Performance of GIC-derived active RC biquads with variable gain

GIC-derived second-order active filter sections are known to be `optimal¿ for a certain choice of element values that preset the filters´ signal gain to a fixed value. Further investigations show that, owing to finite amplifier gain-bandwidth products, GIC-derived filters suffer from various deviations, such as severe Q-enhancement, if the gain must be adjustable, so that the optimal low-sensitivity design cannot be implemented. This paper examines this effect and proposes a simple passive compensation technique that requires only a single additional resistor and results in greatly improved GIC-type-filter performance for all gains.