Because op amps are direct-coupled devices with large gains at dc, they are prone to dc
problems. The first such problem is the dc offset voltage. To understand this problem consider
the following conceptual experiment: If the two input terminals of the op amp are tied
together and connected to ground, it will be found that despite the fact that vId = 0, a finite dc
voltage exists at the output. In fact, if the op amp has a high dc gain, the output will be at
either the positive or negative saturation level. The op-amp output can be brought back to its
ideal value of 0 V by connecting a dc voltage source of appropriate polarity and magnitude
between the two input terminals of the op amp. This external source balances out the input
offset voltage of the op amp. It follows that the input offset voltage (VOS) must be of equal
magnitude and of opposite polarity to the voltage we applied externally.