Define GBW and describe its effect on op-amp closed-loop performance.

• A. GBW is the sum of closed-loop gain and bandwidth.
• B. GBW is the product of open-loop gain and open-loop bandwidth.
• C. GBW is the DC gain of the op-amp.
• D. GBW is the product of the closed-loop gain and its bandwidth; it is roughly constant for a simple dominant-pole op-amp.

Answer: (D)

GBW represents the speed-gain trade-off of an op-amp. In a simple dominant-pole design, the gain-bandwidth product stays roughly constant: if you set a higher closed-loop gain, the closed-loop bandwidth shrinks so that the product of the two remains near the GBW. This happens because the op-amp’s internal compensation creates a single dominant pole, giving an open-loop transfer roughly A0/(1 + s/ωp). The unity-gain frequency, which defines GBW, is about f_t ≈ A0·f_p. With feedback, the closed-loop gain is approximately the desired G_cl, and the bandwidth is roughly f_t divided by G_cl. Therefore, G_cl × f_bw ≈ GBW. This means higher closed-loop gain reduces bandwidth, while lower gain increases bandwidth, up to the limit set by the op-amp’s GBW.