Op-amp slew rate is the maximum rate at which the output voltage can change, and it is a large-signal limit that has nothing to do with the small-signal bandwidth figure on the front page. An amplifier with generous bandwidth can still be unable to move a large output swing quickly enough, and when that happens the output stops being an amplified copy of the input.
Two limits with two different mechanisms
Small-signal bandwidth describes how the amplifier’s gain falls with frequency when the signal is small enough that every stage stays in its linear region. Slew rate describes what happens when it is not. Inside a typical amplifier a compensation capacitor must be charged and discharged to move the output, and the current available to do that is finite, set by the input stage’s tail current. Once the input difference is large enough to divert all of that current, the capacitor charges at a constant rate and the output moves at a constant slope regardless of how much larger the input error becomes. At that point the feedback loop is effectively open and the amplifier is no longer doing arithmetic on the input; it is simply ramping as fast as it can.
Why large signals distort before small ones
The required rate of change of a sinusoid is proportional to both its amplitude and its frequency. Double the amplitude and you double the demand; double the frequency and you double it again. So an amplifier can reproduce a small signal faithfully at a given frequency and grossly distort a large one at exactly the same frequency. This is why datasheets quote a full-power bandwidth separately from the gain-bandwidth product: it is the highest frequency at which the rated output swing can be produced without exceeding the slew rate. The distortion this produces is distinctive. A sine wave becomes visibly triangular as its peaks are replaced by straight ramps, and a square wave shows sloped edges whose slope is the same regardless of how fast the input edge was.
What slewing does to accuracy and settling
While an amplifier is slewing, it is not regulating. The loop cannot correct errors, so the output position during that interval is determined by the ramp rather than by the input. After the ramp ends the amplifier must recover linear operation and settle to its final value, and that settling takes additional time that the datasheet may specify separately to a stated accuracy. In a sampled system this is critical. A buffer driving an analog-to-digital converter must slew to the new voltage and then settle to better than the converter’s resolution before the sample is taken, and a part chosen on bandwidth alone may do the first but not the second. The same applies to multiplexed channels, where the amplifier must handle a full-scale step between adjacent samples.
Slew rate also interacts with input stage overload
During slewing the two inputs are, by definition, far apart, and that condition can stress the input stage. Some amplifiers show increased input bias current, or even input stage damage, when a large differential voltage appears across the inputs, which is why many datasheets specify a maximum differential input voltage and why some parts include internal clamp diodes. Those clamps limit the differential voltage but also draw current from the source when they conduct, loading the preceding stage in a way that only occurs during fast transients. The practical effect is a distortion mechanism that appears only on fast edges and disappears in steady-state testing, making it easy to miss unless you deliberately exercise the circuit with large fast signals.
Checking both limits properly
Test with the signal amplitude the application actually uses, not a convenient small one. Apply a fast square-wave step covering the full expected output swing and observe the edge on a scope: a straight ramp means you are slew limited, and its slope is the achieved slew rate under your load. Then apply a sine wave at the highest operating frequency and full amplitude and look for triangular peaks. Do both with the real load connected, since capacitive loading can reduce the achievable rate substantially, and at the temperature extremes, because slew rate varies with temperature. When selecting a part, compute the required rate from the maximum amplitude and frequency, then choose an amplifier with comfortable margin above it rather than one that just meets the number.
What to check before you act
- Compute the required rate from your maximum amplitude and frequency.
- Test with a full-amplitude step, not a small-signal input.
- Include settling time, not just slew time, in a sampled system.
- Repeat the test with the real load and at temperature extremes.
Common questions
Will a higher gain-bandwidth amplifier always slew faster?
Usually but not reliably. The two specifications come from related but distinct mechanisms, and some wide-bandwidth parts have modest slew rates. Check both figures on the datasheet.
Why does my square wave have sloped edges even at low frequency?
Slew limiting depends on amplitude and edge speed, not on repetition rate. A large step at any frequency will show the same maximum slope.
The practical takeaway
Bandwidth tells you about small signals and slew rate tells you about large ones, so check both against the amplitude your circuit actually produces. Test with full-scale steps into the real load, allow for settling as well as slewing, and choose with margin rather than exactly meeting the calculated requirement.
References and further reading
- Department of Energy: how electronics losses turn into wasted energy and heat
- FCC: electromagnetic compatibility and radio frequency safety questions answered
- CPSC: safety guidance for electronics and electrical products indoors
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
