A study of the limitations on the collector efficiency of class-D voltage switching amplifiers

This thesis examines the transistor switching phenomena which are instrumental in limiting the collector efficiency of tuned, switched-mode (Class-D) power amplifiers. The relative importance of transistor storage times, rise times, and saturation voltages are considered as a function of two possible amplifier drive waveforms; a square wave of oltage applied via a capacitor-resistor speed-up circuit (quasi-voltage) and a sinusoidal current drive.Excess base charge equations are given and it is shown that the solutions of the equations adequately predict the base storage times. Further, it is shown that with a simple modification to the basic amplifier circuit, high efficiencies can be achieved even with large base storage times.Collector storage effects are investigated and found to severely limit amplifier power output, particularly for amplifiers using non-epitaxial transistors and having a quasi-voltage drive.It is found that the efficiency of the quasi-voltage driven amplifier is limited at the higher frequencies by collector storage effects and for the sinusoidally driven amplifier, by transistor rise times. However, efficiencies for both types of drive are shown to be comparable.