PARTIALLY ACTIVE-R GROUNDED-CAPACITOR

Transcription

1 Active and Passive Elec. Comp., 1996, Vol. 19, pp Reprints available directly from the publisher Photocopying permitted by license only (C) 1996 OPA (Overseas Publishers Association) Amsterdam B.V. Published in The Netherlands under license by Gordon & Breach Science Publishers SA Printed in Malaysia PARTIALLY ACTIVE-R GROUNDED-CAPACITOR CFOA-BASED SINUSOIDAL OSCILLATORS MUHAMMAD TAHER ABUELMA ATTI AND MUHAMMAD HAROON KHAN King Fahd University of Petroleum and Minerals Box 203 Dhahran Saudi Arabia (Received June 12, 1995; in final form September 1, 1995) New grounded-capacitor CFOA-based sinusoidal oscillator circuits with independent frequency control are presented. Experimental results are included. INTRODUCTION At present, there is a growing interest in using the current-feedback operational amplifier (CFOA) in various analoge signal processing applications [1-5]. This is attributed to its larger bandwidth and higher slew rate compared to the conventional voltage feedback operational amplifier (VFOA). Thus, analoge signal processing circuits built around the CFOA are expected to operate at higher frequencies than the VFOA-based circuits. Recently, a number of CFOA-based sinusoidal oscillators have been proposed [6-10]. In some of these circuits, the CFOA pole is used to advantage. The CFOA-based RC-oscillators of References [6,7] exhibit good performance but require equal-valued resistors and capacitors. In the active-r multiphase sinusoidal oscillator of Reference [8], the frequency of oscillation and the condition of oscillation are coupled, thus a change in the frequency of oscillation cannot be obtained without disturbing the condition of oscillation. In the active-r realizations of Reference [9], while the frequency of oscillation can be controlled without disturbing the condition of oscillation by using a grounded (or floating) resistor, the condition of oscillation cannot be adjusted without disturbing the frequency of oscillation. The partially active-r oscillator realization of Reference [9] uses only four passive elements. However, its frequency of oscillation cannot be controlled without disturbing the condition of oscillation. In Reference [10], a number of RC-oscillators using ideal CFOA and enjoying uncoupled frequency and condition of oscillations are proposed. However, in some circuits the control of the frequency or the condition of oscillation requires adjusting a floating resistor or a floating capacitor. Moreover, all the realizations [8-10] assume that terminal Z is accessible. This is not the case for all CFOAs. And thus, the proposed circuits can be realized using only the AD844 CFOA; other CFOAs with no access to point Z cannot be used. In this paper, new grounded-capacitor CFOA-based partially active-r sinusoidal oscillator circuits are presented. The proposed circuits utilize the CFOA-pole to advantage and their frequencies and conditions of oscillation can be independently controlled. 105

2 106 M.T. ABUELMA ATTI AND M. H. KHAN PROPOSED CIRCUITS Consider the oscillator structure shown in Fig. 1. A simplified equivalent circuit for a typical CFOA, the AD844, is shown in Fig. 2 [11]. In this equivalent circuit, Ry 10MI2, R z 3M12, Rx 5012 and Cy C z 4.5pF. Routine analysis yields the characteristic equation of the circuit of Fig. 1, which can be expressed as ( -I- $ Cyea) (1 + sct, Ro) (1 + sc Rc) 13(1 + scp Rp) 0 (1) where R a R2//Ry, Rt, Rp//Rx, Rc R1//R z and [3 RcRy/(R2 + Ry)(Rx + Rp). Using the Barkhausen principle, by equating the real and imaginary parts of (1) to zero, and if Cv>> C z and Cv > Cy, the frequency of oscillation and the condition of oscillation of the circuit of Fig. 1 can be expressed as 2 1 RI[R p to o (2) c,, + cy ) and From (2) and (3), one can see that the frequency of oscillation can be adjusted by tuning the resistor R2 and/or the grounded capacitor Cp without disturbing the condition of oscillation. Moreover, if R2 0 then (2) reduces to R 2 Output R 1 FIGURE Partially active-r Oscillater Circuit.

