Slew rate

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slew rate effect on a square wave: red= desired output, green= distorted output

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:

$\mathrm{SR} \ge 2\pi f \times V_{\mathrm{pk}},$

where f is the frequency, and $V pk$ 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.

[edit] 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.

$\mathrm{SR} = \max\left(\left|\frac{dv_\mathrm{out}(t)}{dt}\right|\right)$

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

[edit] 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.

[edit] 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:

$\mathrm{SR} = \frac{I_\mathrm{sat}}{CA_{2}}$