Explicit-current-output sinusoidal oscillators employing only a single current-feedback op-amp

Transcription

1 Explicit-current-output sinusoidal oscillators employing only a single current-feedback op-amp R. Senani a) and R. K. Sharma Analog Signal Processing Research Lab., Division of Electronics and Communication Engineering, Netaji Subhas Institute of Technology (formerly, Delhi Institute of Technology), Sector 3, Dwarka, New Delhi , India a) senani@nsit.ac.in Abstract: Although a number of sinusoidal oscillators have been described in earlier literature which employ only a single current feedback op-amp (CFOA) to generate voltage-mode sinusoidal oscillations, any sinusoidal oscillator circuit, which can provide a current-mode output signal explicitly (i.e. from a high output impedance node) while using only a single CFOA, has not been reported till date. This letter describes two such circuits and demonstrates their practicability through experimental results based upon AD844 type IC CFOAs. Keywords: Sinusoidal oscillators, Current mode circuits, Current feedback operational amplifiers Classification: Integrated circuits References [1] R. Senani, New types of sine wave oscillators, IEEE Trans. Instrum. Meas., vol. 34, no. 3, pp , [2] S. J. Azhari and H. Kaabi, AZKA cell, the current-mode alternative of Wheatstone bridge, IEEE Trans. Circuits Syst. I, vol. 47, no. 9, pp , [3] J. A. Svoboda, L. McGory, and S. Webb, Application of a commercially available current conveyor, Int. J. Electron., vol. 70, pp , [4] S. Celma, P. A. Martinez, and A. Carlosena, Current feedback amplifiers based sinusoidal oscillators, IEEE Trans. Circuits Syst. I, vol. 41, no. 12, pp , [5] S. Celma, P. A. Martinez, and J. Sabadell, On the design of CFA-based sinusoidal oscillators, 20th International Conference on Microelectronics (MIEL 95), NIS, Serbia, Sept , pp , [6] S. I. Liu, S. C. Chang, and D. S. Wu, Sinusoidal oscillators with single element control using current feedback amplifier, Int. J. Electron., vol. 77, no. 6, pp , [7] S. I. Liu, C. C. Chang, and D. S. Wu, Active-R sinusoidal oscillators using CFA-poles, Int. J. Electron., vol. 77, no. 6, pp , [8] R. Senani and V. K. Singh, Synthesis of canonic single resistancecontrolled oscillators using a single current feedback amplifier, IEE Proc. Circuits, Devices Syst., vol. 143, no. 1, pp ,

2 [9] M. T. Abuelma atti, A. A. Farooqi, and M. S. Al-Shahrani, Novel RC oscillators using the current feedback operational amplifier, IEEE Trans. Circuits Syst. I, vol. 43, no. 2, pp , [10] P. A. Martinez, S. Celma, and J. Sabadell, Designing sinusoidal oscillators with current feedback oscillators, Int. J. Electron., vol. 80, no. 5, pp , [11] R. Senani, Realisation of a class of analog signal processing/signal generation circuits: novel configurations using current feedback operational amplifiers, Frequenz, vol. 52, no. 9/10, pp , [12] M. T. Abuelma atti and M. S. Al-Shahrani, A novel low-componentcount single-element-controlled sinusoidal oscillator using the CFOApole, Int. J. Electron., vol. 80, no. 6, pp , [13] M. T. Abuelma atti and M. S. Al-Shahrani, New CFOA-based sinusoidal oscillators, Int. J. Electron., vol. 82, no. 1, pp , [14] S. S. Gupta and R. Senani, State variable synthesis of single-resistancecontrolled grounded-capacitor oscillators using only two CFOAs: additional new realizations, IEE Proc. Circuits, Devices Syst., vol. 145, no. 6, pp , [15] E. O. Gunes and A. Toker, On the realisation of oscillators using state equations, Int. J. Electron., vol. 56, no. 5, pp , [16] A. Toker, O. Çìçekoğlu, and H. Kuntman, On the oscillator implementations using a single current feedback op-amp, Comput. Electr. Eng., vol. 28, pp , [17] A. M. Soliman, Current feedback operational amplifier based oscillators, Analog Integr. Circuits Signal Processing, vol. 23, pp , [18] R. Senani and S. S. Gupta, Novel SRCOs using first generation Current Conveyors, Int. J. Electron., vol. 87, no. 10, pp , [19] S. S. Gupta and R. Senani, Comment-Differential difference Current Conveyors and their applications, IEE Proc Circuits, Devices Syst., vol. 148, no. 6, pp , [20] R. Senani, New RC active oscillator configurations employing unity gain amplifiers, Electron. Lett., vol. 21, no. 20, pp , [21] R. Senani, Realisation of sinusoidal oscillators, Chapter-4 in Realisation of some classes of active networks. Ph.D Thesis, Faculty of Engineering, University of Allahabad, India, pp , [22] N. Boutin, Synthesis of oscillator circuits using only unity gain amplifiers, Electron. Lett., vol. 22, no. 1, pp , [23] R. Senani, Network transformations for incorporating non-ideal simulated immittances in the design of active filters and oscillators, IEE Proc. Part-G, vol. 134, no. 4, pp , [24] M. T. Abuelma atti, Ten faster op-amp oscillators, Electron. Wireless World, vol. 93, no. 1620, pp , Introduction Sinusoidal oscillators find numerous applications in instrumentation, measurement and communication systems as test oscillators or signal generators, for example, testing of radio receivers, measurement of SWR and signal-tonoise ratio, etc. Traditionally, op-amp-based oscillators have been considered for such applications and a wide variety of these have been known in literature, for 15

