slew rate

Slew rate is the fastest your amplifier's output can change, usually quoted in volts per microsecond. Picture a bucket being filled by a hose with one fixed setting: no matter how urgently you need the water level to jump, it can only rise as fast as that one flow allows. Inside the amplifier the bucket is a capacitor (often the compensation cap, or whatever load hangs on the output), and the hose is a fixed bias current. When the input suddenly demands a big swing, the amplifier dumps all the current it has into that capacitor — and that is all it has. The output then ramps in a straight line at a constant speed, and that maximum speed is the slew rate: SR = I / C.

Here is the part that trips people up: slew rate is a large-signal limit, and it is a different animal from your small-signal bandwidth. Bandwidth (and gain–bandwidth product) tells you how the amplifier behaves for tiny wiggles, where everything stays linear and the math of gm and poles applies. But ask for a big, fast step and the input pair saturates — one side hogs the whole tail current — so the output can no longer track the ideal exponential and instead crawls up that fixed-slope ramp. A part can have plenty of bandwidth on paper yet still slew-limit on large signals, turning a clean sine into a lopsided triangle and adding distortion.

Because SR = I / C, you buy more slew rate by spending more current or shrinking the capacitor you must charge — both of which cost something elsewhere (power, or stability margin). That is why a slewing op-amp is often a sign you are asking one block to be both low-power and fast at full swing, and you cannot have both for free.