Every op-amp datasheet lists a headline bandwidth number — the gain-bandwidth product (GBW), also called unity-gain bandwidth. It's the frequency at which the open-loop gain has fallen to exactly 1 (0 dB). What trips up beginners is thinking that's the bandwidth they'll actually get in their circuit. It isn't. The moment you close the feedback loop with any real gain, your usable bandwidth collapses proportionally.

The reason is baked into the compensation network inside the op-amp. Internally-compensated op-amps roll off at −20 dB/decade from a very low dominant pole (often below 100 Hz) all the way up to the unity-gain frequency. This straight-line slope on a Bode plot means one thing: gain times bandwidth is a constant. If you burn gain, you lose bandwidth in exact trade.

The rule of thumb is dead simple:

Closed-loop bandwidth = GBW ÷ Closed-loop gain

Take a classic LM358 with a GBW of 1 MHz. Build a non-inverting amplifier with a gain of 100 (40 dB). Your −3 dB bandwidth is 1 MHz ÷ 100 = 10 kHz. Try to amplify a 20 kHz audio signal and it's already attenuated by 6 dB. Want to amplify a 100 kHz signal with gain of 100? You'd need an op-amp with at least a 10 MHz GBW — and realistically 30-50 MHz to keep phase margin healthy and the response flat rather than drooping near the corner.

Real-world example: A photodiode transimpedance amp reading a fast optical pulse. You need gain of 1 V/µA (a 1 MΩ feedback resistor) and want to resolve a 500 kHz pulse. If your amp uses an OPA320 with 20 MHz GBW, your naive bandwidth would be 20 MHz ÷ (noise gain, which for TIAs includes the C_in multiplication term). Once you factor in the photodiode capacitance forming a zero with R_f, your noise gain at high frequency easily reaches 20-50×, giving you maybe 400 kHz — right at your signal edge. This is why TIAs almost always demand op-amps with 10-100× more GBW than you'd think.

Design implications:

• Never operate near unity-gain bandwidth — response is already 3 dB down and phase shift approaches 90°.
• Design for at least 10× headroom between your highest signal frequency and the closed-loop bandwidth to keep the response flat and phase manageable.
• For high gain at high frequency, cascade two stages of moderate gain rather than one high-gain stage. Two stages of gain 10 with a 10 MHz GBW amp gives you 1 MHz bandwidth; one stage of gain 100 gives only 100 kHz.
• Watch out for decompensated op-amps (stable only above some minimum gain like 5 or 10) — they offer higher GBW for the same silicon by trading away unity-gain stability.