Op-Amp Slew Rate Explained (with Examples)

Hey friends, welcome to the Kohiki.com Website  ALL ABOUT ELECTRONICS. So, in this article, we will learn about the Op-Amp Slew Rate. So here a Simple Image To Understand Op-Amp Slew Rate. So let’s start To Understand About Op-Amp Slew Rate.

What is Slew Rate of an Op-Amp

  • So, the slew rate of the op-amp can be defined as the maximum rate at which the output of the op-amp can change. Or in another way, we can say that how fast the op-amp is able to respond.
  • So, let’s understand this slew rate (Op-Amp Slew Rate) by taking one example. So, let’s say, we have one op-amp which is configured as a unity follower. That means the output of the op-amp will follow the input signal. And let’s assume that at the input side, we have applied the square wave.
  • So, this square wave is varying from zero to 5V. So, at the output side also we should get a square wave pulse which is varying from 0 to 5V. Now, here, ideally, the output should go from zero to 5V in no time. But actually, if you see, the output will take some time to reach from 0 to 5V. And if you see the waveform, then the actual waveform will look like this. So, the time which is being taken by the op-amp to reach this 5V depends upon the slew rate of the op-amp.
  • So, the unit of this slew rate Op-Amp Slew Rate is Volt per microsecond. So, let’s say for one particular op-amp slew (Op-Amp Slew Rate) rate is 1 V per microsecond, it means that the output will change by one volt in one microsecond. And for that op-amp, to reach from 0 to 5V,it will take around 5 microseconds of time. Now, here the different op-amp has the different value of slew rate. And the value of slew rate varies from 0.1V per microseconds up to 1000V per microseconds.
  • So, here is the list of few op-amps and corresponding slew rates (Op-Amp Slew Rate). So, as you can see, for the general purpose741 series op-amp, the value of the slew rate (Op-Amp Slew Rate) is 0.5 V per microsecond. While fro the op-amp from linear technology, the slew rate can be as fast as 1000 V per microsecond.
  • So, depending upon your application you need to select the op-amp with the specific (slew rate Op-Amp Slew Rate) so that your output will not get distorted. And very shortly we will see how the slew rate (Op-Amp Slew Rate) can affect your output signal and how we can avoid that. But before we understand that, let’s quickly see, why every op-amp has a finite amount of slew rate. And why every op-amp is not able to respond very quickly. So, the reason for this slew rate is the internal compensation capacitor of the op-amp.

Causes of Slew Rate in Op-Amp

we have already talked about it in the last article. So, we had seen that this compensation capacitor is used to enhance the stability of the op-amp at high frequencies.

  • So, this capacitor is the intermediate stage of the op-amp. And the voltage that is developed across this capacitor is amplified at the final stage.
  • So, the slew rate of the op-amp depends upon how fast this capacitor charges or discharges. So, we already know that the capacitor currents can be given by the expression, C times DVC/dtSo, we can say that the rate of change of voltage will be equal to the charging currents divided by the capacitance of this capacitor.
  • So, let’s say, for the given op-amp if the charging current is 200 micro-ampere and the value of this compensation capacitor is 30 pF, in that case, the slew rate (Op-Amp Slew Rate) will be equal to 6.66 Mega Volt divided by second. Or we can say that it is equal to 6.66 Voltper micro-second.
  • So, in this way, the slew rate of the op-amp depends upon this charging and discharging of this internal compensation capacitor. Now, for most of the op-amps, the value of this slew rate (Op-Amp Slew Rate) is identical for the positive as well as the negative voltage swing. That means, whether your output is going from negative to positive or positive to negative, the value of the slew rate is identical in both directions.

Effect of Slew Rate on Pulse input

  • So, now that is being said, let’s see how the slew rate can affect your output signal. So, first of all, let’s see the effect of slew rate (Op-Amp Slew Rate) on the square wave pulse.
  • So, here assume that the output of one op-amp is a square wave pulse which is varying from 0 to 5V and the time period of the squarewave pulse is 40 microseconds. And the ON time of the square wave is 20 microseconds.
  • So, in the ideal case, the output of this op-amp should go from 0 to 5V in no time. But because of this slew rate, let’s see, how the output will be get affected.
  • So, here, assume that the slew rate of the op-amp is 0.5 Volt per micro-second. And for our understanding, let’s take only this first pulse.
  • So, here we are considering only a single pulse and in ideal condition, the response of the op-amp should look like this. But here we know that in actual condition, the slew rate of the op-amp is 0.5 Volt per micro-second. So, this op-amp can change the output voltage only by 0.5 Volt in one microsecond.
  • so, it will take around 10 microseconds to reach the 5V. So, actually, if you see, it will take 10microseconds to reach 5V. And then after it will remain flat up to the20 microseconds. And then after again it will take 10 microseconds to reach the 0V.

