GB1240335A - Improvements in or relating to frequency-selective resistance-reactance networks - Google Patents

Abstract

1,240,335. Active filters; transistor oscillators and amplifiers. ADVANCE ELECTRONICS Ltd. 9 May, 1969 [10 May, 1968], No. 22379/68. Headings H3T and H3U. A frequency selective network comprises a two section resistance-reactance ladder network C1R1, C2R2, Fig. 3, with the outputs from the two sections 2, 3 fed to a combining circuit 7. Buffer amplifiers 19, 20 may be included between the networks and the combining circuit and a further buffer (21, Fig. 5, not shown) may be included between the two C.R. networks. The resistances and reactances may be interchanged, Fig. 4 (not shown). The network is basically bandpass, but may be modified by the provision of various feedback paths. As shown in Fig. 3, a resistive network 12, 17, 18 and a switch 16 are added. With the switch in position c the basic bandpass response is obtained, in position b an allpass response is obtained, and in position a bandstop is obtained. In order to obtain a high pass or low pass response additional feedback paths are provided from amplifiers 19, 20 via a network 30, Fig. 9, and are combined with the input signal 26 in a further combining stage 28. Positive feedback may be added to improve the Q factor, Fig. 6 (not shown). The network may be used in a phase shift oscillator by connecting the terminal 8 of Fig. 3 back to terminal 1 with the switch in the "allpass " position, Fig. 7 (not shown). The various amplifiers in such an oscillator may be realized by transistor circuits, Fig. 10 (not shown). In all the circuits, the C.R. networks may be replaced by LR ones.