Design of a GaN wide bandgap power amplifier

0 Introduction

Semiconductor power devices have generally gone through three stages according to the material classification. The first generation of semiconductor power devices is mainly represented by Si bipolar power transistors, which are mainly used in the S band and below. The pulse output power of Si bipolar power transistors in the L band can reach hundreds of watts, while the pulse power in the S band is close to 200W. The second generation of semiconductor power devices is represented by GaAs field effect transistors, and its maximum operating frequency can reach 30 to 100 GHz. The maximum output power of GaAs field effect transistors in the C band is close to 100W, and in the X band it can reach 25W. The third generation of semiconductor power devices is mainly represented by SiC field effect transistors and GaN high electron mobility transistors. Compared with the first and second generation semiconductor materials, SiC and GaN semiconductor materials have the advantages of wide bandgap, high breakdown field strength, high saturation electron drift rate and strong radiation resistance. They are particularly suitable for high-frequency, high-power, radiation-resistant power devices, and can work in high temperature and harsh environments. Due to these advantages, wide bandgap semiconductor power devices can significantly improve the performance of electronic information systems and are widely used in important fields such as artificial satellites, rockets, radars, communications, fighter jets, and marine exploration.

This paper designs and implements a high-efficiency GaN wide-bandgap power amplifier based on Agilent ADS simulation software. The design steps are explained in detail and the amplifier is tested. The results show that the amplifier can achieve an output power of more than 15W in the range of 2.3 to 2.4 GHz with an additional efficiency of more than 67%.

1 Design of GaN wide bandgap power amplifier

1.1 Amplifier Design Specifications

In the 2.3-2.4 GHz operating frequency band, the amplifier is required to operate in continuous wave mode, with an output power greater than 10 W and an additional efficiency greater than 60%.

1.2 Selection of power tube

According to the design indicators required by the amplifier, the design uses a SiC-based GaN wide bandgap power tube provided by an imported company. Its main performance parameters are shown in Table 1.

1.3 Amplifier Circuit Design

Figure 1 is a block diagram of the power amplifier. In Figure 1, IMN & Bias and OMN & Bias are the input matching network and input bias circuit and the output matching network and bias circuit, respectively, and VGS and VDS are the gate-source operating voltage and drain-source operating voltage, respectively. The design ideas adopted are: DC analysis of the power tube to determine the static operating voltage of the amplifier; stability analysis and design; use the source pull (Source Pull) and load pull (Load Pull) methods to determine the optimal source impedance ZS and optimal load impedance ZL of the power tube matching circuit (see Figure 1 for the definition of ZS and ZL); design the input and output matching circuits and bias circuits based on the obtained source impedance and load impedance; processing, debugging and revision.

1.3.1 DC Analysis

The purpose of DC analysis of power amplifier is to determine the static working voltage of power tube through the current-voltage (IV) curve of power tube. Since the manufacturer provides the ADS model of power tube, the model is directly used for simulation design in the design (the same below).

Figure 2 shows the results of DC analysis of the device model in Agilent ADS software. According to the device specifications given by the manufacturer and the IV curve in Figure 2, VDS = 28 V and VGS = -2.5 V are selected as the operating voltage of the amplifier. In order to achieve higher efficiency of the amplifier, the static voltage is selected here to make the amplifier work under Class C conditions.

1.3.2 Stability analysis

Stability is one of the key factors to be considered in amplifier design, which depends on the S parameters of the transistor and the terminal conditions. The instability of the power amplifier will produce undesirable parasitic oscillations, resulting in distorted results or even design failure. Therefore, stability analysis and design must be performed before designing the amplifier impedance matching circuit.

Figure 3 shows the curve of the power tube stability factor changing with frequency. In Figure 3, the stability factors K and D are defined as:

It can be seen from Figure 3 that within the design frequency band, the stability coefficients K and D satisfy the conditions of being greater than 1 and less than 1 respectively, so the power tube is unconditionally stable.

