Explicit-Current-output Single Cfoa Based Oscillator

IEICE Electronics Express, Vol.2, No.1, 14–18

Explicit-current-outputsinusoidal oscillatorsemploying only a singlecurrent-feedback op-amp

R. Senani a) and R. K. SharmaAnalog Signal Processing Research Lab., Division of Electronics and Communica-

tion Engineering, Netaji Subhas Institute of Technology (formerly, Delhi Institute of

Technology), Sector 3, Dwarka, New Delhi 110 075, India

a) [email protected]

Abstract: Although a number of sinusoidal oscillators have been de-scribed in earlier literature which employ only a single current feedbackop-amp (CFOA) to generate voltage-mode sinusoidal oscillations, anysinusoidal oscillator circuit, which can provide a current-mode outputsignal explicitly (i.e. from a high output impedance node) while usingonly a single CFOA, has not been reported till date. This letter de-scribes two such circuits and demonstrates their practicability throughexperimental results based upon AD844 type IC CFOAs.Keywords: Sinusoidal oscillators, Current mode circuits, Currentfeedback operational amplifiersClassification: Integrated circuits

References

[1] R. Senani, “New types of sine wave oscillators,” IEEE Trans. Instrum.Meas., vol. 34, no. 3, pp. 461–463, 1985.

[2] S. J. Azhari and H. Kaabi, “AZKA cell, the current-mode alternativeof Wheatstone bridge,” IEEE Trans. Circuits Syst. I, vol. 47, no. 9,pp. 1277–1284, 2000.

[3] J. A. Svoboda, L. McGory, and S. Webb, “Application of a commerciallyavailable current conveyor,” Int. J. Electron., vol. 70, pp. 159–164, 1991.

[4] S. Celma, P. A. Martinez, and A. Carlosena, “Current feedback amplifiersbased sinusoidal oscillators,” IEEE Trans. Circuits Syst. I, vol. 41, no. 12,pp. 906–908, 1994.

[5] S. Celma, P. A. Martinez, and J. Sabadell, “On the design of CFA-basedsinusoidal oscillators,” 20th International Conference on Microelectronics(MIEL ’95), NIS, Serbia, Sept. 12-14, pp. 731–736, 1995.

[6] S. I. Liu, S. C. Chang, and D. S. Wu, “Sinusoidal oscillators with sin-gle element control using current feedback amplifier,” Int. J. Electron.,vol. 77, no. 6, pp. 1007–1013, 1994.

[7] S. I. Liu, C. C. Chang, and D. S. Wu, “Active-R sinusoidal oscillatorsusing CFA-poles,” Int. J. Electron., vol. 77, no. 6, pp. 1035–1042, 1994.

[8] R. Senani and V. K. Singh, “Synthesis of canonic single resistance-controlled oscillators using a single current feedback amplifier,” IEE Proc.Circuits, Devices Syst., vol. 143, no. 1, pp. 71–72, 1996.

c© IEICE 2005DOI: 10.1587/elex.2.14Received November 18, 2004Accepted December 22, 2004Published January 10, 2005

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[9] M. T. Abuelma’atti, A. A. Farooqi, and M. S. Al-Shahrani, “Novel RC os-cillators using the current feedback operational amplifier,” IEEE Trans.Circuits Syst. I, vol. 43, no. 2, pp. 155–157, 1996.

[10] P. A. Martinez, S. Celma, and J. Sabadell, “Designing sinusoidal oscil-lators with current feedback oscillators,” Int. J. Electron., vol. 80, no. 5,pp. 637–646, 1996.

[11] R. Senani, “Realisation of a class of analog signal processing/signal gen-eration circuits: novel configurations using current feedback operationalamplifiers,” Frequenz, vol. 52, no. 9/10, pp. 196–206, 1998.

[12] M. T. Abuelma’atti and M. S. Al-Shahrani, “A novel low-component-count single-element-controlled sinusoidal oscillator using the CFOA-pole,” Int. J. Electron., vol. 80, no. 6, pp. 747–752, 1996.

[13] M. T. Abuelma’atti and M. S. Al-Shahrani, “New CFOA-based sinusoidaloscillators,” Int. J. Electron., vol. 82, no. 1, pp. 27–32, 1997.

