Synthesis of Active RC Networks

Passive filters using only resistive and capacitive elements are attractive for reasons of size, cost and reliability. Their use has been limited because the network complexity of RC filters (due to restrictions on impedance functions realizable with R's and C's only) is greater than that of equivalent RLC filters. This defect can be overcome by using active elements in addition to passive RC networks. Active RC networks are particularly attractive for achieving exacting performance at low frequencies, where it is not practical to use either inductors or crystals. The practicality of active networks is a direct result of the availability of precision resistors and capacitors of small size having small drifts and low temperature coefficients, as well as the development of reliable junction transistors. In fact, it is not unusual to find that the active element's drift with time and temperature is no worse than that of a passive element. There are several techniques available for synthesis of transfer functions by active RC networks. 1,2 ' 3,4,5 The active element used is either a stabilized high-gain feedback amplifier or a negative-impedance converter. W i t h the feedback amplifier, one RC network is used to produce the zeros of the transfer function and another RC network produces the poles of the function. The number of passive elements required in this 1269