Op Amp Slew Rate

Overview of Op-amp slew rate, what it is, how it affects operational amplifier circuit and an op amp rate calculator.

The slew rate may be important parameters in applications where the output is required to switch from one level to another quickly. In these applications the rate at which the op amp can change between the two levels is important.

OP Amp slew rate basics

The slew rate of an op amp or any amplifier circuit is the 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/us or V/ms

 Op amp slew rate illustration

A typical general purpose device may have a slew rate of 10V/ms, this means that when a large step change is placed on the input, the 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 1000V/ms.

Op amp may have different slew rates for positive and negative going transitions because of the circuit configuration. They 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.

Slew rate rationale

The slew rate issues arise from the internal circuitry within the op amp. There are two main 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 occurs at the output, and hence it affects the overall op amp slew rate.

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.

Slewing 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

 

 

Maximum rate of change of sine wave occurs at 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.

 

Op amp slewing distortion (limit)

 

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.