The RC amplifier-type active filter: A design method for optimum stability

The RC amplifier filter has many advantages over its passive RLC counterpart for low-audio frequency or narrow-band applications. The major disadvantage to an active filter has been its sensitivity to component tolerance and drift, particularly where high Q\´s are involved. A design method is presented which optimizes transmission stability when the probable "drift" (tolerances, temperature and aging effects, tracking errors) in each circuit parameter is known. Design equations are derived for a group of two-pole networks with external zeros. Two experimental high-Q filters are discussed including a continuously tunable four-pole filter which has attenuation of 80 db at twice and at one-half center frequency.