Slew rate
From Wikipedia, the free encyclopedia
In electronics, the slew rate is a nonlinear effect in amplifiers. It represents the maximum rate of change of signal at the amplifier output.
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.
[edit] 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.
where vout(t) is the output produced by the amplifier as a function of time t.
Slew rate is typically expressed in units of V/µs.
[edit] Measurement
The slew rate can be measured using a function generator and oscilloscope.
[edit] Origin of slew rate limiting in operational amplifiers
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, amongst 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 a compensation capacitance C and gain A2, then slew rate in this example can be expressed as:
where Isat is the output current of the first stage in saturation.
