Filter circuit

A filter circuit, comprising an input terminal for receiving an input signal;

• first and second transistor means, each having base, emitter and collector electrodes, said first and second transistor means being connected in differential amplifier configuration and having their emitter electrodes connected to a common connection;
• variable current source means connected to said common connection;
• said input terminal being connected to the base electrode of said first transistor means;
• a source of reference potential;
• reactance means coupled between the collector electrode of said second transistor means and said source of reference potential;
• third transistor means having base, emitter and collector electrodes, the base electrode of said third transistor means being connected to the collector electrode of said second transistor means and the emitter electrode of said third transistor means being connected to the base electrode of said second transistor means;
• an additional current source connected to the emitter electrode of said third transistor means;
• resistance means connected between the emitter electrode of each of said first and second transistor means and said common connection;
• and an output terminal connected to the emitter electrode of said third transistor means, whereby the cut-off frequency of said filter circuit is a function of the current produced by said variable current source.