A filter circuit (406; 706) comprising: a first input terminal (420; 720); a reference terminal (422; 722), wherein the filter circuit is configured to receive an input voltage waveform across the first input terminal and the reference terminal; a current regulation device (424; 724); and a current sensing element (426; 726), wherein: the current regulation device (424; 724) comprises a conduction channel and a control terminal, wherein the current regulation device is configured to regulate current flow through the conduction channel in accordance with a regulation-control-signal received at its control terminal; and (i) the current sensing element (426; 726) and (ii) the conduction channel (i) the current sensing element (426; 726) and (ii) the conduction channel of the current regulation device, are connected in series with each other between the first input terminal (420; 720) and the reference terminal (422; 722); characterised by the filter circuit further comprising: a first feedback circuit configured to: receive a sensed-voltage-level-signal representative of a sensed voltage across the current sensing element (426; 726); receive a voltage-set-point-signal (428; 728); and set the regulation-control-signal such that the sensed-voltage-level-signal tends towards the voltage-set-point-signal (428; 728); and a second feedback circuit configured to: receive a predetermined-threshold-signal; receive a regulation-control-voltage-signal, representative of a voltage level of the regulation-control-signal; and adjust the voltage-set-point-signal (428; 728) in accordance with a comparison between the regulation-control-voltage-signal and the predetermined-threshold-signal. a first input terminal (420; 720); a reference terminal (422; 722), wherein the filter circuit is configured to receive an input voltage waveform across the first input terminal and the reference terminal; a current regulation device (424; 724); and a current sensing element (426; 726), wherein: the current regulation device (424; 724) comprises a conduction channel and a control terminal, wherein the current regulation device is configured to regulate current flow through the conduction channel in accordance with a regulation-control-signal received at its control terminal; and (i) the current sensing element (426; 726) and (ii) the conduction channel (i) the current sensing element (426; 726) and (ii) the conduction channel of the current regulation device, are connected in series with each other between the first input terminal (420; 720) and the reference terminal (422; 722); characterised by the filter circuit further comprising: a first feedback circuit configured to: receive a sensed-voltage-level-signal representative of a sensed voltage across the current sensing element (426; 726); receive a voltage-set-point-signal (428; 728); and set the regulation-control-signal such that the sensed-voltage-level-signal tends towards the voltage-set-point-signal (428; 728); and a second feedback circuit configured to: receive a predetermined-threshold-signal; receive a regulation-control-voltage-signal, representative of a voltage level of the regulation-control-signal; and adjust the voltage-set-point-signal (428; 728) in accordance with a comparison between the regulation-control-voltage-signal and the predetermined-threshold-signal. a first feedback circuit configured to: receive a sensed-voltage-level-signal representative of a sensed voltage across the current sensing element (426; 726); receive a voltage-set-point-signal (428; 728); and set the regulation-control-signal such that the sensed-voltage-level-signal tends towards the voltage-set-point-signal (428; 728); and receive a sensed-voltage-level-signal representative of a sensed voltage across the current sensing element (426; 726); receive a voltage-set-point-signal (428; 728); and set the regulation-control-signal such that the sensed-voltage-level-signal tends towards the voltage-set-point-signal (428; 728); and a second feedback circuit configured to: receive a predetermined-threshold-signal; receive a regulation-control-voltage-signal, representative of a voltage level of the regulation-control-signal; and adjust the voltage-set-point-signal (428; 728) in accordance with a comparison between the regulation-control-voltage-signal and the predetermined-threshold-signal. receive a predetermined-threshold-signal; receive a regulation-control-voltage-signal, representative of a voltage level of the regulation-control-signal; and adjust the voltage-set-point-signal (428; 728) in accordance with a comparison between the regulation-control-voltage-signal and the predetermined-threshold-signal.