Palaniappan C <palani@orizin.net> wrote:
> Maximum power transfer theorem states that maximum power
> is supplied to the load when load resistance is equal to output
> impedance. so in that case , efficiency will be only 50% .
> 
> But class C ,D  amplifiers claim more than 50% efficiency. 
> If we match impedance , will it reduce their efficiency ?

Yes.

But be careful about what "efficiency" you're referring to in each case.

When talking about the efficiency of a power amplifier, you're comparing
the power supplied to the amplifier from some AC or DC source with the
power delivered to the load. Linear amplifiers (class A, B and to some
extent, class C) have a certain irreducible loss associated with
converting the raw power supply into signal power. Class D amplifiers
are theoretically perfectly efficient at this, assuming zero losses in
the switches and filtering components. In practice, they typically get
numbers in the 80 - 95% range.

When talking about the efficiency of power transfer from a source to a
load, you're comparing the signal power dissipated within the source
(because of its source impedance) to the power dissipated within the
load. Real amplifiers (both linear and class D) have some definite, but
small output impedance. If you match a load to this impedance, and
assuming the amplifier can deliver the necessary current, you will
indeed get the maximum possible power to the load, but you'll also be
dissipating the same amount of power in the source impedance of the
amplifier. Even a class-D amplifier will be reduced to 50% overall
efficiency under these conditions.

But audio power amplifiers, for example, aren't designed for these
conditions. Instead, they are designed to supply signal power to some
definite load impedance, usually 4 or 8 ohms. The designers minimize
the source impedance of the amplifier so that the overall efficiency,
not just the signal transfer efficiency, is maximized.

-- Dave Tweed
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