1.3.3 Source-pull and load-pull analysis

Principle of source-pull/load-pull analysis method: When the amplifier is excited by a large signal level, the power tube is analyzed by continuously changing the source impedance/load impedance, and then the equal power curve and equal gain curve are drawn on the Smith impedance circle chart. The optimal source impedance/optimal load impedance is selected according to the design requirements to accurately design a power amplifier that meets the requirements.

The center frequency f = 2.35 GHz is selected in the analysis. In order to accurately obtain the optimal source impedance ZS and the optimal output impedance ZL of the power tube, the efficiency-first strategy is followed in the analysis process, and the following steps are taken:

First, assume that ZS(O) = 10 Ω and perform load-pull analysis to obtain ZL(1); then, perform source-pull analysis based on ZL(1) to obtain ZS(1); then perform load-pull analysis based on ZS(1) to obtain ZL(2), etc. Repeat the source-pull analysis and load-pull analysis until the load impedances ZL obtained twice are equal or differ very little.

FIG4 is a contour diagram of the power tube output power and power added efficiency (PAE) obtained by performing source pull analysis and load pull analysis. In FIG4, the power tube added efficiency is defined as:

Where: POUT, PIN and PDC are the amplifier output power, input power and power consumption respectively; ηPAE represents the power added efficiency.

From Figure 4, we can see that the optimal source impedance and optimal load impedance of the power amplifier are ZS = 2.1-j6.5 Ω and ZL = 13+j7.8 Ω respectively.

1.3.4 Matching network and bias circuit design

The matching circuit is mainly used for impedance transformation, and its ultimate goal is to achieve maximum power transmission. In the simulation design process, it is first assumed that the input and output matching network designs are carried out using the best source impedance and best load impedance obtained under the ideal bias circuit. Then the bias circuit is designed according to the 1/4λ criterion, and the bias circuit meets the requirements of the RF choke by fine-tuning some circuit parameters. In Agilent ADS software, in order to make the design accurately simulate the real situation, it is generally necessary to perform RFMomentum optimization simulation after the circuit design (model-based). Figure 5 shows the amplifier matching network and bias circuit designed by Agilent ADS software. In Figure 5, the microwave circuit substrate material is Rogers' RT/duroid 6002 plate, with a dielectric constant of 2.94 and a thickness of 0.254 mm. In the optimization simulation process, it is found that the efficiency and bandwidth of the amplifier are a pair of contradictions. When the efficiency is improved, the bandwidth becomes narrower, and vice versa.

2. Index test

The actual amplifier is shown in Figure 6.

The designed wide bandgap power amplifier was tested. The test conditions are: continuous wave operation, drain voltage VDS=28 V, gate voltage VGS=-2.5 V. Figure 7 shows the curves of the amplifier output power and additional efficiency changing with input power at a frequency of 2.35 GHz. The test results show that: with the increase of input power, the output power of the amplifier increases approximately linearly, and saturation begins to appear at 26 dBm; with the increase of input power, the amplifier additional efficiency increases, and the maximum additional efficiency reaches 68.5% at 27 dBm. The output power and additional efficiency parameters of the amplifier were also tested in the frequency range of 2.2 to 2.6 GHz (0.5 GHz as a step), and the test results are shown in Figure 8. In the frequency range of 2.25 to 2.5 GHz, the amplifier output power is above 10 W, and the additional efficiency is also over 60%. In the frequency range of 2.3 to 2.4 GHz, the output power exceeds 15 W, and the additional efficiency exceeds 67 9/6, and the amplifier meets the design indicators.

3 Conclusion

An S-band 10 W power amplifier was designed and fabricated using SiC-based GaN wide bandgap power devices. The test results show that the designed amplifier has an additional efficiency of more than 67 9/6 in the range of 2.3 to 2.4 GHz, which also confirms the high efficiency and high gain characteristics of wide bandgap devices.