Simulation Design of S-Band Solid-State Power Amplifier

　　1 Introduction

　　Microwave power amplifiers are widely used in many microwave electronic devices and systems as vital components in transmitter units, such as modern wireless communications, satellite transceiver equipment, radar, telemetry and remote control systems, electronic countermeasures, etc. Traditional high-power amplifiers are implemented with vacuum tubes. With the continuous development of semiconductor devices, the advantages of solid-state devices are becoming increasingly obvious. Microwave solid-state power amplifiers are favored in many fields due to their small size, low operating voltage, high stability, and good repeatability. This paper studies a high-power solid-state amplifier in the S band, with an output power of 180W continuous wave, an operating frequency of 2.0GHz to 2.3GHz, a power gain greater than 13dB, a gain flatness less than +/-1.0dB, an output power of 50dBm at the 1 dB gain compression point, a saturated output power greater than 53.4dBm, and a power added efficiency greater than 48%.

　　2 Design of matching circuit

　　Since the power amplifier works in nonlinearity, the network design method of small signal amplifier is no longer applicable. This paper studies a 180W high power amplifier. The input and output impedance of the amplifier changes with the frequency and input power. There are usually three analysis methods to analyze the matching circuit: dynamic impedance method, large signal S parameter method and load pull method.

　　The dynamic impedance method requires the dynamic input and output impedance (also called the optimal load impedance) under the large signal working state. The principle of dynamic impedance testing is to use a mixer to adjust the power tube to the maximum power output state, and then measure the impedance from the signal source to the input end of the power tube and from the load to the output end. The impedance value is the dynamic input and dynamic output impedance; the large signal S parameters can be used to analyze the power gain and stability of the power amplifier and design the gain and flatness. At the same time, when designing a power amplifier using large signal S parameters, in addition to selecting the load impedance according to the output power, it can also be considered separately according to the two conditions of absolute stability and potential instability. The measurement of large signal S parameters is more difficult, and the dual signal method or large current DC fitting method is usually used to measure large signal S parameters; the load traction method requires different data corresponding to various parameters such as output power, power gain and efficiency, and is comprehensively designed by a computer. Its design system is relatively complex.

　　Usually for high-power transistors, manufacturers often provide the dynamic input and output impedance of the power transistor, so the matching network design also uses the dynamic impedance method for design, and the experiment also uses the dynamic impedance method to design the matching network. Next, we will use this as a basis to design the matching circuit. In this design, the single-chip power amplifier tube provides the dynamic input and output impedance, which is a complex number. The core idea of ​​the impedance matching network design is to match the input and output impedance within the frequency range to near the 50W impedance, that is, on the impedance circle diagram, the input and output impedance is matched to near the center of the impedance circle diagram. If you design a power amplifier for a series power amplifier tube, inter-stage matching is also very important. Generally, conjugate matching is achieved, and in actual situations, multiple methods can be compared to select a more appropriate matching circuit for design.

　　3 Power synthesis technology

　　Due to the limitations of process, design linearity, and working conditions, the output power of a single tube of a microwave RF power amplifier is difficult to meet the design requirements. Therefore, it is necessary to use a multi-tube parallel connection method to synthesize power to meet the design requirements. Power combiners are divided into two-way combiners, multi-way combiners, and chain combiners. Generally, two-way combiners are used for power synthesis. The two-way power dividers commonly used for power synthesis are: WILKSON power divider, 3dB orthogonal power combiner, and anti-phase push-pull power combiner. The main factors affecting the synthesis efficiency of the combiner include the input and output impedance matching of the combiner (input and output voltage standing wave coefficient), amplitude, phase imbalance, insertion loss, and mutual isolation between each channel. The power combiner superimposes the RF output voltages of each module and transmits the sum of the output power of all modules minus the loss of the combiner to a single port. Many power synthesis-distribution structures can be used, and they all show certain different characteristics. Generally, the requirements for power combiners are as follows:

　　(1) The synthesizer should have low insertion loss so that the transmitter power output and efficiency are not affected;

　　(2) The synthesizer should have RF isolation between ports so that a faulty module does not affect the load impedance or power combining efficiency of the remaining working modules;

　　(3) The synthesizer should be able to provide a controllable RF impedance to the amplifier module so that the performance of the amplifier is not degraded;

　　(4) The reliability of the synthesizer should be much higher than that of other transmitting components;

　　(5) The power combiner terminal load should be able to withstand power consumption sufficient to accommodate any combination of amplifier faults;

　　(6) The mechanical packaging of the combiner should facilitate maintenance of the module. The packaging should also provide short, equal-phase, and low-insertion-loss interconnections between the amplifier module and the combiner.

