Altifigence Academy

24 / 32 · Concept

Output range, bandwidth, and slew rate are separate limits

Test DC headroom and dynamic requirements before trusting an ideal gain.

Start with the allowed DC region

An op amp's supply range, input common-mode range, output swing, output current, and stability requirements are distinct specifications. “Rail-to-rail” needs datasheet conditions and does not imply zero distance from both rails under any load. In this educational example, assume 3.3 V supply and usable output 0.2–3.1 V. With Vref = 1.65 V and gain 11, maximum symmetric output excursion is 1.45 V, corresponding to approximately 131.818 mV at the input.

Small-signal bandwidth

For a suitably compensated, approximately single-pole voltage-feedback op amp, estimate closed-loop bandwidth as GBW divided by noise gain. A 1 MHz GBW and noise gain 11 give approximately 90.909 kHz. This is an approximation, not a universal rule for current-feedback or decompensated amplifiers. An inverting signal gain of −10 has noise gain 11, so using 10 for that estimate is incorrect.

At 10 kHz, a one-pole closed-loop stage with 90.909 kHz bandwidth retains approximately 0.9940 of its low-frequency gain and has about −6.28° phase lag. Further poles, capacitive loading, and layout can change that result. Verify the actual device's stability guidance.

Large-signal speed

A sinusoidal output of peak amplitude Vp requires maximum slope 2πfVp2\pi fV_p. At Vp = 1 V and f = 10 kHz, this is 0.062832 V/μs. An assumed slew rate of 0.5 V/μs supports that slope, while 100 kHz would require 0.628319 V/μs and exceed it. A small-signal AC plot does not reveal slew-induced large-signal distortion.

For a 2 V output step, the same slew limit implies at least 4 μs of transition, excluding settling and other dynamics. Check amplitude, bandwidth, and slew rate together; passing one does not pass all three. TI's architecture discussion provides context for the assumptions behind gain-bandwidth relationships. All example limits here are fictional design inputs rather than specifications of a named part.

Try it yourself

At SR = 0.5 V/μs, what is the largest sinusoidal frequency allowed by slew alone for Vp = 2 V?

Read the explanation

f ≤ SR/(2πVp) = 500000/(4π) = 39788.735773 Hz. Output swing, small-signal bandwidth, and distortion requirements can impose lower limits.

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