Chaos in the Colpitts oscillator

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IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-I: FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 41, NO. 11, NOVEMBER 1994 77 1 Express Letters

Chaos in the Colpitts Oscillator Michael Peter Kennedy Abstract-In this work, we present experimental results and SPICE

simulations of chaos in a Colpitts oscillator. We show that the non- linear dynamics of this oscillator may be modeled by a third-order autonomous continuous-time circuit consisting of a linear inductor, two linear capacitors, two linear resistors, two independent voltage sources, a linear current-controlled current source, and a single voltage-controlled

nonlinear resistor. The nonlinear resistor has a two-segment piecewise- linear DP characteristic. With the appropriate choice of parameters, the

piecewise-linear circuit model has a positive Lyapunov exponent.

I. INTRODUCTION In his Ph.D. thesis on ‘‘Monolithic microwave oscillators and amplifiers,” Nguyen refers to the so-called multi-oscillation phe- nomenon in bipolar RF oscillators: “It is a phenomenon in which two or more oscillations exist simultaneously in the steady state. In other words, there are parasitic or unwanted oscillations existing

together with a main oscillation. Due to the multiple oscillations, the resultant steady-state signal is severely distorted and hence has limited application in communication systems.” [ 11

This observation raises an interesting question: is the “multi- oscillation phenomenon” chaos? If so, then perhaps the oscillator

can be exploited as a transmitter in chaotic-carrier communication systems [2]-[4]. The objectives of this work are: (1) to establish that the Colpitts oscillator [5] can exhibit chaos; (2) to show that this chaos is not a par- asitic effect; and (3) to present a third-order autonomous circuit model

containing just one nonlinear element-a two-segment piecewise- linear resistor-which captures the dynamics of the oscillator.

11. EXPERIMENTAL COLPITTS OSCILLATOR

The Colpitts oscillator which we study is shown in Fig. l(a). The circuit consists of a single bipolar junction transistor (BJT) Q which

is biased in its active region by means of \%E, REE, and Vcc. The feedback network consists of an inductor L with series resistance RL, and a capacitive divider composed of C1 and

C2.

Fig. 2 shows a suspected chaotic attractor in the Colpitts oscillator

of Fig. 1 with the following parameters: Vcc = 5 V, RL = 35 0, L

= 98.5 pH, CI = 54 nF, C2=54 nF, REE = 400 0, and T/EE = -5

V. The BJT is type 2N2222.

The observed complex dynamical behavior is robust in the sense that it persists even if the BJT is replaced with one of a different but similar type, e.g. BC108 or 2N3904.

Manuscript received July 22, 1994. This paper was recommended by The author is with the Department of Electronic and Electrical Engineering,

IEEE Log Number 9406582.

Associate Editor Hsiao-Dong Chiang.

University College Dublin, Dublin 4, Ireland.

vcc RL$ ‘L

LL

‘C -n-

vcc ‘7 RLT

‘L

kE? c2

VEE

T

(b) Fig. 1. (a) BJT Colpitts oscillator and (b) its equivalent circuit.

111. SPICE MODEL AND SIMULATION RESULTS

Fig. 3 shows the PSpice [6] deck which was used to simulate the behavior of the circuit of Fig. l(a). The 2N2222 BJT is modeled using

1057-7122/94$04.00 0 1994 IEEE

Authorized licensed use limited to: Donghua University. Downloaded on February 2, 2010 at 08:25 from IEEE Xplore. Restrictions apply. 712 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS-S: FUNDAMENTAL THEORY AND APPLICATIONS, VOL. 41, NO. 11, NOVEMBER 1994

Fig. 2. Horizontal axis: VCE (V); vertical axis VBE (V). Suspected chaotic attractor in the BJT Colpitts oscillator of Fig. 1.

the NPN 2N2222A model parameters supplied with the evaluation version of PSpice. Fig. 4 shows the results of this simulation. The oscillator is kick- started by ramping the positive rail VCC from 0 V to 5 V. The first 3ms of the transient is discarded and the next lms of (steady-state) trajectory is plotted.

Iv. PIECEWISE-LINEAR CIRCUIT MODEL AND SIMULATION

RESULTS

If we assume that the BJT in the Colpitts oscillator shown in Fig. l(a) acts as a purely resistive element, then the circuit may be described by a system of three autonomous state equations:

c1- VCE = IL - IC

dt

- IL - IB C2k = -

VEE

+ VBE

at REE IL L-=v dt

cc

-

VCE

+

VBE - ILRL.

From experimental observations, it is clear that the BJT operates in just two regions: forward active and cutoff. Consequently, we model the transistor as a two-segment piecewise-linear voltage-controlled resistor NR and a linear current-controlled current source, as shown