What does the gain-bandwidth product indicate in op-amp design, and how does it affect multi-stage designs?

• A. GBW is the product of closed-loop gain and bandwidth; the overall bandwidth is limited by the stage with the lowest bandwidth for its gain.
• B. GBW is the sum of gains across stages and sets phase margin.
• C. GBW only applies to single-stage designs.
• D. GBW equals the ratio of input to output impedance.

Answer: (A)

The gain-bandwidth product shows how gain and speed relate for an op-amp in closed-loop operation. In practice, for a given op-amp, if you set a closed-loop gain, the usable bandwidth is roughly GBW divided by that gain, so the product A_cl × f_bw ≈ GBW stays nearly constant. This means pushing for more gain automatically narrows the bandwidth.

When you design with multiple stages, each stage has its own closed-loop gain and bandwidth, and each stage roughly adheres to the same GBW. Because the product is constant, the stage with the smallest bandwidth for its chosen gain becomes the bottleneck. The overall bandwidth of the whole cascade is limited by that weakest stage, so you can’t just stack gain without paying in speed. To maintain performance, designers often buffer or compensate between stages or choose devices whose GBW supports the desired total gain without sacrificing bandwidth.

This is why GBW is defined as a constant product of gain and bandwidth, and why, in multi-stage designs, the stage with the lowest effective gain-bandwidth capability tends to govern the overall response.