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Doherty
amplifiers
Updated July 12,
2008
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New for July 2008! How'd
you like to have an amplifier named after you? William Doherty did
just that when he invented a special amplifier. That's why he's
in our Microwave Hall of Fame!
The Doherty amplifier offers
improved efficiency compared to balanced amplifiers. These amplifiers
are typically used in communications (radios, not radars). The heart
of the Doherty amplifier is the Doherty combiner, shown below.
Need image!
The Doherty amplifier power-combines
two amplifiers, one is called the "carrier" amplifier
while the second is called the "peaking" amplifier. The
two amplifiers are biased differently, the carrier amp is at a "normal"
Class AB (provides gain at any power level) while the peaking amplifier
is at Class C which only conducts at half of the cycle. The beauty
of the Doherty amplifier is improved power-added efficiency, compared
to a balanced amplifier.
The Doherty amplifier pair is
shown in the figure below. On the input the signal is split using
a 3dB quadrature coupler, such as a Lange or branchline hybrid.
The input s the same as a balanced amplifier, and has the same feature
where mismatched amplifiers will have their reflection coefficients
reduced if the reflection coefficients are equal in amplitude and
phase, the reflected waves end up in the load terminating the isolated
port of the coupler.
Need image!
The output of the Doherty pair
is where life gets interesting. The two signals are out of phase
by 90 degrees, but by the addition of a quarterwave transmission
line of the peaking amplifier, they are brought back into phase
and reactively combined. At this point the two signals in parallel
create a Z0/2 impedance. This is stepped up to Z0 by a quarterwave
transformer. In a fifty ohm system the transformer would be 35.35
ohms.
If anyone has an image or test
data on the Doherty amplifier that they can donate to this page,
please contact us!
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