Slew rate is the maximum speed at which an op amp’s output voltage can change, measured in volts per microsecond (V/µs). If your signal demands a faster voltage swing than the op amp can deliver, the output distorts. It’s one of the most important practical limits you’ll encounter when designing with op amps, and it’s completely separate from bandwidth.
How Slew Rate Works
Think of slew rate as a speed limit on the output. When you feed a signal into an op amp, the output needs to follow along. If the input asks for a gentle, slow change, the output tracks it perfectly. But if the input demands a sudden jump, the output can only ramp at its maximum rate. That maximum ramp is the slew rate.
Formally, slew rate equals the maximum rate of change of the output voltage over time (dV/dt). A general-purpose op amp might have a slew rate around 0.5 to 2 V/µs, meaning its output can swing at most 0.5 to 2 volts in a single microsecond. High-speed op amps can reach 32 V/µs or more. To measure it, engineers apply a large step to the input and record how fast the output climbs from 10% to 90% of its final value.
What Limits It Inside the Chip
Inside a typical op amp, two components set the speed ceiling: a small bias current in the input stage and a compensation capacitor. The input stage of most op amps uses a pair of transistors that steer current back and forth. This pair shares a fixed “tail current,” and once the input signal is large enough to steer all of that current to one side, the stage is maxed out. It physically cannot push any harder.
That maxed-out current then has to charge the compensation capacitor, a deliberately large capacitor added inside the chip to keep the op amp stable and prevent oscillation. Charging a capacitor faster requires more current, but the tail current is fixed. So the slew rate boils down to a simple relationship: tail current divided by compensation capacitance. A larger bias current or a smaller capacitor means a faster slew rate, but both come with tradeoffs in power consumption and stability.
Some modern op amps include a “slew boost” circuit that temporarily increases the available current during large signal swings. For example, the OPA2992 from Texas Instruments uses this technique to reach 32 V/µs, well beyond what its normal bias current would allow.
Slew Rate vs. Bandwidth
This is where many people get confused. Bandwidth and slew rate are two different limits, and either one can be the bottleneck depending on your signal.
Bandwidth (specifically, the closed-loop bandwidth) describes how the op amp handles small signals at different frequencies. If your signal frequency exceeds the bandwidth, the output amplitude rolls off, getting smaller and smaller. But the shape of the waveform stays clean. You get a quieter signal, not a broken one.
Slew rate, on the other hand, is a large-signal limit. When the output voltage needs to swing far and fast, the op amp hits its maximum rate of change and clips the waveform into straight ramps instead of smooth curves. The result is visible distortion: a sine wave starts looking like a triangle wave. A signal with a frequency well within the op amp’s bandwidth can still trigger slew-rate distortion if the amplitude is large enough.
The Key Formula: SR = 2πfVp
The relationship between slew rate, frequency, and amplitude is straightforward. For a sine wave, the fastest rate of change happens as the signal crosses through zero, and that rate equals:
Required Slew Rate = 2π × f × Vp
Here, f is the signal frequency and Vp is the peak output voltage. If your op amp’s slew rate is lower than this value, the output will distort.
A practical example: say you’re building an audio preamp that needs to handle frequencies up to 20 kHz with a 10 V peak output swing. Plugging in the numbers gives 2π × 20,000 × 10 = 1.26 V/µs. Any op amp you choose for this circuit needs a slew rate above 1.26 V/µs, or the loudest high-frequency notes will come out distorted. A basic op amp rated at 0.5 V/µs would fail here, while one rated at 2 V/µs would handle it comfortably.
Full-Power Bandwidth
Rearranging that same formula gives you the full-power bandwidth (FPBW), which is the highest frequency an op amp can output at its maximum voltage swing without slew-rate distortion:
FPBW = Slew Rate / (2π × Vp)
This number is often much lower than the small-signal bandwidth listed on the datasheet. An op amp might have a 1 MHz bandwidth but a full-power bandwidth of only 16 kHz when swinging its full output range. If you’re working with large signals, FPBW is the number that actually matters.
Asymmetric Slew Rates
One detail that catches people off guard: the positive and negative slew rates of an op amp aren’t always identical. The output stage may source current faster than it sinks it, or vice versa. Datasheets typically list the slower of the two values, since that’s the one that limits your design. If your application involves signals that swing equally in both directions (like audio), the slower direction is your bottleneck.
Choosing the Right Op Amp
When selecting an op amp, start by identifying the highest frequency and largest voltage swing your circuit needs to produce. Plug those into the formula SR = 2π × f × Vp and add some margin, at least 50% above the calculated minimum. This ensures clean output even under worst-case conditions, since real-world slew rates can vary slightly with temperature and supply voltage.
For low-frequency or small-signal applications like sensor amplifiers or DC-level shifting, slew rate rarely matters. A basic 0.5 V/µs part works fine. For audio circuits, you’ll typically need 1 to 5 V/µs. For video signals, fast data converters, or pulse-shaping circuits, look for parts rated at 10 V/µs or higher. High-speed op amps designed for RF or communications work can exceed 1,000 V/µs, though they demand more careful circuit layout to stay stable.
The key takeaway is that bandwidth alone doesn’t tell you whether an op amp can handle your signal. Slew rate is the constraint that determines whether large, fast-moving signals come out clean or distorted, and checking it against your actual signal conditions takes about ten seconds with a calculator.