Achieve High Power Output from CMOS Power Amplifiers Using RF Power Combining Techniques

Key Takeaways

• Power amplifiers in wireless RF communication systems should be of high output power and high average efficiency. 

• Power amplifiers are also required to exhibit amplitude and phase linearity.

• The transformer-based RF power combining technique helps achieve high output power, high peak efficiency, power control abilities, and power amplifier linearization. 

Power amplifiers are crucial to any wireless communication transmitter chain

As RF and microwave wireless communication systems grow in popularity, it is increasingly important for the electronics market to strive to reduce the cost and size of circuits used in signal transmission systems. One way this can be achieved is through the use of power amplifiers. 

Power amplifiers are crucial to any wireless communication transmitter chain. The realization of power amplifiers using CMOS technology helps keep transmitter circuits low-cost and compact in size. However, it is difficult to attain high power output using CMOS power amplifiers, as the breakdown voltage of devices is low in such circuits. This drawback of CMOS technology means that RF power combining techniques are required to mitigate these issues. 

Using RF power combining techniques is a straightforward way to combine the power output of multiple power amplifiers to achieve the total required output power. These power combining techniques increase the reliability of wireless communication systems and help control power output. 

Let’s take a look at the basics of RF power combining and discuss the transformer-based power combining network.

The Advantages of RF Power Combining Techniques

Power amplifiers in wireless RF communication systems should be of high output power and high average efficiency. Power amplifiers should also exhibit amplitude and phase linearity. 

At RF frequencies, it is challenging to generate high power, especially with CMOS power amplifiers, as they have low breakdown voltage and high energy loss of on-chip impedance transformation. RF power combining techniques can be utilized to combine the output of several smaller power amplifiers into one single-ended output. 

There are two parts in a power combining structure:

1) Power amplifier block 

2) Combining block 

The output power from each power amplifier is combined using a combining network. The advantages of RF power combining techniques in a power combining structure are:

1. Combined high output power and high efficiency.

2. Higher efficiency at power back-off. 

3. Capable of power amplifier linearization. 

In the upcoming section, we will discuss the transformer-based RF power combining technique.

The Transformer-Based RF Power Combining Technique

The transformer-based RF power combining technique is a straightforward way to combine the power from several power amplifiers using transformers. At RF GHz frequencies, it is possible to design low-loss transformers to combine power from CMOS power amplifiers. These low-loss transformers are implemented in CMOS technology using one or two thick metal layers. Low-loss transformer-based power combining networks allow power control, as each stage works independently and the power amplifiers can be turned off at any instant. 

The output power can be increased by either increasing the number of parallel stages, N, or the turns ratio, m. The insertion loss increases with any increase in the turns ratio, m. Therefore, it is recommended to increase the parallel stages N rather than increasing m. 

The transformer-based RF power combining network supports power control and power back off. When one or more amplifiers are turned off, the total output power decreases. The rest of the active amplifiers operate close to their saturated output power, and these amplifiers exhibit high efficiency. This shows that the transformer-based RF power combining technique improves the efficiency of power amplifiers at lower output power levels. 

The transformer-based RF power combining technique offers power control and power amplifier linearization. By rapidly switching between different power levels, the amplitude information is reconstructed at the output by using either non-linear or saturated amplifiers. In short, the transformer-based RF power combining network helps to achieve high output power, high peak efficiency, power control abilities, and power amplifier linearization.

The challenges of designing high power output CMOS power amplifiers in wireless communication systems can be overcome by using RF power combining techniques, and this technique will continue to be a critical part of linearized transmitters. 

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