3 SINUSOIDAL OSCILLATOR 107 R z o V y c Ry Vy Cy FIGURE 2 Simplified equivalent Circuit for the CFOA AD R/Rp 0o= (4) C.C R R From (4), one can see that the frequency of oscillation of the resulting oscillator circuit, using only three passive elements, can be adjusted by tuning the grounded capacitor Cp without disturbing the condition of oscillation. EXPERIMENTAL RESULTS The proposed sinudoidal oscillator circuits were experimentally tested using the AD844 CFOA. Fig. 3 shows the results obtained from the circuit of Fig. 1-with R 20012, Rp and Cp 200pF. A typical output waveform obtained from the circuit of Fig. 1 using the AD844 CFOA with R 20012, R2 3k12, Rp 20312, Cp 200pF is shown in Fig. 4. The de power supply is +15V and the output frequency is 3.12 MHz. CONCLUSION New grounded-capacitor partially active-r CFOA-based sinusoidal oscillator circuits are presented. In one of the circuits, the frequency of oscillation can be changed by tuning a grounded capacitor without disturbing the condition of oscillation. The circuit uses only three passive elements. The use of grounded capacitors is another attractive feature for integration. It is worth mentioning that, while the experimental results reported here were obtained from an AD844-based realization, the prop.osed circuits can be realized using any other commercially available CFOAs.

4 108 M.T. ABUELMA ATTI AND M. H. KHAN Frequency MHz R 2 k.1.. FIGURE 3 Measured frequency of Oscillation from the Circuit of Fig. with: R , Rp , Cp 200 PE FIGURE 4 Typical Output Waveform from the Circuit of Fig. 2 with: R , Rp , R2 3KII, Cp 200 PF DC supply +_15V. Frequency 3.12 MH

5 SINUSOIDAL OSCILLATOR 109 REFERENCES 1. A. Fabre, Gyrator implementation from commercially available transimpedance amplifiers, Electronics Letters, Vol. 28, 1992, pp A. Fabre, Insensitive voltage-mode and current-mode filters from commercially available transimpedance opamps, IEE Proceedings, Part G, Vol. 140, 1993, pp C.-M. Chang, C.-S. Hwang and S.-H. Tu, Voltage-mode notch, lowpass and bandpass filter using current-feedback amplifiers, Electronic Letters, Vol. 30, 1994, pp C.-M. Chang and C.-S. Hwang, Comment on "Voltage-mode notch, lowpass and bandpass filter using current-feedback amplifiers", Electronics Letters, Vol. 31, 1995, p S.-I. Liu and J.-J. Chen, Realisation of analogue divider using current feedback amplifiers, IEE Proceedings-Circuits Devbices Systems, Vol. 142, 1995, pp S. Celma, EA. Martinez and A. Carlosena, Current feedback amplifiers based sinusoidal oscillators, IEEE Transactions Circuits and Systems-I: Fundamental Theories and Applications, Vol. 41, 1994, pp S. Celma, A. Carlosena and EA. Martinez, Current feedback amplifiers based sinusoidal oscillators, International Symposium on Circuits and Systems, Vol. 5, 1994, pp D.-S. Wu, S.-I. Liu, Y.-S., Hwang and Y.-E Wu, Multiphase sinusoidal oscillator using the CFOA pole, IEE Proceedings-Circtlits Devices Systems, Vol. 142, 1995, pp S.-I. Liu, C.-C. Chang and D.-S. Wu, Active-R sinusoidal oscillators using the CFA pole, Electronics, Vol. 77, 1994, pp S.-I. Liu, C.-S. Shih and D.-S. Wu, Sinusoidal oscillators with single element control using a current-feedback amplifier, Electronics, Vol. 77, 1994, pp J.A. Svoboda, Comparison of RC op.-amp, and RC current conveyor filters, International Journal of Electronics, Vol. 76, 1994, pp

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