Bandwidth Revisited

Because open-loop gain of op-amps is a function of frequency, bode plot and bandwidth are often used to describe op-amp's AC response.
Frequency bandwidth is measured at the point where the op-amp gain falls to 0.707 or 1/√2 of its maximum value. This point is usually referred to as the -3 dB point of the amplifier, indicating that the gain at that point has dropped by 3 dB from its maximum value.

The -3 dB points at some different cases are shown in the accompanying image.

The open-loop configuration is going to have a higher Direct Current (DC) gain. However, as we can see from the figure, this configuration is extremely bandwidth limited. That means the gain starts decreasing (rolling off) at only a few hertz, or that the -3 dB frequency is only a few hertz.

In closed-loop configuration, the DC gain of the amplifier or the gain of the amplifier at zero frequency is reduced. However, the op-amp bandwidth is much wider. It means that the frequency at which the op-amp starts rolling off has increased. The frequency at which the op-amp gain is only 1 or at 0 dB, is called unity gain frequency (bandwidth).