Analysis of BJT Colpitts Oscillators - Empirical and Mathematical Methods for Predicting Behavior

Oscillator circuits perform two fundamental roles in wireless communication –the local oscillator for frequency shifting and the voltage-controlled oscillator formodulation and detection. The Colpitts oscillator is a common topology used for theseapplications. Because the oscillator must function as a component of a larger system, theability to predict and control its output characteristics is necessary. Textbooks treating thecircuit often omit analysis of output voltage amplitude and output resistance and theliterature on the topic often focuses on gigahertz-frequency chip-based applications.Without extensive component and parasitics information, it is often difficult to makesimulation software predictions agree with experimental oscillator results.The oscillator studied in this thesis is the bipolar junction Colpitts oscillator in thecommon-base configuration and the analysis is primarily experimental. Thecharacteristics considered are output voltage amplitude, output resistance, andsinusoidal purity of the waveform. The contributions of each of the components of theoscillator to the output voltage waveform are investigated and methods to predict andcontrol amplitude are discussed. The relationships of the output resistance and waveformshape to the inductor are also studied. Two example Colpitts oscillators with differentdesign criteria are constructed for the benefit of the reader and to test the methodsidentified in the work