International Journal of Computer and Electrical Engineering, Vol. 1, No. 3, August 2009 1793-8163 - 307 - Abstract—In this paper a new second–order voltage– and current–mode universal biquadratic frequency filters using only single current conveyor transconductance amplifier (CCTA) and four passive elements are presented. The proposed voltage–mode universal filter has been transformed into current–mode universal filter using adjoint transformation method. Proposed circuits enables to realize the low–, band–, high–pass, band–stop and all–pass responses without changing the circuit topology and the possibility of independent control of the quality factor Q using single passive element. No component matching is required and all the passive and active sensitivities are low. PSPICE simulations and experimental results based on commercially available amplifiers OPA861 and MAX435 are included verify the theoretical conclusions of the proposed circuits. Index Terms— Active filter, adjoint transformation, CCTA, current– and voltage–mode, universal biquadratic filter. I. INTRODUCTION Frequency filters are linear electric circuits, which are used in wide area of electronics and also are basic building blocks in analogue signal processing. The analogue frequency filters are the most often used in high–speed data communication systems defined by ITU standards, for signal processing in cable modems, in regulation and measurement techniques, in electro acoustics or in automobile industry [1]. In the last decade, the current–mode signal processing circuits have received increasing attention, due to their advantages [2–3] and the highest attention is paid to universal filter realization that can realize all filter functions such as low–, band–, high–pass, band–stop, and all–pass filter response. A number of papers concerning this issue can be found [4–11]. These papers presents voltage– or current–mode universal filters using single CFA (current–feedback amplifier) [4–5], second–generation current conveyor (CCII) [6], second–generation current–controlled conveyor (CCCII) [7], fully–differential second–generation current conveyor (FDCCII) [8], differential–difference current conveyor (DDCC) [9], current differencing buffered amplifier (CDBA) [10] and operational transresistance amplifier (OTRA) [11]. Manuscript received May 28, 2009. The paper has been supported by the Czech Science Foundation project GACR 102/09/1681 and Ministry of Education of the Czech Republic project No. MSM0021630513. Norbert Herencsar, Jaroslav Koton, and Kamil Vrba are with the Department of Telecommunications, Faculty of Electrical Engineering and Communication, Brno University of Technology, Purkynova 118, 612 00 Brno, Czech Republic (phone: +420–541–149–190; fax: +420–541–149–192). However, the proposed circuits require certain component matching conditions. Hence, the active and passive component sensitivities are larger than unity in absolute value. In this work, we have proposed a novel voltage–mode universal filter using single current conveyor transconductance amplifier (CCTA) [12] and four passive elements. Using the adjoint transformation method [13] the current–mode frequency filter has been designed. The proposed structures have been simulated using PSPICE and experimentally measured to verify the theoretical analysis. The active and passive sensitivities of the presented structures are low. II. DESCRIPTION OF THE CCTA AND FILTER DESIGN The current conveyor transconductance amplifier (CCTA) [12] is a combination of the third–generation current conveyor (CCIII) [14] and balanced-output operational transconductance amplifier (BOTA) [15]. In [16] the CCIII in input stage of the CCTA has been replaced by CCCII because of more flexibility in frequency filter and oscillator design. In this work the CCIII has been replaced by CCII [17]. The schematic symbol of the CCTA is shown in Fig. 1. Relations between the individual terminals of the CCTA can be described by following hybrid matrix: y y x x z z o+ o+ m o o m 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 v i i v v i v i g v i g - -                         =       +             -       .(1) The proposed voltage–mode filter is shown in Fig. 2 (a) employs only one CCTA and four passive elements. The output voltage V o of this circuit is given as follows: 2 1 2 1 2 1 2 2 m 3 2 m 4 2 1 2 2 1 2 m i i i i o CC CG +G g Cg CC CG Gg + - = + + s V s V V s V V s s . (2) Fig. 1 Schematic symbol of the CCTA Single CCTA–Based Universal Biquadratic Filters Employing Minimum Components Norbert Herencsar, Jaroslav Koton, and Kamil Vrba