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.

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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.

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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!