GaN Breaks Barriers—RF Power Amplifiers Go Wide and High

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Design Considerations

There are different topologies and design considerations for the IC designer to use when choosing how to start a design to optimize power, efficiency, and bandwidth. The most common type of monolithic amplifier design is a multistage, common-source, transistor-based design, also known as a cascade amplifier design.

Here the gain multiplies from each stage, leading to high gain and allowing us to increase output transistor sizes in order to increase the RF power. GaN offers benefits here because we are able to greatly simplify the output combiners, reducing loss, and thereby improving efficiency, as well as shrinking the die size, as shown in Figure 2. As a result, we are able to achieve wider bandwidths and improve performance.

A less obvious benefit of going to GaN devices from GaAs is to achieve a given RF power level, perhaps 4 W—the transistor size will be less, resulting in higher gain per stage. It will lead to fewer stages per design and ultimately higher efficiency. The challenge with this cascade amplifier technique is that it is difficult to achieve bandwidths over an octave without significantly compromising the power and efficiency, even with the help of GaN technology.

Lange Coupler

One approach to achieve wide bandwidth design is to implement a balanced design with Lange couplers on the RF input and output, shown in Figure 3. Here the return loss is ultimately dependent on the coupler design, as it becomes easier to optimize the gain and power response over frequency without also needing to optimize the return loss. Even while using Lange couplers, it becomes more difficult to achieve bandwidths over an octave, but they do offer a very nice return loss for the design.

The next topology to consider is the distributed power amplifier shown in Figure 4. The benefit of a distributed power amplifier is accomplished by incorporating the parasitic effects of the transistor into the matching networks between devices. The input and output capacitances of the device can be combined with the gate and drain line inductance, respectively, to make the transmission lines virtually transparent, excluding transmission line loss.

Benefits of the Distributed Amplifier

By doing this, the gain of the amplifier should only be limited by the transconductance of the device and not the capacitive parasitics associated with the device. This only happens if the signal traveling down the gate line is in phase with the signal traveling down the drain line, so that each transistor’s output voltage adds in phase with the previous transistors output.

The signal traveling to the output will constructively interfere so that the signal grows along the drain line. Any reverse waves will destructively interfere since these signals will not be in phase. The gate line termination is included to absorb any signals that are not coupled to the gates of the transistors. The drain line termination is included to absorb any reverse traveling waves that could destructively interfere with the output signal and improve the return loss at low frequencies.

As a result, multiple decades of bandwidth are able to be realized from kHz to many GHz. This topology is popular when more than an octave of bandwidth is needed and there are some nice benefits, such as flat gain, good return loss, high power, etc. An illustration of a distributed amplifier is shown in Figure 4.

One challenge here with distributed amplifiers is that the power capability is dictated by the voltage applied to the device. Since there is no narrow-band tuning capability, you are essentially providing a 50 Ω impedance to the transistor or close to it. When we consider the equation for average power out of a power amplifier, the average power of PA, RL or optimum load resistance, essentially becomes 50 Ω. Therefore, the achievable output power is set by the voltage applied to the amplifier such that if we want to increase the output power, we need to increase the voltage applied to the amplifier. P_out = V_DD2 /2 x R_L

This is where GaN becomes very helpful, as we can quickly go from a 5 V supply voltage with GaAs to a 28 V supply voltage in GaN, and the achievable power goes from 0.25 W to almost 8 W simply by changing from GaAs to GaN technology. There will be other considerations to look at such as the gate length of the process available in GaN and if they can achieve the gain you need at the high frequency end of the band. As time progresses, more of these GaN processes become available.

The fixed R_L of 50 Ω for distributed amplifiers is different compared to the cascaded amplifier where we change the resistance value presented to the transistor by matching networks to optimize the power from the amplifier. There is a benefit in optimizing the resistance value presented to the transistor with cascade amplifiers in that it can improve the RF power.

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