Published in IET Circuits, Devices & Systems Received on 3rd August 2009 Revised on 10th December 2009 doi: 10.1049/iet-cds.2009.0213 ISSN 1751-858X Current-mode operational transconductance amplifier-capacitor biquad filter structures based on Tarmy– Ghausi Active-RC filter and second-order digital all-pass filters D.V. Kamat 1 P.V. Ananda Mohan 2 K. Gopalakrishna Prabhu 3 1 Department of Electrical and Computer Engineering, Manipal Institute of Technology, Manipal 576104, Karnataka, India 2 R&D, ECIL, 2nd Floor, 30/1 Cunningham Road, Bangalore 560052, Karnataka, India 3 Department of Biomedical Engineering, Manipal Institute of Technology, Manipal 576104, Karnataka, India E-mail: anandmohanpv@live.in Abstract: The Tarmy– Ghausi (TG) active resistor– capacitor (RC) filters using op-amps and its modification by Moschytz are well known to active RC filter designers. These use first-order all-pass networks in a negative feedback loop. New current-mode universal operational transconductance amplifier-capacitor (OTA-C) biquad filters based on the TG active RC filter are considered here. These are based on a recently proposed OTA-C based first-order all-pass network. Three different feedback arrangements are investigated in the proposed filter structure so as to reduce the pole-Q sensitivity. The proposed biquad filters are shown to implement all different types of filters like low-pass, high-pass, band-pass, symmetric notch, all-pass, low-pass notch and high-pass notch. The synthesis of the general biquad is carried out in a novel way by invoking the analogy with direct-form digital filter structures. The special case of all-pass filter realisation derived from the proposed universal filter needs additional hardware for realising the feed-forward coefficients. Hence alternative OTA-C based all-pass filter implementations based on Mitra–Hirano and Gray–Markel second-order digital filter structures are derived in which the coefficients that are used to realise the denominator are shared in the realisation of numerator also. All the proposed circuits are compared with the other structures available in the literature. The simulation results of the proposed circuits are also presented. 1 Introduction The operational transconductance amplifier-capacitor (OTA-C) approach [1–26] is one of the most successful design methods for continuous-time integrated filter design at high frequencies. At present there is growing interest in current-mode signal processing [3] because of its advantages like increased bandwidth, high dynamic range and reduced power supply requirements. The single-output OTAs (SO-OTA) have only one current output and hence may be restricted in their use in applications requiring current feedback to realise filtering functions. Hence, the current-mode active filters employing active elements like dual-output OTAs (DO-OTA) and multiple-output (≥2) OTAs (MO-OTA) [5–26] have become an attractive research topic. These filters are generally based on two integrator-loops. The all-pass filters are widely used in analogue signal processing as group delay equalisers in order to shift the phase while keeping the amplitude constant. First-order all-pass filter can also be used as a building block to realise second-order filters [27–36] and quadrature oscillators. The Tarmy–Ghausi (TG) [28] biquad is one of the popular biquad filter realisation method in which two identical first-order all-pass networks are cascaded in a 346 IET Circuits Devices Syst., 2010, Vol. 4, Iss. 4, pp. 346–364 & The Institution of Engineering and Technology 2010 doi: 10.1049/iet-cds.2009.0213 www.ietdl.org