An amplifier suitable for use as an active filter such as a low-pass filter, a high-pass filter or a phase shifter, which can be easily formed as part of an integrated circuit. The amplifier includes first and second transistors, which together form a first differential amplifier for converting the voltage of the input signal thereto to a signal current. It also includes third and fourth transistors, which together form a second differential amplifier for dividing the signal current from the second transistor in a given ratio. A capacitor is connected to the collector of the fourth transistor, and first and second resistors are connected, respectively, to emitters of the first and second transistors. The current division ratio of the current flowing through the third and fourth transistors is controlled by a control voltage DC supplied to the base of the third transistor and the base of the fourth transistor. In the amplifier, the signal current shunted in the current division ratio K between the third and fourth transistors flow through the load capacitor so that the time constant of the active filter is controlled, the ratio K being controlled by the control voltage V.sub.c. The control voltage V.sub.c also compensates for the inaccuracy of the values of the resistors and capacitors.