Evaluation of an Existing Approach for Oscillator Power Optimization

Abstract

Oscillators nowadays are indispensable components in most of electronic devices. Different theories have been developed to design and analyze oscillators. However, due to their nonlinearity and structure complexity, the design of an optimal Radio Frequency (RF) or microwave oscillator is not simple, and is subjected at the end to practical experiments. This thesis evaluates one of the approaches used to optimize the oscillator output power on the basis of maximizing the negative real part of the small-signal output immittance. The main goal is to examine whether the output power can be maximized by maximizing the small-signal negative resistance/conductance or not. And in more general words, we want to check if we are capable to predict or control a large-signal parameter (as output power) by just using a small-signal parameter (as negative resistance or conductance). To reach our goal, a computer aided design tool was used, and also practical experiments were carried out for a BJT Clapp oscillator. The negative real part of output immittance at startup was recorded as a function of variable feedback reactances, and the corresponding output power was also recorded while the load was kept always at its optimal value. This procedure was performed for parallel- and series-load orientations and a range of coupling capacitor ratios.

The study revealed disagreement with this approach that aimed to maximize the output power by maximizing the small-signal negative resistance/conductance. The maximum output power was not delivered by maximizing the output small-signal negative resistance or conductance, but it was delivered at a less negative resistance/conductance value. The results suggest not to rely on this approach for optimizing the oscillator output power. Moreover, we recommend for further analytic studies to explore the behaviour of the negative resistance or conductance in terms of the oscillation amplitude, especially when reaching the steady-state point.