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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 in the 1930s. That's
why he's in our Microwave Hall
The Doherty amplifier originally
was intended for radio transmitters. Below we can see that the life
of the radio engineer in the 1930s wasn't as glamorous as it is
today. If someone can tell us why the tubes are blowing out when
the music gets loud, we'd love to know!
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.
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, at backed off power levels. If you will
always operate at saturated power (like in many radars) you have
no business using the Doherty.
The Doherty amplifier pair works
as follows: On the input the signal is split using a 3 dB quadrature
coupler, such as a Lange or branchline hybrid (branchline is shown
in our schematic). The input behaves 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.
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
If anyone has an image or test
data on the Doherty amplifier that they can donate to this page,
please contact us!