Detailed Explanation of Main Characteristics and Characteristics of High Power Gan Amplifier

Gallium nitride (GAN) power semiconductor technology has made great contributions to improving the performance level of RF / microwave power amplification. Gan transistors have achieved higher output power density, wider bandwidth and better DC to RF efficiency by reducing parasitic components of devices, adopting shorter gate length and higher operating voltage. For example, the radar system application demonstration of GaN based X-band amplifier has been made in 2014, which can provide 8kw pulse output power to replace traveling wave tube (TWT) devices and TWT amplifiers. 32kw versions of these solid-state Gan power amplifiers are expected in 2016. While looking forward to these amplifiers, we will investigate some main characteristics and characteristics of high-power Gan amplifiers.

Not long ago, Gan was also the preferred technology for reflected frequency electronic warfare (crew) applications, and thousands of amplifiers have been delivered for practical use. Now, the technology is also deployed in the field of airborne electronic warfare. The amplifier under development can provide hundreds of watts of output power in multiple octaves of RF / microwave range. Several versions of such broadband EW power amplifiers will be released this year.

Subsequent research includes improving the linearity of peak to average power ratio (PAPR) waveform, which is adopted by many military communication systems, including universal data link (CDL), broadband network waveform (WNW), soldier radio waveform (SRW) and broadband satellite communication (SATCOM) applications. ADI's "bit to RF" program will integrate the company's advantages in baseband signal processing and Gan power amplifier (PA) technology. By using techniques such as predistortion and envelope modulation, this integration will help to improve PA linearity and efficiency.

Gan devices released in the past few years include both discrete field effect transistors (FETs) and single-chip microwave integrated circuits (MMICs), which have been widely used in high-power microwave amplifier systems. Such devices can be provided by many wafer factories and device manufacturers, and are usually made of 100mm silicon carbide (SIC) wafer. Gallium nitride on silicon process is also under consideration, but the thermal conductivity and conductivity of silicon are relatively poor, offsetting its cost advantage in high-performance and high reliability applications. The gate length of these devices is as small as 0.2 μ m. It supports working in millimeter wave band. In many high-frequency applications and all low-frequency applications (except the most cost sensitive applications), GaN based devices have largely replaced gallium arsenide (GaAs) and silicon transverse diffusion metal oxide semiconductor (LDMOS) devices.

RF power amplifier designers pay attention to Gan devices because they support very high operating voltage (three to five times higher than GaAs), and the allowable current per unit FET gate width is roughly twice that of GaAs devices. These characteristics are of great significance to PA designers, which means that higher load impedance can be supported at a given output power level. The output impedance of previous designs based on GaAs or LDMOS is often extremely low (relative to the typical system impedance of 50 Ω or 75 Ω). Low device impedance will limit the achievable bandwidth, that is, as the impedance conversion ratio between the amplification device and its load increases, the number of elements and insertion loss will also increase. Due to this high impedance, the early users of such devices only installed one device in a mismatched test fixture, applied DC bias, and driven the device with RF / microwave test signals in some cases.

Due to these operating characteristics and their exceptionally high reliability, Gan devices are also suitable for high reliability space applications. Several device suppliers have conducted life tests at 225 ° C or higher junction temperature, and the results show that the average time before failure (MTTF) of a single device exceeds one million hours. Such high reliability is mainly due to the high band gap value of GaN (GaN is 3.4 and GaAs is 1.4), which makes it particularly suitable for high reliability applications.

The main obstacle to expanding the use of Gan in high-power applications is its relatively high manufacturing cost, which is usually two to three times higher than GaAs and five to seven times higher than Si LDMOS devices. This hinders its use in cost sensitive applications such as wireless infrastructure and consumer handheld devices. Now with the gallium nitride on silicon process, although there are the performance problems mentioned above, the devices produced by this process may be most suitable for cost sensitive applications. In the near future, as the manufacturing of Gan devices turns to larger wafers (with a diameter of 150mm and larger, at present, several leading Gan device foundries are under development), the cost is expected to be reduced by about 50%.

Currently deployed radar systems for weather forecasting and target acquisition / recognition rely on TWT power amplifiers operating at C-band and X-band frequencies. These amplifiers operate at high supply voltage (10 kV to 100 kV) and high temperature, and are easy to be damaged due to excessive shock and vibration. The field reliability of these TWT amplifiers is usually only 1200 to 1500 hours, resulting in high maintenance and spare parts costs.

As an alternative to high-power TWT amplifier, ADI company has developed an 8kw solid-state X-band power amplifier based on GaN technology. The design uses an innovative hierarchical merging method to add up the RF / microwave output power of 256 MMICs, and each MMIC produces an output power of about 35 W. When individual MMICs fail, this combination method ensures that the output performance will not degrade sharply. TWT amplifier is not so. Because of its low redundancy, a single fault often leads to catastrophic device failure. For this solid-state Gan power amplifier, the RF / microwave combining architecture must strike a reasonable balance between the required isolation between MMICs and the RF / microwave insertion loss of the whole network.

