Implementation Of A 450 MHz Doherty Amplifier

With the rapid development of modern communication systems, the power amplifier, which plays an important role, is required to work more and more excellently. On the other hand, as the technology develops rapidly, the data transmission rate in limited spectrum bandwidth becomes higher and higher, but the spectrum resource is already extremely scarce, so several kinds of modulations are used, such as the WCDMA(Wideband Code Division Multiple Access) in the third generation mobile system and the OFDM(Orthogonal Frequency Division Multiplexing) in the fourth generation mobile system. The high peak-to-average ratio signals in these modulations require high linearity of the amplifiers.To satisfy this, the amplifiers usually operate back-off from the peak output power, which results in low efficiency. Therefore, how to enhance the efficiency of the amplifiers becomes a hotspot. Currently, the Doherty amplifier, one of the efficiency enhancement techniques is widely used with the DPD(Digital Pre Distortion) so as to satisfy the linearity requirement, it becomes the technical mainstream.This paper mainly describes a Doherty amplifier for 419MHz-481 MHz, its saturated output power can reach 250 W, unlike the conventional Doherty amplifier, a 3dB hybrid rather than the one-quarter wavelength microstrip is used to achieve the impedance transformation, which can also reduce the circuit size.The simulation results of the ADS(Advanced Design System) proves that a 3dB hybrid can achieve the impedance transformation as well as the one-quarter wavelength microstrip. And the test result of the Doherty amplifier shows that it works well, the PAE(Power added efficiency) is 14% higher than the traditional class AB amplifier at 7 dB back-off from the peak output power.This paper consists of four chapters, the first chapter is the introduction, it mainly introduces the main technical indicators of the amplifier and several kinds of amplifier efficiency enhancement techniques. In the second chapter, the basic circuit principle of the Doherty amplifier is presented. In the third chapter, the 3 dB hybrid is proved that it can achieve the impedance transformation and active load modulation by using the ADS simulation software, meanwhile, the particular simulation process and the simulation figures is showed. Then in the fourth chapter, it shows the actual debugging of the Doherty amplifier and the test results. Finally, it summarizes the whole article and points out the developing direction.