The Doherty Power Ampliier

The Doherty Power Amplifier (DPA) was invented in the far 1936 by W. H. Doherty, at the Bell Telephone Laboratories of Whippany, New Jersey (Doherty, 1936). It was the results of research activities devoted to find a solution to increase the efficiency of the first broadcasting transmitters, based on vacuum tubes. The latter, as it happens in current transistors, deliver maximum efficiency when they achieve their saturation, i.e. when the maximum voltage swing is achieved at their output terminals. Therefore, when the signal to be transmitted is amplitude modulated, the typical single ended power amplifiers achieve their saturation only during modulation peaks, keeping their average efficiency very low. The solution to this issue, proposed by Doherty, was to devise a technique able to increase the output power, while increasing the input power envelope, by simultaneously maintaining a constant saturation level of the tube, and thus a high efficiency. The first DPA realization was based on two tube amplifiers, both biased in Class B and able to deliver tens of kilowatts. Nowadays, wireless systems are based on solid state technologies and also the required power level, as well as the adopted modulation schemes, are completely different with respect to the first broadcasting transmitters. However, in spite of more than 70th years from its introduction, the DPA actually seems to be the best candidate to realize power amplifier (PA) stage for current and future generations of wireless systems. In fact, the increasing complexity of modulation schemes, used to achieve higher and higher data rate transfer, is requiring PAs able to manage signals with a large time-varying envelope. The resulting peak-to-average power ratio (PAPR) of the involved signals critically affects the achievable average efficiency with traditional PAs. For instance, in the European UMTS standard with W-CDMA modulation, a PAPR of 5-10 dB is typical registered. As schematically reported in Fig. 1, such high values of PAPR imply a great back-off operating condition, dramatically reducing the average efficiency levels attained by using traditional PA solutions. 6