The 8kw amplifier topology is modular, including four 2kW amplifier components, and its output power is combined by waveguide structure (Fig. 1). The amplifier can be installed in a standard 19 inch housing. The current design of the amplifier (Figure 2) uses water cooling, and other versions using air cooling are under development. Table 1 gives a summary of the performance of water-cooled 8 kW Gan PA.

Figure 1. GaN based solid-state power amplifier can provide 8 kW output power and operate at X-band frequency

Figure 2. Block diagram reflecting the structure and devices of GaN and X-band solid-state power amplifiers

The 8 kW SSPA supports the combination of multiple modular SSPAs to produce higher power levels. An amplifier with three such 8 kW SSPA modules is currently being developed, which can achieve a peak output power level of 24 kW over the same frequency range. Other configurations to achieve a 32 kW power level are also feasible and are currently under consideration for further evaluation.

ADI is currently developing an advanced power module, which is also based on GaN technology. Its RF / microwave output power will be twice that of the current module. The module is sealed to support operation in extreme environments. Combined with the next generation combined structure and lower insertion loss (compared with the current method), it will increase the pulse output power of RF / microwave frequency to nearly 75 kW to 100 kW. These advanced high-power SSPAs will include control and processor functions, support fault monitoring, built-in test (bit) function, remote diagnostic test, and control of fast real-time bias control circuits for MMIC devices that power amplifiers.

This kind of Gan solid-state power amplifier is designed to meet the industry's demand for wide instantaneous bandwidth and high output power amplifier. Some systems attempt to meet these requirements using channelization or multiple amplifiers, each covering a portion of the required spectrum and fed into a multiplexer. This increases cost and complexity and results in voids at the frequency crossing point of the multiplexer. A more effective alternative solution is to continuously cover a wide frequency range at a higher power level, which has been achieved by two different Gan amplifiers covering VHF to L-band frequencies and 2 GHz to 18 GHz.

For VHF to S-band frequencies, ADI has developed a very small size, rich functions and multiple frequency range amplifier, which can provide 50 W output power in the range of 115 MHz to 2000 MHz. In the full frequency range, the amplifier can achieve an output power level of 46 DBM (typical 40 W) when feeding a nominal input signal of 0 DBM.

The amplifier uses a size of 7.3“ × 3.6" × 1.4 "compact package, with bit function, can provide heating and current overload protection and telemetry report, and integrate DC-DC converter to achieve the best RF performance. The input power range is 26 VDC to 30 VDC. Figure 3 shows the photo of the amplifier, and the relationship between typical measured performance data of output power and frequency is shown in Figure 4.

Figure 3. Continuous wave (CW), 50 W, solid state power amplifier, operating frequency range 115 MHz to 2000 MHz

Figure 4. Relationship between output power and frequency of 50 W, 115 MHz to 2000 MHz power amplifiers

Figure 5. 50 W, CW output power amplifier, operating frequency range 2 GHz to 18 GHz

For broadband applications above 2 GHz, ADI has also developed a GaN amplifier, which can generate 50 W continuous wave (CW) in the frequency band of 2 GHz to 18 GHz Output power. This amplifier uses a commercial 10 W Gan MMIC, and its output power contribution is combined through a broadband low loss combining circuit. Multiple such amplifiers can also be combined to produce up to 200 W output power in the same 2 GHz to 18 GHz bandwidth. The drive amplifier chain is also based on GaN active devices. The amplifier uses 48 VDC power supply, built-in voltage regulator and high-speed on-off The circuit is closed, supports pulse operation, and has good pulse fidelity and fast rise / fall time. The specifications of this amplifier are listed in Table 2. Figure 5 shows a photo of the amplifier, and Figure 6 shows the functional relationship between the output power and frequency (2 GHz to 18 GHz).

Figure 6. Relationship between output power and frequency of 50 W, 2 GHz to 18 GHz power amplifier

This 50 W amplifier is one of a series of amplifiers in the 2 GHz to 18 GHz band. ADI has also developed a compact desktop amplifier with 12W output power (Figure 7) and a rack mounted unit with 100W output power (Figure 8) Other amplifiers in the frequency range from 2GHz to 6GHz and from 6GHz to 18GHz are under development. ADI is also working to increase the output power of these broadband amplifiers from the current level to 200 W and higher. In order to achieve a higher output power level, ADI is developing high output power modules and broadband RF power combiners, which will greatly improve the combining efficiency and reduce the loss Lower than current power combiner.

Figure 7. Broadband 2 GHz to 18 GHz power amplifier, generating 12 W CW output power in the full frequency range

Figure 8. 2 GHz to 18 GHz solid state power amplifier, generating 100 W CW output power in the full frequency range

The above are some examples of the performance levels that can be achieved with Gan solid-state amplifiers. As more GaN semiconductor suppliers turn to larger wafers and the yield of each wafer continues to improve, the unit cost of such amplifiers is expected to be reduced in the future. With the shortening of gate length, GaN based SSPA will be able to support higher operating frequencies, so there will be more and more Gan amplifiers It is obvious that the current trend of Gan to improve performance and reduce cost should continue for some time.

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