# Slew Rate for OP-AMP, Square and Sinusoidal Wave – How to Calculate It

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## What is Slew Rate

#### The maximum possible rate of change of output voltage of Op-Amp with respect to time is referred to as Slew Rate. In other words, it can be defined as the parameter that describes the rate of variation of output voltage per specified unit time, when the inputs change rapidly.

Fig. 1 – Introduction to SR

#### The unit of Slew Rate is V/µsec (Volts/microsecond). For Example – If a device has a SR of 10 V/ µsec, it simply means that the voltage can be varied by 10 V in 1 μs. SR is typically measured by applying large signal step to the Op-Amp and the rate of change of output signals amplitude from 10 % to 90 % is measured. It is directly proportional to temperature i.e. as the temperature rises, SR decreases. Amplifiers with higher SR have higher current consumption.

Fig. 2 – Signal Distortion by Slewing

## How is Slew Rate Calculated

#### We can conclude that for the same rise time, Slew Rate can be 10 times larger due to the change in the Voltage.

Fig. 3 – SR Calculation

## Slew Rate for Pulse Signals

#### To avoid this, the Op-Amp’s internal circuit contains a compensation capacitor i.e. a capacitor is connected inside of the op-amp IC, which improves the stability of the amplifier and prevents the oscillations across the output. This compensation limits the SR of the Operational Amplifier and since this is internal, it cannot be altered.

Fig. 4 – Schematic Representation of Output Compensation

## SR for Sinusoidal Waveform

#### We can infer that Slw Rate is directly proportional to frequency and amplitude of the waveform. Hence high amplitude, high frequency signals require op-amps with high SR. It is used to determine performance of the amplifier, usually by defining how fast a signal can slew at the output of the amplifier. Full Power Bandwidth (FPBW), also known as large signal bandwidth, is the highest frequency (fmax) that the amplifier can produce at its largest output voltage swing without Slewing Induced Distortion. FPBW or fmax can be found from rearranging the above equation:

Fig. 5 – SR for Sinusoidal Waveform

## Datasheet of Op-Amp

#### The Datasheet of an Operational Amplifier specifies SR for “Rising Signal” called as “Positive SlewRate” and “Falling Signal” called as “Negative SlewRate”. Datasheets of some devices might not differentiate between rising and falling signal as shown in the Fig. 6.

Fig. 6 – Datasheet of Different Amplifiers