Slew rate

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:

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.

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.

where v_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 A₂, then slew rate in this example can be expressed as:

where I_sat is the output current of the first stage in saturation.

Slew rate helps us to identify what is the maximum input frequency applicable to the amplifier such that the output is not distorted. Thus it becomes imperative to check the datasheet for the device's slew rate before using it for high frequency applications.