R-C ACTIVE BANDPASS FILTERS

Abstract

The design of an R-C active bandpass filter is discussed. The filter exhibits double "humping" effects similar to these of two tightly coupled identically tuned parallel L-C circuits. The degree of humping can be controlled to give good amplitude flatness and phase linearity within the passband. The active element of the filter is a do feedback amplifier using Ge transistors. The feedback network consists of two identically tuned R-C twin-T's in cascade. Control of the humping is achieved by varying the complex impedance bridging one of the twin-T's. When the source and load impedances are pure resistances a rigid relationship between the source load product and the twin-T parameters is neseeseary for symmetrical skirt-response. By making the source and load impedance complex, the skirt-response is always symmetrical provided the R-C product of the source and load impedances is identical to that of the twin-T. R-C active bandpass filters have been designed to work in the audio and sub-audio frequency ranges. They have a pass-to-reject ratio in excess of 40db with a bandwidth of approximately 40 per cent of the centre frequency using an amplifier with a voltage gain of 250. In a modified version using an *active twin-T" a pass-to-reject ratio in excess of 45db and a bandwidth of 6 per cent of the centre frequency is obtained. The band edges of the modified version are mach steeper. Design-charts showing bandwidth and amplifier-gain for various twin-T parameters have been constructed to suit different amplifier input and output impedances.