New Current Mode Biquad Filter Using CFOAs

J. of Active and Passive Electronic Devices, Vol. 2, pp. 293–298Reprints available directly from the publisherPhotocopying permitted by license only

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New Current Mode Biquad Filter Using CFOAs

D. R. BHASKAR∗ AND DINESH PRASAD

aDepartment of Electronics and Communication EngineeringFaculty of Engineering and Technology, Maulana Mohammed Ali Jauhar Marg

Jamia Millia Islamia, Jamia Nagar, New Delhi-110025, India

A new current-mode universal biquad filter using only two currentfeedback operational amplifiers (CFOAs) and five passive elements ispresented. The proposed configuration realizes the second order lowpass, high pass and band pass responses simultaneously. The circuitemploys both grounded capacitors and offers very low active and passivesensitivities. The validity of the proposed configuration is establishedthrough SPICE simulations.

Key words: Current feedback operational amplifiers, current mode signalprocessing, current- mode filters.

1 INTRODUCTION

Owing to the well-known advantages of current mode signal processing[1], many active filters employing Current Conveyors (CC) and operatingin current mode with single-input-multi-output (SIMO)-type, have beenproposed in the literature, see [2–5] and references cited therein. Currentfeedback operational amplifiers (CFOA) such as AD844 (which contain aplus-type CC (CCII) followed by a voltage buffer) have been receivingconsiderable attention in the context of various signal processing/signalgeneration circuits due to their potential advantageous features over thetraditional voltage mode op-amps, see [6–8] and references cited therein.

Recently, in [9], a good current-mode multifunction (SIMO)-type filterconfiguration using three CFOAs and five passive elements was proposed.The purpose of this paper is to propose a new circuit configuration, which,by contrast, employs only two CFOAs and five passive elements to achievemost of the desirable features including the employment of two groundedcapacitors, as preferred for integrated circuit implementation.

∗Corresponding author: E-mail: [email protected]

293

294 BHASKAR AND PRASAD

FIGURE 1

2 PROPOSED CONFIGURATION

The proposed configuration is shown in Figure 1. Assuming the standardnotation, the port relations of a CFOA can be characterized by Vx =Vy, iy = 0, iz = ix and Vw = Vz . A routine analysis of the circuit showsthat the transfer functions realized by the configuration are given by:

T1(s) = IO3

Iin= −

1C1C2 R2 R3

s2 + s(

1C1 R1

)+ 1

C1C2 R1 R2

(1)

T2(s) = IO2

Iin= −

sC1 R2

s2 + s(

1C1 R1

)+ 1

C1C2 R1 R2

(2)

T3(s) = IO1

Iin= s2

s2 + s(

1C1 R1

)+ 1

C1C2 R1 R2

(3)

Taking into account the non-idealities of the CFOAs, namely Iz = αIx ,Vx = βVy and Vw = γ Vz , where α = 1 − ε1 and ε1 (ε1 � 1) denotes thecurrent tracking errors, β = 1 − ε2(ε2 � 1) is the input voltage trackingerror and γ = 1 − ε3(ε3 � 1) is the output voltage tracking error of the

NEW CURRENT MODE BIQUAD FILTER USING CFOAS 295

CFOAs, the natural frequency ω0 and Q0 are given by:

ω0 =√

α1α2β1β2γ1

C1C2 R1 R2(4)

Q0 =√

C1 R1α1α2β1β2γ1

C2 R2(5)

The sensitivities of ω0 and Q0 to active and passive components are

Sω0C1,C2 ,R1,R2

= −1

2, Sω0

α1,α2,β1,β2,γ1= 1

2(6)

SQ0C2 ,R2

= −1

2, SQ0

C1,R1,α1,α2,β1,β2,γ1= 1

2(7)

all of which are seen to be very low.

3 SIMULATION RESULTS

To verify the theoretical analysis, the proposed circuit has been simulatedusing SPICE circuit simulation programme. The component values usedwere C1 = 0.1 nF, C2 = 0.2 nF, the CFOAs were biased with ±12 voltsD.C. power supplies with R1 = R2 = 20K� and R3 = 19.8K�. AD844from Analog Devices was used to implement CFOAs. Figure 2 shows the

FIGURE 2

296 BHASKAR AND PRASAD

FIGURE 3

simulated filter responses of low pass, high pass and band pass. Theseresults, thus confirm the validity of the proposed configuration.

In addition to the above simulated results, the performance of theproposed multifunction (SIMO)-type filter was compared with the second-order Sallen-Key [10] high pass filter shown in Figure 3. The simulations wereagain carried out using SPICE for the same values of passive componentsas those used in the proposed configuration. From the simulation results,which are shown in Figure 4, it can be seen that the proposed filter has a

FIGURE 4

NEW CURRENT MODE BIQUAD FILTER USING CFOAS 297

wider bandwidth, greater linearity and much sharper pass band than theconventional op-amp filter.

4 CONCLUDING REMARKS

In this paper a new current-mode biquad filter configuration has beenproposed which employs only two CFOAs and five passive elements, incontrast to a circuit recently published in [9]. The proposed filter structurerealizes the second order low pass, high pass and band pass responsessimultaneously. The configuration employs two grounded capacitors andoffers very low active and passive sensitivities. To justify wider bandwidth,greater linearity and much sharper pass band than the conventional op-ampfilters, a comparison with a second order Sallen-Key high pass filter hasalso been carried out. SPICE simulations have established the workabilityof the proposed formulation.

ACKNOWLEDGEMENT

The authors gratefully acknowledge Prof. R. Senani, Head, Division ofElectronics and Communication Engineering, NSIT, Azad Hind Fauj Marg,Sector-3, Dwarka, New Delhi, India, for useful discussions/ suggestionsand his help in the preparation of this manuscript.

REFERENCES

[1] Toumazau, C., Lidgey, F. J. and Haig, D. G. (1990). Analogue IC Design: Thecurrent- mode approach. London: Peter Peregrinus Limited.

[2] Senani, R. (1996). A simple approach of deriving single-input-multiple outputcurrent-mode biquad filters. FREQUENZ, 50, 124–127.

[3] Senani, R. (1992). New current-mode biquad filter. Int. J. Electronics, 73(4), 735–742.

[4] Ozoguz, Toker, A., and Cicekoglu, O. (1998). High output impedance current-modemultifunction filter with minimum number of active and reduced number of passiveelements. Electronics Lett., 34(19), 1807–1809.

[5] Chang, C. M. (1993). Novel universal current-mode filter with single-input andthree-outputs using only five current conveyors. Electronics Lett., 29(23), 2005–2007.

[6] Senani, R. (1998). Realization of a class of analog signal processing/signal generationcircuits: Novel configurations using current feedback op-amps. FREQUENZ, 52(9–10),196–206.

[7] Bhaskar, D. R. (2003). Realization of second-order sinusoidal oscillator/ filters withnon-interacting controls using CFAs. FREQUENZ, 57(1–2), 12–14.

298 BHASKAR AND PRASAD

[8] Sharma, R. K., and Senani, R. (2004). On the realization of universal current-modebiquads using a single CFOA. Analog integrated circuits and signal processing, 41,65–78.

[9] Ahmad, S. N., Khan, M. R., and Moinuddin (2004). A new current-mode universalfilter. FREQUENZ, 58(7–8), 195–196.

[10] Huelsman, L. P. (1993). Active and Passive Analog Filter Design. New York:McGraw-Hill. Inc.