Direct-coupled amplifier
A direct-coupled amplifier or DC amplifier is a type of amplifier in which the output of one stage of the amplifier is coupled to the input of the next stage in such a way as to permit signals with zero frequency, also referred to as direct current, to pass from input to output. This is an application of the more general direct coupling. Current. The common use of the term "DC amplifier" does not mean "direct current amplifier", as this type can be used for both direct current and alternating current signals. The frequency response of  the direct coupled amplifier is similar to low pass filter and hence it is also known as "Low-Pass Amplifier". The amplification of DC (zero frequency) is possible only by this amplifier, hence it later becomes the building block for differential amplifier and operational amplifier. Furthermore, monolithic integrated circuit technology does not allow the fabrication of large coupling capacitors. Drift. Direct-coupled amplifiers constructed in the conventional form of single-ended amplifiers connected in cascade suffer from drift of the output voltage with time and temperature. In many high-gain applications it is necessary to provide offset adjustments to deal  with drift. The drift problem can be overcome by using differential amplifiers. Application. Direct-coupled amplifiers are used in voltage regulators, servo drives and other instrumentation amplifiers. It also forms a building block for differential amplifiers and operational amplifiers.