  • So, in this way because of this slew rate (Op-Amp Slew Rate) you’re the output waveform will be get distorted. Now, here, the distortion of the output signal depends upon the input signal frequency as well as the amplitude of the output voltage.
  • So, now let’s understand how the frequency and the amplitude will affect your output signal. So, now instead of this 40-microsecond time period, let’s say, our new input signal has 20 microseconds of a time period. Or we can say that the ON time of that signals 10 microseconds. And let’s find the response of the op-amp for this particular condition.
  • So, now if you see, it will take 10 microseconds to reach up to the 5V. Because here, the slew rate of the op-amp is 0.5 V per microseconds. And then after it will again take 10 microseconds to reach from 5V to 0V. So, in this way, your output waveform will like a triangular wave. Similarly, let’s see how the amplitude of the signal will also affect your output.
  • So, now, here, instead of 0 to 5V, the new signal is going from 0 to 10 v. And the ON time of this square wave pulse is 10 microseconds. Now, here the slew rate of the op-amp is 0.5V per microsecond.
  • So, in 10 microseconds, it can reach up to 5V. So, if you see here, in 10 microseconds, the output will reach 5V. And then after again it will go from 5V to0V in 10 microseconds. So, as you can see here, because of this slow slew rate (Op-Amp Slew Rate), the output of the op-amp is not even able to reach this 10V.
  • So, based on your input frequency and the amplitude, you should decide the op-amp with a specific slew rate (Op-Amp Slew Rate), so that your output will have a minimum amount of distortion.

  • So, in this case, suppose if we choose the op-amp which is having a slew rate of let’s say 20 V per microseconds, in that case, if you see the response, then the response will look like this. Because now, the op-amp is able to reach this10 V in only 0.5 microseconds.
  • So, in this way, if we use this op-amp then the distortion in the output will be minimum. So, based on your application, you should decide on an op-amp with a specific slew rate. So, now this slew rate (Op-Amp Slew Rate) not only affects the square wave pulses, but it can also affect the sine waves.

Effect of Slew Rate on Sinusoidal Signal

So, now let’s see the effect of this slew rate (Op-Amp Slew Rate) on the sinusoidal signals. So, in the case of these sine waves also, your output signal will get distorted if the signal is changing faster than the slew rate of the op-amp.

  • So, let’s say, we have on the sine wave, which can be expressed by the expression, V(t), which is equal to Vm* sin(wt)So, now the rate of change of the signal, that is equal to dV/dt = Vm*w*cos(wt)So, here, the rate of change of signal will be maximum when the value of this cos(wt) is 1.
  • So, the maximum rate of change of signal will be equal to the VM multiplied by the 2*Pi times this frequency f. So, the value of the slew rate for the op-amp should be at least equal to this maximum rate at which this sine wave is changing.
  • So, we can say that the slew rate should be greater than or equal to this VM, multiplied by the 2*Pi times this frequency f. So, whenever, this slew rate (Op-Amp Slew Rate) is greater than this value, in that case, it is ensured that you will not see any distortion in the output.
  • So, let’s take the worst case that is, the slew rate (Op-Amp Slew Rate) that is equal to this maximum rate of change of sine wave. So, from this, we can find the maximum frequency for which you will not find any kind of distortion. And that can be given by the expression, Slew rate (Op-Amp Slew Rate)divided by (2*Pi) times this peak voltage of this sine wave.
  • So, up to this frequency, you will not see any kind of distortion in the output. And this frequency is some ties also known as the power bandwidth of the op-amp. Now, here do not get confused with the unity-gain bandwidth of the op-amp. (Op-Amp Slew Rate) Because this unity-gain bandwidth is defined for the small signals, which are in the range of millivolts. While this power bandwidth is defined as for the large signals, which are in the range of volts.
  • So, for the small signals, whenever your input signal frequency is greater than this unity gain frequency, in that case, you will see the attenuation in the output. While in the case of these large signals, whenever your input signal frequency is greater than this maximum frequency, in that case, you will see the distortion in the output.
  • So, whenever, this condition is not satisfied, in that case, you will see the distortion in the output. So, this is the case when your slew rate (Op-Amp Slew Rate) is less than the maximum rate at which your signal is changing. So, now based on this concept, let’s take some examples based on this slew rate.

