Minimally active RC variable equalizers

Novel variable equalizer structures (Canadian and U.S. patent application pending.) based on the concept of inductorless simulation of classical Bode variable equalizers are discussed. It is shown that one can remove the need for negative resistance (in a full range equalizer) by switching between negative and positive feedback, thus saving an operational amplifier. A number of typical equalizer responses with a variable range of 10 dB are realized with components and a single operational amplifier. The measured responses are shown to agree closely with the computer-simulated nominal network responses. In a particular example, a 10 dB variable bump equalizer is realized with only two operational amplifiers in contrast to four required in a recent realization.