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.