Using RF Envelope Detection to Implement Drain Modulation System

The wireless device industry strives to reduce device cost, size, and power consumption, and improving the power added efficiency (PAE) of high power amplifiers remains a challenging goal. There are many technologies under development. In most cases, the commercialization of any technology will depend on the development of breakthrough technologies. This article focuses on some of the technologies used to improve PAE and some of the RF signal processing blocks that support this technology.

Peak-to-average ratio

Figure 1 shows the temporal envelope of a 20MHz bandwidth orthogonal frequency division multiplexing (OFDM) signal. The signal consists of a large number of orthogonal QAM modulated subcarriers with relatively low symbol rates. OFDM-based wireless transmission is becoming increasingly popular, in part because the low symbol rate subcarriers are relatively insensitive to fading. It is currently used in wireless LAN and WiMax systems and will also be used in the next generation Long Term Evolution (LTE) mobile data and voice systems. Advanced OFDM systems allow the modulation of the subcarriers to change as operating and environmental conditions change. For example, if a user is located at the edge of a cell, the system may decide to modulate the subcarriers using quadrature phase shift keying, which requires a relatively low signal-to-noise ratio for successful demodulation. The trade-off is a relatively low data rate. On the other hand, if the user is closer to the center of the cell and requires a high data rate, a higher order modulated subcarrier can be transmitted, resulting in a higher data rate.

Figure 1: Time envelope of a 20MHz bandwidth OFDM carrier.

Higher order QAM signals such as 64QAM and 128QAM have high peak-to-average ratios, and OFDM signals can easily include 1024 subcarriers, so the peak-to-average ratio of OFDM signals is also high. This is clearly shown in Figure 1. You can also see from Figure 1 that the signal also has some deep valleys. So, although the peak-to-average ratio is generally discussed, we will see later that the peak-to-minimum ratio of the signal (which can be as high as 40dB) is also important when designing higher efficiency power amplifiers.

Figure 2 shows the most basic block diagram of a power amplifier system. The current supplied to the load is sourced from the high power amplifier (HPA) power supplies (±4V in this case). The rms output signal has an rms level (VRMS) and a peak level (VPEAK). For good signal fidelity, there must be enough headroom between the output signal and the power supplies so that the signal peaks are not clipped. This headroom requirement introduces a weakness in the system that can cause inefficiencies. If the signal has a high peak-to-average ratio, the power supplies must be biased to support the peak level instead of the rms level.

Figure 2: Power amplification of a high peak-to-average ratio signal.

Assume the output rms level is 1Vrms and the peak-to-average ratio of the signal is 4, or 12dB. This means that the peak of the signal is 4V and the peak-to-peak swing is 8V. Therefore, the absolute minimum supply voltage of the system may be ±4V (8V for a single-supply system). The power delivered to the load is equal to 20mW (1V×1V/50), and the load current is equal to 20mA. However, the power delivered by the power supply is equal to 8mW (4V×20mA). Therefore, the efficiency is only 25% (100×(20mW/80mW)).

While the above example is not truly representative of an actual system, it does illustrate that transmitting a high peak-to-average ratio signal will naturally reduce the efficiency of the power amplification system.