Controllable filter arrangement

摘要

A controllable filter arrangement comprises a filter network for filtering an input signal of the filter arrangement, and a transistor, the transistor being configured as a voltage-follower having a controllable current source in its output circuit for providing a controllable DC transistor output current, an output terminal of the voltage-follower being connected to the filter network such that a cut-off frequency of the filter can be controlled by variation of the DC current supplied by the current source. The filter network may be a single-pole RC network and the output of the follower may be connected to an additional capacitor (or resistor) of the network which is coupled to the junction of the existing capacitor and resistor. The effect of the filter arrangement is to create a filter which is controllable between two values of cut-off frequency, depending on whether the additional capacitor (or resistor) is effectively driven or not by the voltage-follower, or which is continuously controllable over a range of cut-off frequencies, depending on the value of the current provided by the current source.