Basic Op-amp Circuits: Bandwidth

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 and indicates the gain at the point where it 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 Figure 1, this configuration is extremely bandwidth limited. That means the gain starts 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. This basically means the frequency at which the op amp starts rolling off has increased (remember that this is the -3 dB point.)

The frequency at which the op-amp gain is only 1 or at 0 dB.

Application Challenge

A closed-loop system is a more desirable configuration when using an amplifier unless the amplifier is used as a comparator. (A comparator is a device that compares two voltages and switches its output to indicate which is larger.)