Doherty power amplifier research and design

RF power amplifiers are widely used in various wireless communication transmitting equipment. Linear power amplifiers in base stations in the percentage of the costs account for about 1 / 3, how effective, low-cost solution to the power amplifier linearization issues become very important. High linearity power amplifier efficiency is a hot research topic, especially in recent years, power amplifier for WCDMA. Currently more than capable of producing 10 W WCDMA power amplifier manufacturers only a few companies, because the WCD-MA power amplifier linearity requirements on higher. The use of ordinary regression line with the production of the WCDMA power amplifiers can only be a few watts indicators, the power used in the base station is not enough, only a small repeater with the general.

Power amplifier linearity and efficiency are the focus of power amplifier design. Linearity, the feed-forward structure, the structure is relatively mature, widely used in modern communication systems, digital pre distortion in the industry were considered in the direction of the linear power amplifier. With the development of modern communication, efficiency, began more and more attention. Doherty is considered the most promising to improve the efficiency of a structure. Doherty feedforward structure combined with structure or combination of digital pre-distortion and the Doherty structure of great value.

1 Doherty power amplifier design

1.1 Doherty amplifier PRINCIPLES

Doherty amplifier structure consists of two components: a main amplifier, an auxiliary amplifier, main amplifier working in class B or AB, auxiliary power amplifier work in the C category. The work of the two amplifiers are not rotating, but the main amplifier has been working, auxiliary amplifier to set the peak before work (this amplifier is also called the peak ampli-fier). 90 ° behind the main amplifier quarter wavelength impedance transformation line is aimed at supporting amplifier work, the play will be the main power amplifier decreases as the impedance of the role, work to ensure the time and auxiliary amplifier circuit composed of the back Active low load impedance changes, so the main amplifier output current to become larger. As the main amplifier with a quarter wavelength behind the lines, in order for the two amplifier output with the phase front of the auxiliary amplifier is also required 90 ° phase shift.

The main amplifier working in Class B, when the input signal is small, only the main amplifier in a working state; when the tube's output voltage reaches the peak point of saturation, the theoretical efficiency can reach 78.5%. If this time will encourage increased twice, then the tube is half the peak occurs when filled to capacity, efficiency 78.5% maximum, this time also began auxiliary amplifier to work with the main amplifier (C category, threshold is set to half the excitation signal voltage). The introduction of auxiliary amplifier, making the angle from the main amplifier, the load decreases, because the role of auxiliary amplifier to the load is equivalent to connecting a negative resistance, so even if the main amplifier output voltage saturation constant, but the output power because the load The decrease has continued to increase (the load current flows through the larger a). When the peak inspiration, the auxiliary amplifier has reached its maximum point of efficiency, so that together the two amplifier's efficiency is much higher than the single class B power amplifier efficiency. Class B power amplifier single maximum efficiency appears in the peak of 78.5%, 78.5% current efficiency at half the peak appeared. Therefore, this system architecture to achieve high efficiency (each amplifier reached the maximum output efficiency).

1.2 Doherty Amplifier Design

Under the rated power 30 W, output gain 50 dB, frequency of 2 110 ~ 2 170 MHz as the design requirements such as:

Doherty power amplifier should be designed, first of all should choose components, and then select the appropriate DC operating point, set the bias circuit. Further impedance matching, and finally design the combiner 90 ° to the main and auxiliary power amplification circuit synthesis.

1.2.1 Select and amplifier bias power amplifier design according to the indicator, the first selected power amplifier. RF power amplifiers used Motorola LDMOS main tube, and its market share above 70%, while the LDMOS device is also suitable for CDMA, W-CDMA, TETRA, digital terrestrial television frequency range needed, high linearity and the use of life of demanding applications. In addition, as the total gain of the amplifier output to reach 50 dB, so it should use multi-stage power amplifier.

Doherty power amplifier for maximum efficiency is about 6 dB when the rollback to, so I chose two Motorola power amplifier tube MRF21060, they work at maximum power, the total power of 120 W, back to back 1 / 4 (6 dB) that are 30 W.

Doherty power amplifier to design, we need to make one of the pipe work in the AB class B type of work or the state, while the other tube static operating point chosen state in the C class work.

LDMOS tube MRF21060 on static work point scanning, selected a suitable quiescent point, shown in Figure 2.

Doherty power amplifier research and design

According to Doherty technical requirements, the choice of power amplifier bias, should be made when the main amplifier saturation, when auxiliary amplifier to start work, so as to achieve very high efficiency.

Select MRF21060 this kind of work in the AB state, the static operating point at about 3.7 V Department. Select C class working condition, select the work in the 2.1 V. Two kinds of bias state quiescent current and voltage shown in Figure 3.

Doherty power amplifier research and design

1.2.2 Matching Circuit Design

LOAD PULL (traction load) measurement method that enables designers to determine the load impedance ZL. RF power amplifiers in large-signal work, the optimal load impedance of the transistor with the input signal power will increase along with changes, so must the Smith chart, for different input power units, each given an input power value gave draw such different load impedance when the output power curve (power contours), to help find the maximum output power when the load impedance, this method is called LOAD PULL.

LOAD PULL is the basic test tube power amplifier input and output load characteristics of the method, in theory, LOAD PULL load changes can be obtained under various load characteristics, and the same characteristics as the composition of the gain equal to a similar point can map the contour lines the circle diagram. Such as the gain circle, and so the efficiency of circle, line graph, etc., etc., get the information, can be requested in the design trade-off design. For example, in the specific request gain and linear, so you can find the corresponding linear gain circles and the other round the middle of the intersection region as the impedance matching of the target location. According to the simulation results: the partial din specified, if the input signal is 30 dBm, the maximum 42.84% power added efficiency, maximum output power of 14.34 dBm. Select the most optimal load for the 3.530-j2.811.

You can use Motorola typical reference circuit input and output matching circuit. The recommended circuit match the value shown in Table 1.

Doherty power amplifier research and design

Zsource = Test circuit impedance as measured from gate to ground

Zload = Test circuit impedance as measured from drain to ground

After completion of the matching circuit, but also the design of the combiner 90 ° and 90 ° offset with an impedance converter is used to synthesize the main and auxiliary amplifier.

Thus a complete Doherty Power Amplifier on the design is complete.

2 Experimental simulation

Following on the simulation results of the Doherty power amplifier with class AB power amplifier relatively common. Figure 4, shown in Figure 5.

Doherty power amplifier research and design

Doherty power amplifier can be seen power-added efficiency than ordinary class AB power amplifier added efficiency increased.

3 Conclusion

As expected, like, Doherty power amplifier can improve the power amplifier power added efficiency, but the linearity will be worse. This is where the auxiliary amplifier working in Class C state of the. So Doherty also based on the generally linear increase in the circuit to improve power amplifier linearity.