Abstract: A filter circuit comprising: a signal path for carrying a signal from an input to an output; the signal path comprising a first reactive component; a first node on the signal path; a first series resonant circuit comprising at least a second reactive component in series with a third reactive component, the first series resonant circuit being connected between the first node and a ground; an active circuit; the active circuit comprising a voltage controlled current source (VCCS) arranged to change the current flow through the second reactive component in dependence on a voltage sensed (or measured) on the signal path. The first series resonant circuit forms a single harmonic trap with a notch frequency defined by the component values of its reactive components. The effectiveness of the series resonant circuit is dependent upon the strength with which it draws current from the signal path at its resonant frequency.

Abstract: An amplifier, including: an amplifying element, having a voltage input across a first terminal and a third terminal and a voltage controlled current path between a second terminal and the third terminal; and a trifilar transformer having a primary winding, a secondary winding and a tertiary winding; wherein the primary winding is connected to the third terminal, the secondary winding is connected to the first terminal and the tertiary winding is connected to the second terminal; wherein the primary winding and the secondary winding are mutually coupled in inverting relationship; wherein the primary winding and the tertiary winding are mutually coupled in non-inverting relationship; wherein the secondary winding and the tertiary winding are mutually coupled in inverting relationship; and wherein the tertiary winding is between the amplifier output and the second terminal.

Abstract: A trifilar transformer comprising: a first winding; a second winding; and a third winding, wherein one winding is mutually coupled to each of the other two windings, and wherein said other two windings are substantially not coupled to each other. At least one of the first winding, the second winding and the third winding may comprise a figure-of-eight winding, e.g. a clockwise loop and an anti-clockwise loop. In some embodiments, the trifilar transformer may comprise: a first winding; a second winding concentric or interwound with the first winding; and a third winding formed from a first winding part in series with a second winding part, the first winding part having a shape corresponding to the first winding and the second winding part having a shape corresponding to the second winding.

Abstract: An amplifier, comprising: an amplifying element having an input side and an output side; a first transformer on the output side of the amplifying element arranged to mutually couple a fraction of the output current from the amplifying element onto the input side of the amplifying element; a second transformer on the input side of the amplifying element arranged to increase the input voltage on the input side via mutual coupling of its primary and secondary windings; wherein a primary winding of the first transformer is connected to an output of the amplifying element; wherein a secondary winding of the first transformer is ac connected to a secondary winding of the second transformer; and wherein the primary winding of the first transformer is dc blocked from the secondary winding of the second transformer. The negative and the positive reactive feedback loops are not formed from the same trifilar transformer.

Abstract: A circuit comprising: a passive reactive component; and an active circuit, the active circuit arranged to increase the ac voltage difference across the reactive component by changing the current at an input to the reactive component and the current at an output of the reactive component by equal and opposite amounts. By increasing the current on one side of the resonant circuit and decreasing the current on the other side of the resonant circuit, the amount of current flowing through the resonant circuit is increased and thus the ac voltage difference across the inductor of the LC resonant circuit is increased. The Q of an inductor (the ratio of its imaginary to real impedance) is increased. In a filter, the improved Q provides a sharp, high rejection notch and faster pass-band to stop-band roll-off, thus improving the frequency response of the circuit.

Abstract: An amplifier, including: an amplifying element, having a voltage input across a first terminal and a third terminal and a voltage controlled current path between a second terminal and the third terminal; and a trifilar transformer having a primary winding, a secondary winding and a tertiary winding; wherein the primary winding is connected to the third terminal, the secondary winding is connected to the first terminal and the tertiary winding is connected to the second terminal; wherein the primary winding and the secondary winding are mutually coupled in inverting relationship; wherein the primary winding and the tertiary winding are mutually coupled in non-inverting relationship; wherein the secondary winding and the tertiary winding are mutually coupled in inverting relationship; and wherein the tertiary winding is between the amplifier output and the second terminal.

Abstract: A trifilar transformer comprising: a first winding; a second winding; and a third winding, wherein one winding is mutually coupled to each of the other two windings, and wherein said other two windings are substantially not coupled to each other. At least one of the first winding, the second winding and the third winding may comprise a figure-of-eight winding, e.g. a clockwise loop and an anti-clockwise loop. In some embodiments, the trifilar transformer may comprise: a first winding; a second winding concentric or interwound with the first winding; and a third winding formed from a first winding part in series with a second winding part, the first winding part having a shape corresponding to the first winding and the second winding part having a shape corresponding to the second winding.

Abstract: An amplifier, comprising: an amplifying element having an input side and an output side; a first transformer on the output side of the amplifying element arranged to mutually couple a fraction of the output current from the amplifying element onto the input side of the amplifying element; a second transformer on the input side of the amplifying element arranged to increase the input voltage on the input side via mutual coupling of its primary and secondary windings; wherein a primary winding of the first transformer is connected to an output of the amplifying element; wherein a secondary winding of the first transformer is ac connected to a secondary winding of the second transformer; and wherein the primary winding of the first transformer is dc blocked from the secondary winding of the second transformer. The negative and the positive reactive feedback loops are not formed from the same trifilar transformer.

Abstract: A circuit comprising: a passive reactive component; and an active circuit, the active circuit arranged to increase the ac voltage difference across the reactive component by changing the current at an input to the reactive component and the current at an output of the reactive component by equal and opposite amounts. By increasing the current on one side of the resonant circuit and decreasing the current on the other side of the resonant circuit, the amount of current flowing through the resonant circuit is increased and thus the ac voltage difference across the inductor of the LC resonant circuit is increased. The Q of an inductor (the ratio of its imaginary to real impedance) is increased. In a filter, the improved Q provides a sharp, high rejection notch and faster pass-band to stop-band roll-off, thus improving the frequency response of the circuit.

Abstract: An amplifying circuit arranged for converting an input signal into an amplified output signal comprising: an input node (11) at an input side of said circuit for receiving said input signal (pi); an output node (9) at an output side of said circuit for outputting said amplified output signal (io); a first gain element (M1) connected between said input and output nodes and provided for converting an input voltage taken from said input signal into a current for forming said amplified output signal; a negative feedback loop (3) over said first gain element, said negative feedback loop having first elements (5, 6) arranged for providing input matching; and a positive feedback loop (2) over said first gain element, said positive feedback loop having second elements (7, 8) arranged for providing additional input matching and gain enhancement of said first gain element.

Abstract: An amplifying circuit arranged for converting an input signal into an amplified output signal comprising: an input node (11) at an input side of said circuit for receiving said input signal (pi); an output node (9) at an output side of said circuit for outputting said amplified output signal (io); a first gain element (M1) connected between said input and output nodes and provided for converting an input voltage taken from said input signal into a current for forming said amplified output signal; a negative feedback loop (3) over said first gain element, said negative feedback loop having first elements (5, 6) arranged for providing input matching; and a positive feedback loop (2) over said first gain element, said positive feedback loop having second elements (7, 8) arranged for providing additional input matching and gain enhancement of said first gain element.