Offset free Automatic frequency tuning circuit for Gm-C filter

摘要

PURPOSE: An automatic frequency tuning circuit for correcting an influence of offset for transistor conductance-capacitor filter is provided to control correctly a band frequency of a filter by correcting offset of a tuning circuit. CONSTITUTION: A reference power supply portion(100) provides reference power. A Gm amplifier(5) has. The first integrator(200) has a Gm amplifier(5) having the same as a Gm value of a controlled filter in order to generate periodically charge voltage and discharge voltage proportional to offset of the Gm amplifier(5) in response to the reference power. The second integrator(300) has a Gm amplifier(16) having the same as a Gm value of a controlled filter in order to generate periodically charge voltage and proportional to offset of the Gm amplifier(16) in response to the output voltage(Vo1) of the first integrator(200) and discharge voltage proportional to offset of the Gm amplifier(16) in response to the reference power. A sampling portion(400) samples periodically the output voltage of the second integrator(300). An output portion(500) outputs the sampled voltage of the sampling portion(400) as a tuning signal and feed back the tuning signal to the Gm amplifier(5) in order to control a transfer conductance value of the Gm amplifier(5).