For a dominant-pole op-amp, what is the relationship between closed-loop gain and closed-loop bandwidth?

• A. Bandwidth increases with gain.
• B. In a dominant-pole op-amp, as closed-loop gain increases, the corresponding closed-loop bandwidth decreases; the product GBW remains roughly constant.
• C. Bandwidth is independent of gain.
• D. GBW increases with gain.

Answer: (B)

In a dominant-pole compensated op-amp, the open-loop response is dominated by one slow pole. When you close the loop with feedback, you effectively turn the open-loop gain into a single-pole closed-loop response. The higher the desired closed-loop gain, the lower the frequency at which the loop can maintain that gain without losing phase margin, so the closed-loop bandwidth shrinks as gain increases. The gain-bandwidth product stays roughly constant, so the product of the closed-loop gain and the closed-loop bandwidth remains about the same. In other words, doubling the closed-loop gain halves the closed-loop bandwidth, within the linear operating range.