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The output of an operational amplifier can only change by a certain amount in a given time: this limit is called the slew rate of the op-amp.
Although slew rate is not always mentioned when undertaking an electronic circuit design, it can be a critical factor in ensuring that an amplifier is able to provide an output that is a faithful representation of the input..
Operational amplifier slew rate can limit the performance of a circuit if the slew rate requirement is exceeded. It can distort the waveform and prevent the input signal being faithfully represented at the output if the slew rate is exceeded.
One of the figures quoted in the data sheets for operational amplifiers is the slew rate, and this needs to be checked and some calculations made to ensure that the particular op amp device can handle the output change rate demanded of it.
In certain applications where speed is required and the output needs to change quickly, the slew rate of the operational amplifier can have a significant effect on the overall performance of the electronic circuit, and the design needs to accommodate this.
Video: Operational amplifier slew raten
Op amp slew rate basics
The slew rate of an op amp or any amplifier circuit is the rate of change in the output voltage caused by a step change on the input.
It is measured as a voltage change in a given time - typically V / µs or V / ms.
A typical general purpose device may have a slew rate of 10 V / microsecond. This means that when a large step change is placed on the input, the electronic device would be able to provide an output 10 volt change in one microsecond.
The figures for slew rate change are dependent upon the type of operational amplifier being used. Low power op-amps may only have figures of a volt per microsecond, whereas there are fast operational amplifiers capable to providing rates of 1000 V / µsecond.
The slew rate is governed by the operational amplifier itself and as a result the slew rate performance of the whole electronic circuit design is not affected by the feedback applied.
The chief causes for the slew rate limitations are caused by the internal frequency compensation included in most operational amplifiers to provide stability, especially at high frequencies.
Another contributory factor is the small internal drive currents, as well as any limitations int he output stage. These all combine together to limit the rate at which the output can change from one level to another.
Op amps may have different slew rates for positive and negative going transitions because of the circuit configuration.
Op amps have a complementary output to pull the signal up and down and this means the two sides of the circuit cannot be exactly the same. However it is often assumed that they have reasonably symmetrical performance levels.
There are still some operational amplifiers that are not internally compensated and need external electronic components to provide the compensation.
These can be optimised to provide the best balance between stability and slew rate. In this way the best overall performance can be obtained.
Slew rate rationale
The slew rate issues arise from the internal circuitry within the op amp. There are various reasons for the limitations of most chips:
- Frequency compensation: The capacitors used within the chip to reduce the high frequency response have a marked effect on the slew rate. Limiting the frequency response also limits the rate of change that can occur at the output, and hence it affects the overall op amp slew rate. However to ensure that operational amplifiers remain stable the frequency compensation components are always included, and the resulting effect on slew rate needs too be accommodated in the overall circuit design.
Output driver limitations: Within the chip, and particularly within the output driver, the low current levels limit the rate at which change can occur. This limits the slew rate of the op amp. It is found that this is the area of the performance where rise and fall slew rates may be different.This results from the different ways that the chip increases and decreases the output voltage. For example the output may employ a form of complementary output stage. The slightly different characteristics of each half will cause a small amount on difference between the rise and fall slew rate capabilities.
- High gain input stages: Operational amplifiers use high gain differential input stages. The high gain, and the fact that they are transconductance amplifiers where a voltage input produces a current output means that there is a possibility that the signals can saturate causing the amplifier to act as a constant current source. When this happens the output change rate of the amplifier is severely limited.
It can be seen that the slew rate tends to be governed by factors within the operational amplifier chip itself. Accordingly it is necessary to select a chip for the electronic circuit design that can provide the slew rate that is needed. Calculation of the required slew rate for a given circuit scenario means that any issues can be addressed at the deign stage of the circuit rather than discovering a problem later.
Slew rate distortion
If an op amp is operated above its slew rate limit, signals will become distorted. The easiest way to see this is to look at the example of a sine wave.
The maximum rate of voltage change occurs at the zero crossing point.
It is possible to find the maximum frequency or voltage that can be accommodated. A sine wave with a frequency of f Hertz and peak voltage V volts requires an operational amplifier with a slew rate of 2 x Π x f x V volts per second. This is required to ensure the maximum slew rate requirement which occurs at the zero crossing point can be met.
As can be seen in the diagram, in the limit, the op amp slewing distortion will result in the creation of a triangular waveform. If the frequency is increased the op amp will be even less able to keep up and therefore the amplitude of the output waveform will decrease.
The slew rate may also not be linear over the whole range. As a result the waveform may exhibit a faster rise for the first part of the change, then reverting to the more expected slew rate.
Also note that slew rate is normally specified for a circuit acting as a voltage follower with unity gain and with a full swing step input. This means that there is a large differential drive and as a result large amounts of current flow. For different configurations where there is a small input voltage and larger gain, the slew rate will be much less.
Slew rate calculation & formula
It is relatively easy to calculate the slew rate of an amplifier that is required for a given application or electronic circuit design from a knowledge of the maximum voltage and frequency required.
To give distortion free operation, the slew rate of the amplifier, the simple formula below can be used.
slew rate is measured in volts / second, although actual measurements are often given in v/µs
f = the highest signal frequency, Hz
V = the maximum peak voltage of the signal.
As an example, take the scenario where an op amp is required to amplify a signal with a peak amplitude of 5 volts at a frequency of 25kHz. An op amp with a slew rate of at least 2 π x 25 000 x 5 = 0.785V/µs would be required.
Slew rate calculator
While it is possible to manually calculate the slew rate required for an op-amp or other amplifier manually, the simple op-amp slew rate calculator below provides a much easier and faster option.
Op Amp Slew Rate Calculator
Op amp slew rate may not affect some electronic circuit designs, but for others it may introduce significant levels of distortion. Accordingly it is always best to check that the slew rate limits for the chip will not be exceeded and opt for another device if this is likely for any electronic circuit being designed. There are many high slew rate devices on the market and these can normally be dropped into the circuit in place of a slower device with little modification to the rest of the circuit.
By addressing the op amp slew rate issues at the initial electronic circuit design concept stages, it is possible to ensure that it is not an issues under the circuit conditions that are anticipated to be encountered.
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