Nonlinear Dynamics of a BJT Based Colpitts Oscillator with Tunable Bias Current
Suvra Sarkar1, Sandeepa Sarkar2, Bishnu Charan Sarkar3
1Ms Suvra Sarkar,  Electronics Department, Burdwan Raj College, Burdwan, India.
2Ms Sandeepa Sarkar,  Harisabha Hindu Girls High School, Burdwan,  India.
3Bishnu Charan Sarkar,  Physics Department, Burdwan University, Burdwan, India.
Manuscript received on May 12, 2013. | Revised Manuscript received on June 13, 2013. | Manuscript published on June 30, 2013. | PP: 12-18 | Volume-2, Issue-5, June 2013. | Retrieval Number: E1662062513/2013©BEIESP

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Abstract: The effect of bias current variation in the dynamics of a conventional BJT based Colpitts Oscillator (CO) has been thoroughly examined in this paper. After formulating a suitable ac equivalent model of the CO taking care of the dc bias current, the dynamics of the system has been numerically studied. It is observed that in a CO circuit with given design parameter, a periodic oscillation starts for a critical bias current and with the variation of the bias current to a higher value chaotic oscillations are observed through a period doubling root. A prototype hardware experiment in the low RF band CO supports the observations obtained by the numerical simulations. The change in chaoticity with the variation of the operating bias current is observed by finding Maximum Lyapunov exponent (MLE) from numerically and experimentally obtained time series of the CO output data. The technique of bias current variation could be applicable in any frequency range including Microwave band, in principle and it has important application potential in chaos based communication systems for encoding information bits into chaotic signals.
Keywords: Bias current tuning, Chaotic Colpitts oscillator, Maximum Lyapunov exponent Nonlinear trans-conductance model of BJT, period doubling route to chaos.