　　This design uses 3dB orthogonal power synthesis. The balanced amplifier consists of two identical amplifiers A and B connected in parallel through two 3dB bridges, where the input and output bridges are used as power dividers and power synthesizers respectively. The coupling degree between the through port and the coupled port of the 3dB bridge is 3dB, and the phase difference is 90 degrees. Therefore, the reflected signals of amplifiers A and B at the input and output ends of the balanced amplifier are 180 degrees out of phase and cancel each other out, so the input and output standing wave ratio (VSWR) of the ideal balanced amplifier is equal to 1. It can be seen that the standing wave ratio (VSWR) of the balanced amplifier is only determined by the performance of the 3dB 90-degree bridge and has nothing to do with the performance of the discrete amplifier. The forward transmission signals of amplifiers A and B have the same phase at the output port of the balanced amplifier, and the output signals of amplifiers A and B are added in phase at the output port of the output bridge.

　　4 Considerations for heat dissipation and shielding boxes

　　The power added efficiency of general microwave power amplifiers is low (20% to 40%). The DC power that is not converted into RF power is released in the form of heat inside the power tube. When the power amplifier is working normally, the temperature of the power tube will rise sharply. In order to ensure the stable and reliable operation of the solid-state power amplifier, the heat of the power tube itself should be dissipated in time to keep the temperature of the chip below the maximum allowable junction temperature, which requires a strong heat dissipation capacity. This paper conducts theoretical analysis and calculation of the flange temperature of the power tube, analyzes and obtains the maximum tube shell temperature and flange temperature that the amplifier can withstand when working stably, so as to combine the actual situation to carry out the heat dissipation design of the power amplifier.

　　The power amplifier shielding box mainly plays the role of electrical shielding. It should meet certain electromagnetic compatibility conditions and minimize the impact of the microwave radiation signal of the power amplifier circuit on the entire circuit. Usually, the microstrip circuit (including active and passive components) is placed in the box and works below its cutoff frequency, which will reduce the impact of the microwave components caused by the radiation signal (such as reducing feedback, gain fluctuations, and improving isolation, etc.). The S-band power amplifier designed in this paper has an operating frequency half-wavelength of about 5cm. In order to avoid the waveguide transmission effect in the box, the lateral width of the box is designed to be about 5cm, and the power supply part and the RF part are separated by a metal partition according to the actual circuit structure. The width of the RF cavity is about 2.5cm. According to the actual device size, the cavity size is simulated and optimized in the HFSS software, and the structural model of the power amplifier box is designed.

　　5 Power Amplifier Simulation

　　This paper uses Agilent ADS software to simulate the 180W power amplifier. The large signal gain characteristics of the circuit are shown in Figure 1 and Figure 2. With a 36dBm input power signal, the output power gain can reach 14.7dB in the 2.0-2.3GHz frequency band. In the 2.05-2.25GHz frequency band, the gain fluctuation is less than 0.2dB. The input and output return loss is less than -23dB.

　　The power efficiency characteristics of the circuit are shown in Figure 3. The frequency band of P1dB is 1.94~2.3GHz, the output power is greater than 50dBm, and the efficiency is greater than 45%; the power frequency characteristics of the circuit are shown in Figure 4. In the working band of 2.0~2.3GHz, the output power P1dB is greater than 50.5dBm when the input is 36dBm, and the power frequency curve is very flat, which meets the design requirements; the Two-Tone intermodulation characteristics of PA are shown in Figure 5. The first carrier frequency is 2.13808GHz, and the second carrier frequency is 2.14192GHz. The designed PA Two-Tone has an average output power of 45dBm and IM3 is less than -35dBc, which can meet the CDMA application requirements. The characteristics of PA gain, efficiency and output power are shown in Figure 6. The selected frequency is 2.14GHz. It can be seen from the figure that the saturated output power of the 180W solid-state power amplifier reaches 53dBm and the power added efficiency reaches 60%.

　　Figure 1 Gain characteristics under large signal conditions

　　Figure 2 Return loss at input and output

　　Figure 3 Input and output power and efficiency characteristics

　　Figure 4 Power-frequency characteristics when input power is 36dBm

　　Figure 5 Third-order intermodulation characteristics

　　Figure 6 Power gain efficiency characteristics

　　6 Conclusion

　　This paper uses power synthesis technology to design a high-power amplifier with an S-band output power of 180W, and fully considers the design of heat dissipation and shielding box. Combined with the software Agilent ADS simulation, a power amplifier that meets the technical indicators is designed. The 3dB orthogonal power synthesis used in the paper realizes power synthesis, which has the advantages of low loss and good consistency. And the shielding box is designed with HFSS, making the design of the shielding box relatively simple.