[14] S. S. Gupta and R. Senani, “State variable synthesis of single-resistance-controlled grounded-capacitor oscillators using only two CFOAs: addi-tional new realizations,” IEE Proc. Circuits, Devices Syst., vol. 145, no. 6,pp. 415–418, 1998.

[15] E. O. Gunes and A. Toker, “On the realisation of oscillators using stateequations,” Int. J. Electron., vol. 56, no. 5, pp. 317–326, 2002.

[16] A. Toker, O. Cıcekoglu, and H. Kuntman, “On the oscillator implemen-tations using a single current feedback op-amp,” Comput. Electr. Eng.,vol. 28, pp. 375–389, 2002.

[17] A. M. Soliman, “Current feedback operational amplifier based oscilla-tors,” Analog Integr. Circuits Signal Processing, vol. 23, pp. 45–55, 2000.

[18] R. Senani and S. S. Gupta, “Novel SRCOs using first generation CurrentConveyors,” Int. J. Electron., vol. 87, no. 10, pp. 1187–1192, 2000.

[19] S. S. Gupta and R. Senani, “Comment-Differential difference CurrentConveyors and their applications,” IEE Proc Circuits, Devices Syst.,vol. 148, no. 6, pp. 335–336, 2001.

[20] R. Senani, “New RC active oscillator configurations employing unity gainamplifiers,” Electron. Lett., vol. 21, no. 20, pp. 889–891, 1985.

[21] R. Senani, “Realisation of sinusoidal oscillators, Chapter-4 in ‘Realisationof some classes of active networks.’ Ph.D Thesis,” Faculty of Engineering,University of Allahabad, India, pp. 255–275, 1987.

[22] N. Boutin, “Synthesis of oscillator circuits using only unity gain ampli-fiers,” Electron. Lett., vol. 22, no. 1, pp. 22–23, 1986.

[23] R. Senani, “Network transformations for incorporating non-ideal simu-lated immittances in the design of active filters and oscillators,” IEEProc. Part-G, vol. 134, no. 4, pp. 158–166, 1987.

[24] M. T. Abuelma’atti, “Ten faster op-amp oscillators,” Electron. WirelessWorld, vol. 93, no. 1620, pp. 1053–1054, 1987.

1 Introduction

Sinusoidal oscillators find numerous applications in instrumentation, mea-surement and communication systems as test oscillators or signal generators,for example, testing of radio receivers, measurement of SWR and signal-to-noise ratio, etc.

Traditionally, op-amp-based oscillators have been considered for such ap-plications and a wide variety of these have been known in literature, for


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instance, see [1] and the references cited therein. However, of late, current-feedback-operational amplifier-based oscillators have attracted prominent at-tention as alternatives because they offer improved performance in compar-ison to their voltage-mode op-amp (VOA)-based counterparts, in terms offrequency accuracy, dynamic range, distortion level and frequency span [3-5].Motivated by these advantages, several circuit configurations for designingvariable frequency oscillators (VFO) employing a single CFOA and a fewexternal passive elements (typically three to four resistors and two to threecapacitors) have been presented in the literature [6-16]. However, all thesesingle-CFOA-based oscillators provide only a voltage mode (VM) output (atthe low-output impedance w -terminal of the CFOA).

In view of the recent interest on current-mode signal processing, theremay be several situations where a sinusoidal current-mode oscillator maybe required (which provides the current-mode test signal from a high outputimpedance node), such as test oscillators for checking performance of current-mode active filters or as sinusoidal current-source in new current-mode activebridges, as in [2].

In principle, current mode (CM) signal can also be obtained from theoscillators of [6-16], but by using an additional CFOA configured as a cur-rent follower (CF) (with x as input terminal, z as output terminal and y

grounded, thereby having a virtual ground at its input) to sense out the cur-rent flowing in one of the passive elements of such an oscillator. The resultingCM-oscillators based upon the VM-oscillators of [6-16] would obviously berequiring two CFOAs. There has also been some attention recently in moredirect realizations of CM-oscillators intended to provide current output(s)explicitly (from the high-output impedance z -terminal of the CFOA), for in-stance see [17-19]. However, all these circuits too either require two CFOAs(as in the oscillator of Fig. 4 (b) of [17] and [18]) or usually require more com-plex active building blocks (ABB) such as differential voltage current con-veyor (DVCC), differential difference current conveyors (DDCC) [19], none ofwhich is yet available commercially as an off-the-shelf component (whereasa CFOA is).

