Class-D Efficiency

Ice Cube Fire

The maximum theoretical efficiency for a linear amplifier is about 78.5%.  This assumes zero idle losses and static supply rails (i.e. not a class-G or class-H amplifier).  The maximum theoretical efficiency of a switching amplifier is 100%.  This assumes zero idle losses and “perfect” switching devices.  Of course you can never really achieve 100% efficiency – the second law of thermodynamics forbids this – but you can achieve 90%, 99%, 99.9% and 99.99% as the technology improves to allow for it.

The determination of efficiency is a fairly straightforward measurement for a given product.  Unfortunately, marketing sometimes gets a hold of it and the following scenario occurs:

Worldly marketing type:  “Hey Bob, sweet amp.”

Innocent engineering type:  “Gee Fred, ya think so?  Thanks!”

Worldly marketing type:  “Yah, but we could really improve our position against XYZ if the efficiency were higher than 75%…”

Innocent engineering type:  “But I’m giving the end-to-end efficiency and XYZ is claiming an efficiency of 99.5% by only including the losses in the power cord!”

Worldly marketing type:  “Uh-huh.”

Innocent engineering type:  [Exasperated] “Okay, fine.  I’ll work on it after I take my Honda in for an oil change at lunch.  How’s the Bimmer working out for ya?”

Worldly marketing type:  “Fine, just fine.”

The bit about the car types was inspired by Guy Kawasaki.

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