Doherty amplifiers

Updated August 20, 2009

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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 of Fame!

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 ohms.

If anyone has an image or test data on the Doherty amplifier that they can donate to this page, please contact us!