NEW QUARTZ CRYSTAL OSCILLATORS USING THE CURRENT-FEEDBACK OPERATIONAL AMPLIFIER

New crystal oscillators using the current-feedback operational-amplifier (CFOA) are presented. Each circuit uses one CFOA, a crystal and, at most, five externally connected passive elements. Experimental results are included.


INTRODUCTION
Quartz crystal oscillators built around various discrete and integrated- circuit active elements are available in the literature [1,2].Of particular interest here are crystal oscillators built around the second-generation current-conveyor (CCII) [3][4][5].Because the CCII minimally degrades the quartz crystal performance, CCII-based crystal oscillators enjoy good frequency stability and are, therefore, very attractive [3].
On the other hand, the current-feedback operational-amplifier (CFOA) is a four-terminal device which is equivalent to a CCII plus an output voltage buffer and, thus, it can easily drive external loads.
Moreover, the CFOA enjoys wide bandwidth which is relatively *Corresponding author.
independent of the closed-loop gain and very high slew rate.Thus, the use of CFOA in realizing crystal oscillators will be attractive.
To the authors knowledge, the use of CFOA is designing crystal oscillators has not yet been reported in the literature.It is, therefore, the purpose of this letter to present a number of CFOA-based crystal oscillators.

PROPOSED CIRCUITS
Consider the oscillator structures shown in Figure 1.Assuming ideal CFOA with iz--i, iy=O, vx= Vy, Vo= Vz routine analysis yields the characteristic equation given by Using the crystal equivalent circuit shown in Figure 2, the ad- mittance Y3 can be expressed as r + sL1 + 1/sC + 1/sCo Y3 r/sCo + Zl/Co + 1/s2C1Co where r is the crystal internal resistance, L1 is the crystal internal inductance, C1 is the crystal internal capacitance, and Co is the elec- trostatic capacitance between the two parallel plates of the crystal.

EXPERIMENTAL RESULTS
The proposed circuits of Figure were tested using the AD844 CFOA and different crystals.A typical output waveform obtained from the circuit of Figure l(a) with R 10kf and 20 MHz crystal is shown in Figure 3.Only three externally connected equal resistances were used.No external capacitances were connected.In fact the parasitic capacitances Cy -3 pF and Cz 4.5 pF at terminals y and z of the CFOA were exploited to advantage.It appears that the experimental results are in excellent agreement with the theory presented.

CONCLUSION
New CFOA-based crystal oscillators have been presented.Each circuit uses one CFOA, a crystal and, at most, five externally connected passive elements.In both circuits the output can be taken from a low impedance outlet, thus avoiding the degradation in the crystal performance resulting from external loading effects.The circuit of Figure l(b) is a "single-pin" oscillator and, therefore, may be more attractive than that of Figure l(a).
for the oscillator structure of Figure l(a), and (Y] + Y2)Y4-Y2Y3 0 (lb) for the oscillator structure of Figure l(b).

FIGURE 2
FIGURE Proposed crystal oscillator structures.