J. of Active and Passive Electronic Devices, Vol. 1, pp. 163-169 Reprints available directly from the publisher Photocopying permitted by license only
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Electronically Tunable High Output Impedance Current-Mode Universal Filter N.A. SHAH, M.F. RATHER* AND S.Z. IQBAL Department of Electronics & Instrumentation Technology University of Kashmir, Srinagar-190 006 (India)
A novel current-mode universal filter using two second generation current controlled conveyors (CCCIIs), one current follower (CF) and two capacitors is presented. The circuit is fully programmable and implements all the five generic filtering functions. The LP, BP, and HP functions can be realized simultaneously while notch and AP responses can be implemented simply by connecting the appropriate node currents. The availability of currents at high impedances, facilitate cascadibility feature. The filter performance factors ω0 and ω0/Q are electronically tunable through separate bias currents of the CCCIIs. The PSPICE simulation results are included to confirm the workability of the proposed circuit. Keywords: Current controlled current conveyors, Current follower, Currentmode active filters.
1. INTRODUCTION Current-mode circuits are receiving significant attention owing to their larger dynamic range, higher band width, greater linearity, simpler circuitry
*Corresponding Author: E-mail:
[email protected]
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and low power consumption vis-a-vis their voltage-mode counterparts [1-4 ]. Universal active filters based on operational transconductance amplifiers (OTAs) have several advantages such as integrability, programmability and simplicity in design implementation. However, these circuits suffer from poor current drive capability and limited dynamic range. CCCII based circuits offer a wide range of electronic tunability of the circuit parameters and a wider frequency range of operation [5-6]. The drawback of these reported circuits is the availability of currents through passive grounded components, entailing the use of active device to obtain the function explicitly. To overcome this limitation Shah and Iqbal [7] introduced a current-mode filter employing three dual output second generation current controlled conveyors (DO-CCCIIs) and two operational amplifiers (OAs). The circuit facilitates integrability and programmability as well as ease of implementation. However, the circuit has a large component count. Here we are proposing a current-mode universal filter which employs only two CCCIIs, one CF, and two capacitors. The proposed filter implements LP, BP, and HP functions from the same configuration. Notch and AP responses can be realized simply by connecting the appropriate node currents. The implementation of these two functions require neither change to be induced in the circuit topology nor additional components nor rotation of components nor imposition of constraints on the parameters of the active and passive elements, as has been the problem with Minaei et al [8, 9]. The circuit permits cascadibility for obtaining higher order filter functions as all the output currents are available at high impedances. The filter performance factors ω0 and ω0 /Q are electronically tunable through the bias currents of the CCCIIs over a wide range of frequencies, a desirable in IC design.
2. CIRCUIT DESCRIPTION Using the standard notation, the CCCII is characterized by the port relations Iy = o, Vx = Vy + RxIx and Iz = ±Ix Where the ± sign depicts the polarity of the CCCII. The parasitic resistance Rx at the x-input terminal of the CCCII, controllable through bias current Io is given by Rx = VTi / 2Ioi
( i = 1,2 )
and the CF is characterized by the following port relations:
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Vx = 0, Vy = 0 and Iz = ±Ix After routine analysis, the circuit yields the following current transfer functions: (1) IHP/IIN = s2C1C2 / D(s) ILP/IIN = (1 / Rx1Rx2C1C2) / D(s) IBP/IIN = (-s / Rx1C1) / D(s)
(2) (3)
Where D(s) = (s2 + s / Rx1C1 +1 / Rx1Rx2C1C2 ) The notch filtering signal can be implemented by connecting IHP and ILP and is given by IN / IIN = ( IHP + ILP) / IIN = (s2+ 1 / Rx1Rx2C1C2 )/D(s)
(4)
Similarly by connecting together IHP , IBP , and ILP, an AP response is obtained IAP / IIN = ( IHP + IBP + ILP ) / IIN = (s2 - s / Rx1C1 +1 / Rx1Rx2C1C2) / D(s) (5) The natural frequency ω0, the bandwidth ω0 / Q and the quality factor Q respectively ω0 = (1 / Rx1Rx2C1C2)1/2 = 2 / VT( Io1 Io2 / C1C2)1/2 ω0 / Q = 2 Io1 / VTC1 Q = (Rx1C1 / Rx2C2)1/2 = ( Io2C1/ Io1C2 )1/2
(6) (7) (8)
Equations (6) and (7) reveal that ω 0/Q and ω 0 can be orthogonally adjusted by bias currents Io1 and I02 in that order. The active and passive sensitivities are small and are given as Sω0 /Rx1Rx2C1C2 = -1/2 Sω0/Q /Rx1C1 = -1 Sω0 /Rx1C1 = -Sω0/Rx2C2 = 1/2
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FIGURE 1 Proposed current-mode universal filter.
3. SIMULATION RESULTS: To verify the validity of theoretical results of the filter in Fig.1, PSPICE simulation was carried out. The CCCII sub-circuit by Fabre et al [ 10 ] was used. The CF was implemented by using AD844 with its y-terminal grounded. The following setting was used to obtain LP, BP, HP, notch and AP functions with a natural frequency fo = 159kHz and Q = 1: Io1 = Io2 =
FIGURE 2 Magnitude responses of LP,BP & HP functions.
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FIGURE 3 Magnitude responses of Notch function.
130µA and C1 = C2 = 10nF. Figs. 2 & 3 respectively depict the magnitude responses of LP, BP, and HP functions and Notch function while Fig. 4 shows the phase response of AP function. The variation of f0 with bias current Io2 is shown in Fig. 5.
FIGURE 4 Phase response of AP function.
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FIGURE 5 Variation of f0 with bias current I02.
4. CONCLUSION: A new current-mode universal filter with a single input and three outputs, implementing all the five generic filtering responses is presented. The circuit enjoys the features of: simultaneous realization of LP, HP, and BP responses, cascadability for higher order filters, no matching constraints are required, filtering characteristics are electronically tunable in an orthogonal manner. The active and passive sensitivity figures are low.
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I. A. Khan and M. H. Zaidi, “Multifunction translinear-C current-mode filter”. Int. J. Electronics., vol. 87, pp.1047-1051, 2000.
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[7]
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[9]
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