A current circuit includes a first group and a second group of transistors whose emitters are connected via respective resistors to a voltage source. The collectors of the first-group of transistors (50, 51) are connected together to an output terminal (43) and those of the second group of transistors (70, 71) are connected together to a current source (74) that produces a constant current (I). The bases of the first and second groups of transistors are connected together to form a current mirror, so that the same constant current is drawn by the first group of transistors to the output terminal. From the output terminal, a current inversely variable with uniform resistance variations is drawn, so that a current supplied from the output terminal is a difference between the constant current and the inversely variable current. The current from the output terminal drives an active filter (10) which includes switching circuits and resistor-capacitor circuitry.