Slew rate

In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers. For a sinusoidal waveform not to be subject to slew rate limitation, the slew rate capability at all points in an amplifier must satisfy the following condition::$mathrm\{SR\}\; ge\; 2pi\; f\; imes\; V\_\{mathrm\{pk,$

where "f" is the frequency, and $V\_\{mathrm\{pk$ is the peak amplitude of the waveform. Slew rate is usually expressed in units of V/µs.

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 output slew-rate of an amplifier or other electronic circuit is defined as the maximum rate of change of the output voltage for all possible input signals.

: $mathrm\{SR\}\; =\; maxleft(left|frac\{dv\_mathrm\{out\}(t)\}\{dt\}\; ight|\; ight)$

where $v\_mathrm\{out\}(t)$ is the output produced by the amplifier as a function of time t.

Measurement

The slew rate can be measured using a function generator (usually square wave) and oscilloscope.The unit of slew rate is V/µs. The slew rate is same for both when feedback is considered or not considered.

Slew rate limiting in 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 modern power amplifiers is usually 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 open loop gain of the amplifier is generated. 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 modern power amplifiers 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 $A\_\{2\}$, then slew rate in this example can be expressed as:

: $mathrm\{SR\}\; =\; frac\{I\_mathrm\{sat\{CA\_\{2$

where $I\_mathrm\{sat\}$ is the output current of the first stage in saturation.