Power Envelope Tracking For Mobile Power Amplifiers

Have you heard the story about the guy who had to recharge his 4G phone twice a day? Unfortunately, he was not happy with his phone.

This story has been happening as the demand for high-speed data increases and battery capacity has not matched advances with communications technology. It’s not the battery at fault but the technology needed to power the amplifier on the phone. For years, normal DC-DC converters managed the battery power flowing in the phone to different chips. This included the power amplifier (PA) for driving the cellular signal back to the base station, which worked fine when the peak-to-average power ratio (PAPR) was relatively small for 2G and 3G signals. As the technology has progressed from GSM to GPRS to WCDMA to HSPA, the PAPR has increased progressively. Now LTE or 4G has a much higher PAPR that impacts the draw of power from the phone. In Figure 1 you can see this progression of technology for a typical PA output based on a given power input from a device battery.

The DC-DC converter pulls power from the device battery at a linear level at the peak of the signal power, which is not efficient. A better way to draw power is to anticipate the peaks of the cellular signal and supply only the needed power to the PA. This method of providing power is called power envelope tracking (ET).