GaN Breaks Barriers—RF Power Amplifiers Go Wide and High

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Theoretically, we can continue increasing the transistor periphery size to continue increasing the RF power, but there are practical limitations to this such as complexity, die size, and combining loss. The matching networks also tend to limit the bandwidth, as they become difficult to provide an optimum impedance over wide frequencies. In the distributed power amplifier, there are only transmission lines whose purpose is to have the signals constructively interfere along the amplifier, rather than matching networks.

There are additional techniques to further improve the power in distributed amplifiers, such as using a cascode amplifier topology to further increase the voltage supply to the amplifier.

Results

We have shown that there are various techniques and semiconductor technologies that offer trade-offs in providing optimum power, efficiency, and bandwidth. Each of these different topologies and technologies will likely have a place in the semiconductor world, as they each provide benefits, which is why they have survived to this day. Here we’ll focus on a few results that we believe show what is possible with these technologies today to achieve high power, efficiency, and bandwidth.

We’ll look at a GaAs-based, distributed power amplifier operating from dc to 30 GHz, which is a product released from Analog Devices, HMC994A. This part is interesting because it covers many decades of bandwidth, lots of different applications, and achieves high power and efficiency. The performance is shown in Figure 5. Here we see a saturated output power covering MHz to 30 GHz with over 1 W of power and a power added efficiency (PAE) of 25% nominal.

Today’s Product Capability made from GaAs and GaN

This particular product also has a strong third-order intercept (TOI) performance of 38 dBm nominal. This result shows that with GaAs-based designs we are able to achieve an efficiency that is close to what is possible with many narrow-band power amplifier designs. Given the positive gain slope with frequency, high PAE, wideband power performance, and strong return loss make the HMC994A an interesting product.

It’s also interesting to see what is achievable with GaN-based technology. Analog Devices offers a standard product, HMC8205BF10, which is based in GaN and combines high power, efficiency, and bandwidth. This product operates from a 50 V power supply and provides 35 W of RF power at 35% nominal efficiency with ~20 dB of power gain covering over a decade of bandwidth. In this case, a single IC is able to provide roughly 10× more power compared to similar approaches in GaAs.

In years past, this would have required a complicated combining scheme of GaAs die that would not have been able to reach the same efficiency. This product demonstrates what is possible with GaN technology covering wide bandwidths and providing high power and efficiency, as shown in Figure 6. It also shows progress in high power electronic packaging technology, as this part is housed in a flange package capable of supporting continuous wave (CW) signals.

Summary

The emergence of new semiconductor materials like GaN have opened the possibilities to reach higher power levels covering wide bandwidths. Shorter, gate length GaAs devices have extended frequency ranges from 20 GHz to 40 GHz and beyond. The reliability of these devices is shown in literature to exceed 1 million hours, making them ubiquitous for modern day electronic systems. We expect the trends of higher frequencies and wider bandwidth to continue into the future.

* Keith Benson is Product Line Director for RF/MW Amplifier and Phased Array ICs at Analog Devices in Norwood / U.S.A.

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