Transistor active filters using twin-T rejection networks

The introduction of a rejection-type network, such as a twin-T or a bridged-T, into the feedback loop of an amplifier leads to bandpass characteristics similar to those of single tuned circuits. It is possible to achieve highly selective circuits at very low frequencies, e.g. in the audio range, where high-Q-factor coils are usually physically large and expensive. Some basic circuits using a symmetrical twin-T network with R and C only in the feedback loop of a transistor amplifier are investigated. Results of measurements are given. Q-factors higher than 50 are obtained at a frequency of 600 c/s.