Slew Rate of Op Amp

We have discussed what is slew rate of op amp in the previous section in brief. But today we are going to learn about the get the deep knowledge about the slew rate of op-amp. So let’s start from the basic definition of slew rate.

Slew Rate of Op Amp

Slew rate is defined as the maximum rate of change of output voltage per unit time as it is expressed in volts/microseconds.

\mathbf{SR=\frac{d V_o }{d t} |_{maximum} \,Volts/\mu sec}

Importance of Slew Rate of Op Amp

The slew rate decides the capability of the op-amp to change its output rapidly, hence it decides the highest frequency of operation of a given op-amp.

The slew rate changes with the change in voltage gain. therefore it is generally specified at unity (+1) gain.

The slew rate should be ideally ∞ and practically as high as possible. The slew rate of op-amp 741 is only 0.5 V/µs, which is its biggest drawback. Therefore it can not be used for high frequency applications.

The circuit used for slew rate measurement is shown in the below figure also the input-output voltage waveforms are shown,

The slew rate is measured from the output voltage waveform as:

\mathbf{Slew\,Rate\, S=\frac{\Delta V_o }{\Delta t}}

Ideally, the slew rate should be infinite and practically it should be as large as possible.

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Also Read: Inverting Operational Amplifier – Inverting Op Amp

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Calculate Maximum frequency using Slew Rate

Assuming that the input signal V_s is a sinewave, we can obtain the value of the maximum frequency for which the amplifier produces an undistorted output.

Let, \mathbf{V_s= V_m \sin \omega t}

As we have used a unity gain non-inverting amplifier, the output is exactly equal to the input.

\therefore \mathbf{V_o= V_m \sin \omega t}

\therefore \mathbf{\frac{d V_o}{dt}= \omega V_m \cos \omega t}

\frac{d V_o}{dt} will be maximum when \cos \omega t = 1 and maximum value of \frac{d V_o}{dt} is nothing but slew rare S.

\therefore \mathbf{S=\frac{d V_o }{d t}(max) = \omega V_m = 2 \pi f_m V_m \,V/sec}

\therefore \mathbf{f_m = \frac{S}{2 \pi V_m}}

This is the maximum frequency f_m for which the amplifier produces an undistorted output. It is also called full power bandwidth.

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Also Read: Virtual Short and Virtual Ground Concept

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The slew rate of Op Amp In Inverting Mode

When an op-amp is operated in the inverting mode, the slew rate corresponding to the positive going swing is the same as that corresponding to the negative going swing. That means we have a symmetrical slew rate in the inverting mode operation.

The slew rate of Op Amp In Non-inverting Mode

When the op-amp operates in the non-inverting mode, the additional parasitic capacitance due to the common mode input swing results in an asymmetrical slew rate.

Effect of Slew Rate Limiting

The effect of slew rate limitation is distortion in the output signal. That means if the slew rate capabilities of an op-amp are exceeded, the output is no longer an exact replica of the input.

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Also Read: What is CMRR? | Common Mode Rejection Ratio

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Example of Slew Rate

Q. The output of an op-amp voltage follower is a triangular wave as shown int he below the figure for the square wave input of frequency 1 MHz and 4V peak-to-peak amplitude. What is the slew rate of op-amp?

Solution: The slew rate is defined as the maximum rate of change of the output. Therefore from the above figure, we can write that,

\therefore \mathbf{S = \frac{4 V}{0.5 \mu S} = 8V/\mu S}

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FAQs

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