Thus, to the best knowledge of the authors, any CM sinusoidal oscillatorwhich can provide sinusoidal current output explicitly (directly from its z -output) while using no more than a single CFOA has not been described inthe literature up until now. This letter describes two such circuits.

2 Proposed CM-sinusoidal oscillators using only a single CFOA

The proposed circuits are shown in Fig. 1 and are obtained by appropriatelyembedding, in two different ways, a passive RC null network into the 4-portCFOA characterized by iy = 0, vx = vy, iz = ix and vw = vz. Analysis showsthat for both the circuits, the condition of oscillation (CO) and the frequencyof oscillation (FO) are given by:

R3 = 6 (R1 + R2) ; provided C1 = C2 = C3 = C (1)c© IEICE 2005DOI: 10.1587/elex.2.14Received November 18, 2004Accepted December 22, 2004Published January 10, 2005

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fo =1

2πC√

3R1R2(2)

Since R1 and R2 share a common terminal, their combination can berealised by a single potentiometer. By doing so, fo can be varied through thispotentiometer by changing ‘n’, the ratio R1/R2, while their sum (R1 + R2),and hence, the CO in equation (1), remains invariant. However, CO canalso be adjusted independently through R3. It may be noted that while thecircuit of Fig. 1 (a) provides an explicit current output Iout into a short circuitto ground, the output Iout in the circuit of Fig. 1 (b) is available with highoutput impedance and thus, can be supplied into any arbitrary load RL.

Fig. 1. The proposed CM oscillators using a single CFOA

3 Experimental results

The workability of both the circuits has been checked by hardware implemen-tations using AD844 type CFOAs biased with ±12 volt DC power supply,choosing the component values as, C1 = C2 = C3 = 1nF, R1+R2 = 11.11KΩby realising (R1 + R2) as a series combination of a resistor 1KΩ placed oneither end of a potentiometer of 9.11KΩ. Accordingly, R3 was realised bya 100KΩ potentiometer to attain a value of around 66.66KΩ as per equa-tion (1). Typical waveforms generated from both the circuits are shownin Fig. 2 (a) and (b) respectively. The waveform of Fig. 2 (a) is the signalpresent at x -input of CFOA, whereas that of the Fig. 2 (b) is obtained by


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terminating z -terminal (Iout) of the CFOA into a load of 10KΩ and thentaking out the signal voltage from w -output. The experimental tests confirmthe workability of the proposed circuits.

The output 9.68V of Fig. 2 (a) is taken from the w -output of the CFOAin Fig. 1 (a) whereas in the case of Fig. 1 (b) 6.72V is the signal level at thex -terminal shown in Fig. 2 (b). Hence, the difference in the output voltagelevels. Since the proposed oscillators provide explicit current outputs, thesehave been measured and have been found to be of the same order (∼= 0.9mAfor CFOAs biased with ±12 V) for both the oscillators. The experimentaltests confirm the workability of the proposed circuits.

Fig. 2. Typical output waveforms generated by the newcircuits(a) Output waveform of the oscillator of Fig. 1 (a);THD = 0.581%(b) Output waveform of the oscillator of Fig. 1 (b);THD = 1.154%

4 Conclusions

While several single CFOA-based VM sinusoidal oscillators were known inliterature [6-14], any single CFOA-based CM sinusoidal oscillator, capableof providing explicit current output, was not described earlier. This letterfilled this void by describing two such circuits 1. The workability of the pro-posed circuits was confirmed by experimental results based upon AD844 typeCFOAs.

It is worth observing that both the described circuits were third orderoscillators. Thus, the problem of devising a ‘single-CFOA-based second ordersinusoidal oscillator with explicit CM output’ is a yet unresolved problem.This is, however, by no means trivial and hence, is open to investigation.

1The genesis of the oscillators described herein was inspired by the ideas in [20-24].


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Explicit-Current-output Single Cfoa Based Oscillator