A direct coupling is one in which the output of one stage of amplifier is connected to the input of second stage in such a way to allow even DC signals from one stage to another. The frequency response of the direct coupled amplifier is similar to low pass filter and is characterized by rise time.
The main disadvantage with DC coupling is, since there is no DC isolation the first stage acts as a biasing circuitry for the second stage, the AC signal current is superimposed on DC quiescent currents and this effect propagates.