What is Slew Rate : Working & Its Applications

Op Amp or operational amplifier is one type of integrated circuit or microchip or chip, used to perform different operations like linear, mathematical, and nonlinear. An operational amplifier is a high gain amplifier by coupling directly and it operates with both AC & DC signals. An operational amplifier includes two terminals namely inverting and non inverting where these two terminals are named depending on the phase relationship in between their particular inputs & outputs. There are different important characteristics available for op-amps like open-loop voltage gain, slew rate, o/p offset voltage, and CMRR (Common Mode Rejection Ratio). This article discusses an overview of a slew rate and its working.

What is Slew Rate?

The slew rate in electrons can be simply defined as the rate of change of the output voltage (Vo) of the op-amp for each unit time and it is denoted with the letter ‘S’. It plays a key role in op-amp to help in identifying the highest input frequency & amplitude suitable such that the output of the op-amp is not much distorted.

So, the slew rate must be high to ensure the highest undistorted o/p voltage swing. It is used to verify whether an operational amplifier can deliver a reliable output to the input or not. This factor will be changed once the voltage gain is changed. Thus, it is usually specified at the condition of unity gain.
For instance, if the general-purpose op-amp slew rate value is 10 V/μs. When a large step i/p signal is given to the input, then the device can generate 10 volts an o/p within 1 microsecond.

Slew Rate Formula & Its Units

The slew rate formula is (S) = ΔVout/Δt. The slew rate units are V/μs or Volts per second.

Slew Rate Circuit Diagram

The slew rate measurement circuit is shown below. The slew rate of an op-amp can be simply measured by providing a step signal toward the input of the operational amplifier. Here, the step signal is a standard signal where this signal exists just for the positive time, and for the negative time, it is zero.

Slew Rate Measurement Circuit
Slew Rate Measurement Circuit

Once the signal is applied, we can measure the rate of change that happens at the o/p from 10% – 90% of the amplitude of the output signal. Usually, the step signal which is applied is large about 1 V. The slew rate can be measured from the o/p voltage (Vo) waveform as:

Slew rate (S) = dVo/dt => Vo(90%)-Vo(10%)/ t(90%)-t(10%)/

To measure the op-amp slew rate, a function generator and a cathode ray oscilloscope are used. The following circuit is mainly used for measuring slew rate.

It mainly decides the op-amp capacity to adjust its output quickly, so it decides the maximum frequency of the function of a given operational amplifier. The op-amp slew rate can limit the circuit performance & it can alter the o/p signal if its limit exceeds.

The Slew rate must be infinite & practically it is high. The Slew rate of 741 IC is simply 0.5 V/μs which is its main disadvantage. So, it cannot be utilized in high-frequency-based applications.

Slew Rate vs Bandwidth

Two significant concepts relate to the op-amp speed like the slew rate & the bandwidth. These two concepts are not easy to understand, especially how they are the same as each other. Both work together to decide the total time settling for a step response. So let’s understand these two concepts.
Slew rate can limit any signal change through an operational amplifier.

The input sinusoidal signal multiplied through the op-amp gain results in a slope that is maximum as compared to the op-amp’s slew rate. Therefore output signal will be a straight line rather than a curving part of the sinusoidal signal. So, slewing can change or distort the signal shape.

Bandwidth is the highest rate at which it can react to small changes within the signal. At DC biases, an operational amplifier can be designed by using quiescent power so that it accepts small signals or quite little amplitude signals.

When these signals are broken down through Fourier Transform, then it gives you the addition of extremely dissimilar frequencies which range from small frequencies to extremely large.

When the bandwidth is higher, the operational amplifier is capable to increase higher frequency signals, so they have high speeds. The frequency at which the gain of the signal is 1/√2 ( 0.707) then it is the perfect bandwidth value of the operational amplifier. So this is the highest frequency at which point an operational amplifier can function through expected behavior.

For instance; OPA333AIDBVT from Texas Instrument has 250 kHz gain bandwidth (BW) for a 1 closed-loop gain. For a 2 gain, it will be 165kHz & so on. So an operational amplifier will become slow including maximum closed-loop gain by the product of the bandwidth & gain constant.

Slew Rate vs Frequency Response

The slew rate of an op-amp is a capacity of how rapidly an op-amp can react to a change within the input level and it is normally calculated in volts for each microsecond. Although this specification looks related to dynamic reaction, it has more to perform through the high-frequency response of the amplifier.

Slew rate is the rate at which point an amplifier can react to a change within the output. In different waveforms with various frequencies and equal amplitude, we can notice that the amplitude in every sine wave is the same; it is the signal with the maximum frequency that has the maximum voltage change for each unit time.

Thus, when frequency increases, then slew rate has progressively effect until a threshold is attained wherever the slew rate cannot continue with the frequency of the waveform,

So after that, the high-frequency response can be affected by a slew rate to cause severe distortion & degradation to the signal over a fixed point.

Slew Rate vs Propagation Delay

Slew rate is the rate of change of o/p voltage with respect to time whereas the propagation delay is at how much speed the op-amp reacts throughout applied input.

Slew Rate Calculation

Find out the slew rate if an op-amp is necessary to amplify a signal through 4 volts of peak voltage at a 30kHz of frequency.

Vm = 4V and fm = 30kHz

Slew rate (S) = 2πfmVm

Substitute the given values in the above equation.

S = 2×3.4x4x30x10^3

= 188.4×10^3 = 188400 V/S or 0.1884V/ μs

What does a high slew rate mean?

The rate of change of the highest amplifier’s output is called as slew rate. When the amplifier power is high then, the high slew rate should be obtained for a similar power bandwidth.

What are the factors which determine slew rate?

The factors to determine slew rate are: compensation of frequency, high gain input stages, and output driver limitations.

What is the slew rate in an opamp?

The slew rate in an op-amp is the highest rate of change in an output voltage. So it indicates how quickly its output voltage (Vo) can vary.

What is slew rate distortion?

If an operational amplifier is operated beyond the limit of its slew rate then the signals will turn distorted. This is called slew rate-distortion.

How the slew rate is represented?

Generally, the slew rate is represented in V/μs. For instance, 1V/μs means that the o/p falls or increases not so fast as 1V each microsecond.

Applications

The applications of slew rate include the following.

  • A slew circuit is used in musical instruments to provide a slide from one letter to another.
  • This circuit is used wherever the voltage control is transitioned slowly to dissimilar values above a certain period.
  • It is used in applications where speed is necessary & the output needs to adjust in a time period.
  • It is used to define an op-amp’s highest rate of output excursion.
  • It also influences achievable performance within filters, D/A output stages, data acquisition & video amplification.

Thus, this is all about an overview of slew rate op-amp, slew rate derivation, and its applications.
It helps us in identifying the highest input frequency & amplitude applicable to the op-amp such that the amplifier’s output is not much distorted. When the slew rate value is high then the output can be changed very fast & it can regenerate high frequency-based signals more easily. Here is a question for you, what are the advantages and disadvantages of the slew rate?

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