3 instance, see [1] and the references cited therein. However, of late, currentfeedback-operational amplifier-based oscillators have attracted prominent attention as alternatives because they offer improved performance in comparison to their voltage-mode op-amp (VOA)-based counterparts, in terms of frequency accuracy, dynamic range, distortion level and frequency span [3-5]. Motivated by these advantages, several circuit configurations for designing variable frequency oscillators (VFO) employing a single CFOA and a few external passive elements (typically three to four resistors and two to three capacitors) have been presented in the literature [6-16]. However, all these single-cfoa-based oscillators provide only a voltage mode (VM) output (at the low-output impedance w-terminal of the CFOA). In view of the recent interest on current-mode signal processing, there may be several situations where a sinusoidal current-mode oscillator may be required (which provides the current-mode test signal from a high output impedance node), such as test oscillators for checking performance of currentmode active filters or as sinusoidal current-source in new current-mode active bridges, as in [2]. In principle, current mode (CM) signal can also be obtained from the oscillators of [6-16], but by using an additional CFOA configured as a current 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 current flowing in one of the passive elements of such an oscillator. The resulting CM-oscillators based upon the VM-oscillators of [6-16] would obviously be requiring two CFOAs. There has also been some attention recently in more direct realizations of CM-oscillators intended to provide current output(s) explicitly (from the high-output impedance z -terminal of the CFOA), for instance 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 complex active building blocks (ABB) such as differential voltage current conveyor (DVCC), differential difference current conveyors (DDCC) [19], none of which is yet available commercially as an off-the-shelf component (whereas a CFOA is). Thus, to the best knowledge of the authors, any CM sinusoidal oscillator which can provide sinusoidal current output explicitly (directly from its z - output) while using no more than a single CFOA has not been described in the 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 appropriately embedding, in two different ways, a passive RC null network into the 4-port CFOA characterized by i y =0,v x = v y, i z = i x and v w = v z. Analysis shows that for both the circuits, the condition of oscillation (CO) and the frequency of oscillation (FO) are given by: R 3 =6(R 1 + R 2 ); provided C 1 = C 2 = C 3 = C (1) 16

4 1 f o = 2πC (2) 3R 1 R 2 Since R 1 and R 2 share a common terminal, their combination can be realised by a single potentiometer. By doing so, f o can be varied through this potentiometer by changing n, the ratio R 1 /R 2, while their sum (R 1 + R 2 ), and hence, the CO in equation (1), remains invariant. However, CO can also be adjusted independently through R 3. It may be noted that while the circuit of Fig. 1 (a) provides an explicit current output I out into a short circuit to ground, the output I out in the circuit of Fig. 1 (b) is available with high output impedance and thus, can be supplied into any arbitrary load R L. Fig. 1. The proposed CM oscillators using a single CFOA 3 Experimental results The workability of both the circuits has been checked by hardware implementations using AD844 type CFOAs biased with ±12 volt DC power supply, choosing the component values as, C 1 = C 2 = C 3 = 1 nf, R 1 +R 2 =11.11 KΩ by realising (R 1 + R 2 ) as a series combination of a resistor 1 KΩ placed on either end of a potentiometer of 9.11 KΩ. Accordingly, R 3 was realised by a 100 KΩ potentiometer to attain a value of around KΩ as per equation (1). Typical waveforms generated from both the circuits are shown in Fig. 2 (a) and (b) respectively. The waveform of Fig. 2 (a) is the signal present at x -input of CFOA, whereas that of the Fig. 2 (b) is obtained by 17

5 terminating z -terminal (I out ) of the CFOA into a load of 10 KΩ and then taking out the signal voltage from w-output. The experimental tests confirm the workability of the proposed circuits. The output 9.68 V of Fig. 2 (a) is taken from the w-output of the CFOA in Fig. 1 (a) whereas in the case of Fig. 1 (b) 6.72 V is the signal level at the x -terminal shown in Fig. 2 (b). Hence, the difference in the output voltage levels. Since the proposed oscillators provide explicit current outputs, these have been measured and have been found to be of the same order ( = 0.9mA for CFOAs biased with ±12 V) for both the oscillators. The experimental tests confirm the workability of the proposed circuits. Fig. 2. Typical output waveforms generated by the new circuits (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 in literature [6-14], any single CFOA-based CM sinusoidal oscillator, capable of providing explicit current output, was not described earlier. This letter filled this void by describing two such circuits 1. The workability of the proposed circuits was confirmed by experimental results based upon AD844 type CFOAs. It is worth observing that both the described circuits were third order oscillators. Thus, the problem of devising a single-cfoa-based second order sinusoidal oscillator with explicit CM output is a yet unresolved problem. This is, however, by no means trivial and hence, is open to investigation. 1 The genesis of the oscillators described herein was inspired by the ideas in [20-24]. 18