A differential circuit having a first positive arm and a second negative arm; wherein the first positive arm comprises at least one first reactive component and the second negative arm comprises at least one second reactive component ; further comprising: an active circuit , the active circuit arranged to increase the ac voltage difference across the first reactive component by changing the current at an input to the first reactive component and the current at an output of the first reactive component by equal and opposite amounts, and the active circuit arranged to increase the ac voltage difference across the second reactive component by changing the current at an input to the second reactive component and the current at an output of the second reactive component by equal and opposite amounts; wherein the active circuit comprises: a first amplifying element (Mi); a second amplifying element (M 2 ); a third amplifying element (M 3 ); a fourth amplifying element (M 4 ); a first current source (I 1 ) arranged to draw current through the first amplifying element (M 1 ) of the active circuit from the output of the first reactive component and arranged to draw current through the second amplifying element (M 2 ) of the active circuit from the input of the first reactive component , the first amplifying element (M 1 ) being driven by a signal input to the first reactive component and the second amplifying element (M 2 ) being driven by a signal input to the second reactive component ; and a second current source (I 2 ) arranged to draw current through the third amplifying element (M 3 ) of the active circuit from the output of the second reactive component and arranged to draw current through the fourth amplifying element (M 4 ) of the active circuit from the input of the second reactive component , the third amplifying element (M3) being driven by the signal input to the second reactive component and the fourth amplifying element (M 4 ) being driven by the signal input to the first reactive component . an active circuit , the active circuit arranged to increase the ac voltage difference across the first reactive component by changing the current at an input to the first reactive component and the current at an output of the first reactive component by equal and opposite amounts, and the active circuit arranged to increase the ac voltage difference across the second reactive component by changing the current at an input to the second reactive component and the current at an output of the second reactive component by equal and opposite amounts; wherein the active circuit comprises: a first amplifying element (Mi); a second amplifying element (M 2 ); a third amplifying element (M 3 ); a fourth amplifying element (M 4 ); a first amplifying element (Mi); a second amplifying element (M 2 ); a third amplifying element (M 3 ); a fourth amplifying element (M 4 ); a first current source (I 1 ) arranged to draw current through the first amplifying element (M 1 ) of the active circuit from the output of the first reactive component and arranged to draw current through the second amplifying element (M 2 ) of the active circuit from the input of the first reactive component , the first amplifying element (M 1 ) being driven by a signal input to the first reactive component and the second amplifying element (M 2 ) being driven by a signal input to the second reactive component ; and a second current source (I 2 ) arranged to draw current through the third amplifying element (M 3 ) of the active circuit from the output of the second reactive component and arranged to draw current through the fourth amplifying element (M 4 ) of the active circuit from the input of the second reactive component , the third amplifying element (M3) being driven by the signal input to the second reactive component and the fourth amplifying element (M 4 ) being driven by the signal input to the first reactive component . <EOS>