Slew rate

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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.

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.

[\mathrm = max\left(\frac(t)}\right)]

where [v_\mathrm(t)] is the output produced by the amplifier as a function of time t.

Slew rate is typically expressed in units of V/µs.

Measurement

The slew rate can be measured using a function generator and oscilloscope.

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 [A_], then slew rate in this example can be expressed as:

[\mathrm = \frac}}]

where [I_\mathrm] is the output current of the first stage in saturation.

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