Twin-T feedback filters†

Abstract
Amplifiers with a single twin-T in the feeback path are analysed using root locus methods. A design approach has been suggested for realizing high-Q transistorised active filters resulting in a three-transistor configuration with a stable Q of 200. The behaviour of parallel and cascade configurations of two twin-Ts are studied with a view to realizing stagger tuned band-pass characteristics. It is found that frequencies of selection of the system are much different from the null frequencies of the two twin-Ts. This behaviour is explained using root locus techniques. The difference frequencies for various separations and loop gains have been tabulated. Finally it is concluded that stagger tuning can be realized only by connecting separate high-Q filters in cascade.

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