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What is the maximum frequency at which an op-amp can be used?

To use an op-amp at high frequency, it is necessary to take two factors into consideration: slew rate and cut-off frequency (unity gain).

• 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 Figure 3.

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

As shown in Figure 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.

• Cut-off frequency (fT): 

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

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

Figure 1 Definition of slew rate
Figure 1 Definition of slew rate

Figure 2 Cut-off frequency and bandwidth
Figure 2 Cut-off frequency and bandwidth

Figure 3 Slew-induced waveform distortion
Figure 3 Slew-induced waveform distortion

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 (fmax) 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 = ΔVo / Δtr   or   ΔVo/ Δtf

 

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 =2πfmaxA

fmax = SR / 2πA

 

If there is no unity gain, the voltage gain of the amplification circuit is multiplied by Av, then Vo = Av x Vin.

The same can be said for unity gain.

dVout / dt = AvAω cosωt

 SR = AvAω =AvA2πfmax

fmax = SR / 2πAAv

In practice, it is necessary to set fmax, allowing an adequate margin for the required frequency.

Relationship between slew rate and signal frequency
Relationship between slew rate and signal frequency

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