Band selection filter with two active elements

Claim 1

A band-selection filter circuit with a center frequency fo and corresponding angular frequency omega o 2 pi fo for realization of a desired transfer function H which contains a conjugate pole-pair in a frequency plane used for the description of the filter circuit comprising a network with at least two types of impedance elements, a first type being a resistor and a second type being lossy resonant circuits, the reactance of said lossy resonant circuits when comprising at least one series resonant circuit and the susceptance of said lossy resonant circuits when comprising at least one parallel resonant circuit being zero at a frequency coinciding with the center frequency fo, and two active elements arranged for compensating the resistive losses of said lossy resonant circuits, said active elements being connected with said impedance elements in such a way that when the transfer function H is expressed in polynomials of gamma S(s, omega o)+ omega o/Qm, where S(s, omega o) is a function of the angular frequency omega o and a complex angular frequency variable s with the dimension of frequency and has the value zero for s + OR - j omega o and is proportional to the reactance and the susceptance respectively of said resonant circuits, and where omega o/Qm represents the resistive losses in said lossy resonant circuits and 1/Qm is an average value of the inverted selectivity factors of said lossy resonant circuits, the realization of said conjugate pole-pair is obtained when the denominator of the transfer function H contains a second-order polynomial gamma 2-(A+B-C). gamma +AB+ D, wherein the coefficient of the linear term in gamma is determined primarily by the sum of the two coefficients A and B and the constant term is determined primarily by the product of said coefficient A and B, the coefficient A being determined by a circuit parameter of one of said two active elements and by the magnitudes of components contained in at least two elements of the said impedance elements, and the coefficient B being determined by a circuit parameter of the other of said two active elements and by the magnitudes of components contained in at least two elements of said impedance elements which are different from the said elements determining the coefficient A, the components of said impedance elements and the circuit parameters of said active elements being so chosen that the coefficients A and B are of equal magnitude.