Attach to A: the open-loop gain. The reason for this name will become obvious later on when we “close the loop” around the op amp and define another gain, the closed-loop gain. An important characteristic of op amps is that they are direct-coupled devices or dc amplifiers, where dc stands for direct- coupled (it could equally well stand for direct current, since a direct-coupled amplifier is one that amplifies signals whose frequency is as low as zero). The face that op amps are direct-coupled devices will allow us to use them in many important applications. Unfortunately, though, the direct-coupling property can cause some serious practical problems, as will be discussed in later section.