Page "Electronic amplifier" Paragraph 21

* They are very inefficient ; a theoretical maximum of 50 % is obtainable with inductive output coupling and only 25 % with capacitive coupling, unless deliberate use of nonlinearities is made ( such as in square-law output stages ).

In a power amplifier this not only wastes power and limits battery operation, it may place restrictions on the output devices that can be used ( for example: ruling out some audio triodes if modern low-efficiency loudspeakers are to be used ), and will increase costs.

Inefficiency comes not just from the fact that the device is always conducting to some extent ( that happens even with class AB, yet its efficiency can be close to that of class B ); it is that the standing current is roughly half the maximum output current ( although this can be less with square law output stage ), together with the problem that a large part of the power supply voltage is developed across the output device at low signal levels ( as with classes AB and B, but unlike output stages such as class D ).

If high output powers are needed from a class-A circuit, the power waste ( and the accompanying heat ) will become significant.

For every watt delivered to the load, the amplifier itself will, at best, dissipate another watt.

For large powers this means very large and expensive power supplies and heat sinking.