Slew Rate Limiting in Amplifiers
There are slight differences between different amplifier designs in how the slewing phenomenon occurs. However, the general principles are the same as in this illustration.
The input stage of modern 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 and gain, then slew rate in this example can be expressed as:
where is the output current of the first stage in saturation.
Slew rate helps us to identify what is the maximum input frequency and amplitude 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.
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