Abstract: A low-distortion transformer-coupled circuit. An amplifier is constructed to have a negative output impedance equal to the resistance of the primary winding of a coupling transformer to which the amplifier is connected thereby cancelling the distortion component produced by the transformer distortion current flowing through the primary winding resistance. The input to the amplifier is thus coupled to the secondary winding of the transformer. A low-frequency feedback circuit is provided between the amplifier output and a non-inverting input thereof. Means is provided for adjusting the negative output impedance of the amplifier to match circuit parameters. Alternatively, the coupling transformer employs two matched primary windings and one secondary winding, and the amplifier is constructed to provide a signal to each primary winding.

Abstract: A dual-stage band pass-band reject filter with feedback. Two state variable band pass filters with identical topology are cascaded as first and second stages. Each stage employs two inverting adders and two inverting integrators in a state variable topology, and each provides a band pass output and a band reject output. Both stages are selectively tuned to the same fundamental frequency. The filter output may be switched from a band pass to a band reject mode. In the band reject mode decreased attenuation of harmonics of the fundamental frequency is achieved by providing a predetermined amount of feedback from either the band reject output or band pass output of the second stage to the input of a respective one of the two adders of the first stage.