Slew rate
|
|
In mechanics the slew rate is given in dimensions 1/T and is associated with the change in position over time of an object which orbits around the observer.
In electronics, the slew rate is a nonlinear effect in operational amplifiers. It represents the inability of the amplifier output to keep up with rapid changes in input.
Definition
The slew-rate of an op-amp is defined as the maximum rate of change of the output voltage for all possible input signals.
- <math>SR = \left.\frac{dV_{o}}{dt}\right|_\max<math>
Slew rate is typically expressed in units of V/μs.
Origin of slewing
There are slight differences between different op-amp designs in how the slewing phenomenon occurs. However, the general principles are the same as in this illustration.
The input stage of an op-amp is a differential amplifier with a transconductance characteristic. This means the input stage takes a differential input voltage and produces an output current into the second stage. The transconductance is typically very high - this is where the large gain of the op-amp arises. This also means that a fairly small input voltage can cause the input stage to saturate. In saturation, the stage produces a nearly constant output current.
The second stage of an op amp is, among other things, where frequency compensation is accomplished. The low pass characteristic of this stage approximates an integrator. A constant current input will therefore produce a linearly increasing output. If the second stage has an input capacitance C and gain <math>A_{2}<math>, then slew rate in this example can be expressed as:
- <math>SR = I_{o1,\mathrm{sat}}CA_{2}<math>
where <math>I_{o1,\mathrm{sat}}<math> is the output current of the first stage in saturation.