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Electronics

Op-amp slew rate explained

An op-amp's slew rate is the fastest its output voltage can change, in volts per microsecond (V/µs). Ask for a faster change and the output can't keep up: a square wave's sharp edges become ramps, and a big, fast sine bends into a triangle. The classic 741 slews at about 0.5 V/µs, so a 10 V jump takes about 20 µs.

Steps become ramps

Feed a ±5 V square wave, a 10 V jump at each edge, into a follower. An ideal op-amp would jump from one level to the other instantly. A real one ramps at its slew rate:

t = ΔV over SR = 10 V over 0.5 V/µs = 20 µs

At 1 kHz, each half-cycle lasts 500 µs and a 20 µs ramp hardly shows. At 10 kHz the half-cycle is 50 µs, and each edge eats 40% of it. At 25 kHz the ramps meet in the middle and the output is a triangle; any faster and it turns round before it gets to the top, a smaller triangle.

Live experiment

Make the output keep up

A follower on ±12 V rails gets a square wave. Its output can't pass ±10.5 V, and it can't move faster than 0.5 V/µs, so every jump becomes a ramp: 10 V takes 20 µs. Raise the frequency until the ramps meet in a triangle.

Try it yourself: free lesson

“Real op-amp limits” is lesson 6 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 40 before it first.

Full-power bandwidth

A sine is steepest as it crosses zero, where its slope is 2π × f × Vpeak. If that's faster than the slew rate, the output can't follow and the sine distorts. The highest frequency a sine can reach at full size is the full-power bandwidth:

fmax = SR over 2π × Vpeak

At 0.5 V/µs and 10 V peak:

fmax = 500,000 V/s over 2π × 10 V ≈ 8 kHz

That's below the top of human hearing, which is why the 741 is a poor choice for loud, full-range audio. Halve the swing and the limit doubles; a smaller signal is less affected.

The other limits

Slew rate is one of three limits that datasheets lead with:

  • Output swing: a classic op-amp's output stops about 1.5 V inside each rail, ±10.5 V on ±12 V. Ask for more and the tops are clipped flat. Rail-to-rail parts get much closer.
  • Gain-bandwidth product: gain falls with frequency, so more gain means a lower corner.
  • Slew rate: the speed limit for big waves.

Typical figures: about 0.3 V/µs for the LM358, about 13 V/µs for the TL072, and far more for video op-amps. Pick the slew rate from your biggest, fastest wave, then allow a margin.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“Real op-amp limits” is lesson 6 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 40 before it first.

Or start from the very first lesson, no account needed

Practice questions

Three quick questions from the lessons. Pick an answer to check it.

  1. Question 1 of 3Ask one op-amp for more gain, and the frequency where its gain starts to fall…
    Show the answer

    Answer: gets lower. Gain and bandwidth trade off, like a bike's gears. Their product, the gain-bandwidth product, stays about the same, so more gain means a lower corner.

    • Why not “stays the same”? Their product stays about the same, so raising the gain moves the corner lower.
    • Why not “gets higher”? Gain and bandwidth trade off: ask for more gain and the corner comes down.
  2. Question 2 of 3What is the period of a 1 kHz sine wave?
    Show the answer

    Answer: 1 ms. T = 1 over f = 1 over 1000 = 0.001 s = 1 ms. A thousand cycles fit in each second.

    • Why not “10 ms”? 10 ms is the period of 100 Hz. At 1 kHz each cycle lasts 1 ÷ 1000 s, which is 1 ms.
    • Why not “1 s”? 1 s is the period of 1 Hz. A thousand cycles a second means each one lasts a thousandth of a second.
  3. Question 3 of 3A sine wave repeats every 20 ms. What is its frequency?
    Show the answer

    Answer: 50 Hz. f = 1 over T = 1 over 0.02 s = 50 Hz. Forgetting to turn 20 ms into seconds gives 0.05 Hz, and 20 Hz is the period's number, not a frequency.

    • Why not “0.05 Hz”? That forgets to turn 20 ms into 0.02 s. f = 1 ÷ 0.02 s = 50 Hz.
    • Why not “20 Hz”? 20 is the period in milliseconds, not a frequency. Frequency is 1 ÷ the period.
Keep practising: free lesson

“Real op-amp limits” is lesson 6 of 6 in Amplifier Peaks, Electronics World 6. Free with an account. Lessons unlock in order, so you’ll do the 40 before it first.

Questions people ask

What is slew rate?

The fastest an op-amp's output voltage can change, usually in volts per microsecond. The classic 741 manages about 0.5 V/µs, so a 10 V step takes about 20 µs. Faster audio op-amps reach 10 V/µs or more.

How do I calculate the full-power bandwidth?

A sine's steepest slope is 2π × f × V_peak. Set that equal to the slew rate and solve: f_max = SR ÷ (2π × V_peak). At 0.5 V/µs and 10 V peak, that's about 8 kHz. Above it, big sines turn into triangles.

What is the difference between slew rate and gain-bandwidth product?

Gain-bandwidth is a small-signal limit: gain falls as frequency rises, whatever the size of the wave. Slew rate is a large-signal limit: it only bites when the wave is both big and fast. An amplifier can pass its bandwidth test and still slew-limit at full output.

What does slew-rate distortion sound like?

Harsh, like clipping, because the waveform's shape changes and new harmonics appear. It hits loud treble first: high frequencies at large amplitudes. A faster op-amp, or a smaller swing, cures it.

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