Example 1

  • so, in the first example, we need to select the op-amp such that it is able to reproduce the 20 kHz of a sinusoidal signal which is having a peak voltage of 10V.
  • So, we need to find the minimum acceptable slew rate for the op-amp.
  • So, here, the frequency of this sine wave which is given is 20 kHz. Now, we know that for the sine wave, the minimum slew rate (Op-Amp Slew Rate) that is required can be given by the expression, Vm multiplied by the (2*Pi)times this frequency f.
  • So, here, if we put all these values then we will get the value of this slew rate as 1.257 MV/sOr we can say that it is equal to1.257 Volt per micro-second.
  • So, to avoid distortion, the slew rate of the op-amp should be greater than this value.
  • So, for this application, we can not use the general-purpose 741 op-amps, because it is having a slew rate of 0.5 V per microsecond. But instead of it, suppose if we use, this-amp which is having a slew rate (Op-Amp Slew Rate) of 15V/us, then we will not face any kind of distortion.

Example 2

So, now let’s see the second example. So, in this example, we have been given this unity follower circuit. And to this circuit, we are applying the squarewave pulse as input. Now, here the input signal is varying from0 to 10V and it is having a duty cycle of 50 percent. And also assume that this input signal is tunable. That means it can be tuned from 1KHz up to1 MHz. Now, apart from that, we have been given that the slew rate of the op-amp is 2V/us.

  • So, for this configuration, we need to find the frequency of the square wave pulse, where the output will be the triangular wave with the peak voltage of 4V.
  • So, here, we need to find the frequency of the square wave pulse so that the output will be a triangular wave which is having a maximum amplitude of 4V. And here, the slew rate of the op-amp is 2V/us. That means the op-amp will reach this 4V in2 us.
  • So, we can say that this time will be equal to 2 us. And the same time will be required to reach from 4V to 0V. That means the entire time period will be equal to 4us.
  • So, from this, we can say that the time period of the square wave pulse, should be equal to 4us.or in frequency, it will be equal to 250 kHz.
  • So, in this way, whenever we apply the squarewave pulse of 250 kHz, then at the output you will get a triangular wave which is having a peak amplitude of 4V. Now, suppose in the same example, if you want a triangular wave with 10V of the output voltage, in that case, this time will be equal to 5us. Because of the slew rate (Op-Amp Slew Rate), this op-amp will require 5us to reach up to the 10V. And the entire time period T1 will be equal to 10 us.
  • So, in frequency, it will be equal to 100KHz. So, as you can see, as the input frequency will increases, then your output will get more and more distorted.
  • So, for the particular application, you need to select an op-amp in such a way that your output will have a minimum amount of distortion. So, this is all about the slew rate (Op-Amp Slew Rate) of the op-amp.

FAQ (People also ask)

What causes slew rate in op amp?

The chief causes for the slew rate limitations square measure caused by the inner frequency compensation enclosed in most operational amplifiers to supply stability, particularly at high frequencies. Another contributive issue is that the little drive currents, yet as any limitations in the output stage.

What is CMRR and slew rate?

Common Mode Rejection magnitude relation (CMRR) is eternity. Slew Rate (SR) is eternity. It means, the best op-amp can turn out an amendment within the output instantly in response to Associate in Nursing input step voltage.

Why should slew rate be high?

A higher slew rate makes for a higher operating current. this suggests higher power consumption. quicker slew rate can build higher bandwidth. it should waste information measure and increase the quality of circuit-style

What is the formula of slew rate?

First, we tend to convert the slew rate from V/μS to V/S by multiplying it by one,000,000. this provides USA a slew rate of five hundred,000V/μS. Then we tend to plug all of this into the slew rate formula, slew rate=2πfV= five hundred,000V/μS= 2(3.14)(30,000Hz)V= 2.65V. that the most voltage that the LM741 will output at 30KHz is two.65V.

What is unit of slew rate?

Slew rate is outlined because the most rate of modification of associate op amp’s output voltage and is given units of volts per unit of time. Slew rate is measured by applying an oversized signal step, such as 1V, to the input of the op-amp, and mensuration the speed of modification from 100 percent to ninetieth of the output signal’s amplitude.

You Tube Video

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Last World About CMRR

So, I hope in this article, you understood the slew rate of the op-amp and how we can avoid the distortion in the output because of this slew rate (Op-Amp Slew Rate).

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