ACTIVE-R GROUNDED-CAPACITOR CFOA-BASED SINUSOIDAL OSCILLATORS

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][2][3][4][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][7][8][9][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.

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 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) 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) 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 20312 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.osedcircuits can be realized using any other commercially available CFOAs.

FIGURE 3
FIGURE 3 Measured frequency of Oscillation from the Circuit of Fig. with: R 200 11, Rp 203 11, Cp 200 PE