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Significance of op amps' slew rate

Posted: 28 Dec 2015     Print Version  Bookmark and Share

Keywords:op amps  slewing  non-inverting circuit  transistors 

Slewing behaviour of op amps is usually misunderstood. It's a meaty topic so let's sort it out.

The input circuitry of an op amp circuit generally has a very small voltage between the inputs—ideally zero, right? But a sudden change in the input signal temporarily drives the feedback loop out of balance creating a differential error voltage between the op amp inputs. This causes the output to race off to correct the error. The larger the error, the faster it goes... that is until the differential input voltage is large enough to drive the op amp into slewing.

If the input step is large enough, the accelerator is jammed to the floor. More input will not make the output move faster. Figure 1 shows why in a simple op amp circuit. With a constant input voltage to the closed-loop circuit there is zero voltage between the op amp inputs. The input stage is balanced and the current IS1 splits equally between the two input transistors. With a step function change in Vin, greater than 350mV for this circuit, all the IS1 current is steered to one side of the input transistor pair and that current charges (or discharges) the Miller compensation capacitor, C1. The output slew rate (SR) is the rate at which IS1 charges C1, equal to IS1/C1.

Figure 1: Simple op amp circuit.

There are variations, of course. Op amps with slew-enhancement add circuitry to detect this overdriven condition and enlist additional current sources to charge C1 faster but they still have a limited slew rate. The positive and negative slew rates may not be perfectly matched. They are close to equal in this simple circuit but this can vary with different op amps. The voltage to slew an input stage (350mV for this design) varies from approximately 100mV to 1V or more, depending on the op amp.

While the output is slewing it can't respond to incremental changes in the input. The input stage is overdriven and the output rate-of-change is maxed out. But once the output voltage nears its final value the error voltage across the op amp inputs reenters the linear range. Then the rate of change gradually reduces to make a smooth landing at the final value.

There nothing inherently wrong with slewing an op amp—no damage or fines for speeding. But to avoid gross distortion of sine waves, the signal frequency and/or output amplitude must be limited so that the maximum slope does not exceed the amplifier's slew rate. Figure 2 shows that the maximum slope of a sine wave is proportional to VP and frequency. With 20% less than the required slew rate, output is distorted into a nearly triangle shape.

Figure 2: The maximum slope of a sine wave is proportional to VP and frequency.


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