, the maximum frequency that does not cause slew-inducted waveform distortion is theoretically calculated as follows, where A is the input amplitude.

SR = Aω ＝2πf_{max}A

f_{max} = SR / 2πA

If there is no unity gain, the voltage gain of the amplification circuit is multiplied by A_{v}, then V_{o} = A_{v} x V_{in}.

The same can be said for unity gain.

dVout / dt = AvAω cosωt

SR = AvAω =A_{v}A2πfmax

fmax = SR / 2πAA_{v}

In practice, it is necessary to set f_{max}, allowing an adequate margin for the required frequency.

The following documents also contain related information:

A new window will open