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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. : where 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 and gain , then slew rate in this example can be expressed as: : where is the output current of the first stage in saturation. |
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