Power amplifiers, power supplies and batteries
AMPLIFIERS have already been discussed on several occasions in this work — FET amplifiers, BJT amplifiers and operational amplifiers. However, the loads driven by these were such that the power delivered was low, just as well in view of their low efficiencies. High efficiencies are vital for power amplifiers, not because of the cost of the wasted power itself, but because it produces heat that must be removed, causing inefficient amplifiers not only to be bulkier than need be but also more expensive. Power amplifiers are divided into classes according to the duty cycle of the output transistors, that is the proportion of the time they spend in the active parts of their output characteristics; the higher the duty cycle, the lower the efficiency. But the most rapidly developing power source is the battery, driven by the consumers demand for mains-free operation of more and more electrical equipment. A move towards greater power density than batteries can provide is leading to significant progress in fuel cells of very low power (a few W), as well as MW installations in the USA. Considerable work has also been done on fuel-cell-powered cars, which are nonpolluting and need no hydrocarbons, but the sales of fuel sales in 2001 were only £150M.
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