Frequency and power limitations of Class-D transistor amplifiers

The transition and saturation loss mechanisms of the two basic tuned Class-D amplifiers are studied in detail, as functions of drive waveform, signal frequency, and type of transistor. The charge control approach is used to derive optimum drive waveforms, together with approximate expressions for the various contributions to transition time that are verified experimentally. It is found that for high-frequency operation, the current-switching system using simple epitaxial transistors provides the highest output power with least likelihood of transistor failure, although with a somewhat lower efficiency than the voltage-switch. The voltage-switching system is found to be preferable when simple epitaxial transistors are not used. An example is given of a current-switching amplifier designed to deliver a continuous power of 1 kW at 500 kHz with an overall system efficiency of greater than 90 percent into a variable load.