How to Choose Between GaN and GaAs Microwave SSPAs?

Choosing between GaN and GaAs SSPAs is tricky. GaN's smaller size is appealing, but its non-linearity can secretly tank your system performance¹. Let’s find the right solution.

Choose GaN for higher power density and a smaller footprint, but be prepared for its non-linearity. To maintain signal integrity, you must implement Digital Pre-Distortion (DPD) and a 3-5 dB power back-off. For applications demanding high linearity without DPD, GaAs remains a solid choice.²

On the surface, the choice seems simple. You either pick GaN for power or GaAs for linearity. But the real engineering challenge lies in the details. The consequences of picking the wrong one, or using one incorrectly, can be costly. We need to look closer at what these differences really mean for your system's performance. Let's break down the critical factors you must consider.

Why Does GaN's Size Advantage Come with a Linearity Cost?

Everyone loves GaN's smaller footprint. But the fear of non-linear effects causing signal distortion is real. Understanding the source of this problem is the first step to solving it.

GaN's superior thermal conductivity³ and high power density allow for smaller heat sinks. However, this same material property contributes to stronger trapping effects, which create non-linear memory effects. This causes AM-AM and AM-PM distortion⁴, negatively impacting your signal quality if not properly managed.

The Trade-Off: Power Density vs. Memory Effects

Gallium Nitride (GaN) has a wider bandgap than Gallium Arsenide (GaAs). This lets it handle higher voltages and temperatures. The result is much higher power density. You can get more watts out of a smaller chip, which means your heat sink can be smaller and your overall SSPA is more compact. I remember a project where switching to a GaN SSPA, like one of our high-power models, cut our heat sink volume by nearly half.

But this advantage has a catch. The high-density operation in GaN creates "trapping effects." Charge carriers get temporarily trapped and released within the semiconductor material. This process is not instant, so the amplifier's response at any given moment depends on the signals that came just before. This is what we call a "memory effect." It leads directly to AM-AM and AM-PM distortions, where amplitude and phase variations are no longer linear.

Here's a simple comparison:

This distortion is why you might see your system's EVM suddenly drop, sometimes to as low as 10%⁵, causing data errors and blocking adjacent channels.

How Can You Use GaN SSPAs Without Sacrificing Signal Integrity?

You need the power and efficiency of GaN for your project. But you can't risk the signal distortion it can cause. Do not worry, a clear two-step strategy fixes this.

To safely use GaN SSPAs, you must actively correct for their non-linearity. This involves implementing Digital Pre-Distortion (DPD) to counteract the amplifier's distortion and applying a 3-5 dB power back-off to operate the SSPA in its more linear region. This combination ensures clean signal transmission.

Step 1: Implementing Digital Pre-Distortion (DPD)

To get GaN's size advantage while maintaining linearity, you need a proactive approach. It's a two-part solution we use consistently in our designs at Safari Microwave. Think of DPD as a smart correction system. The DPD circuit learns the specific non-linear behavior of your GaN SSPA. It then "pre-distorts" the input signal in the exact opposite way. When this modified signal goes through the amplifier, the amplifier's natural distortion cancels out the pre-distortion. This results in a clean, linear output. It is a powerful technique for taming GaN’s memory effects.

Step 2: Applying Power Back-Off

Amplifiers are most non-linear when pushed close to their saturation point. To avoid this, we deliberately operate the SSPA below its maximum rated power. This is called "power back-off." For GaN SSPAs, reserving a 3-5 dB back-off from the 1dB compression point (P1dB) is crucial. It gives the DPD system enough headroom to work effectively and keeps the amplifier in a more stable, linear operating range. I remember a specific case where a 3000W GaN SSPA had terrible EVM near saturation. Just by backing off 4 dB and enabling our DPD, the adjacent channel leakage ratio (ACLR) improved by over 15 dB⁶. This combination gives you GaN's efficiency without the distortion penalty.

Conclusion

GaN provides superior power and size, but requires DPD and power back-off for linearity. GaAs offers simpler, native linearity. Your choice depends on your system's overall design and requirements.