In the past two weeks, there have been some really neat product introductions that can greatly enhance Envelope Tracking (ET) viability in RF Power Amplifiers (PAs).
EPC's eGaN high speed FETs
First, Alex Lidowís company, Efficient Power Conversion (EPC), brought out the EPC8000 eGaN FET transistor series. This ultra-high frequency milestone now enables RF and power system designers with a power device that switches into the GHz region. EPCís packageless technology even makes for better performance since package parasitics cause significant signal degradation at these frequencies. See Figure 1.
The mounting side of the EPC8000 series eGaN FET is shown in this packageless design.
EPC has an application note, AN015 that outlines a 10MHz ET converter design that uses a buck converter with eGaN high speed power FETs. See Figure 2.
An ET buck converter output stage using eGaN high speed power FETs
EPC recommends using TI's LM5113 gate drivers to provide sufficient drive for the eGaN FETs.
ET DC/DC converter
The other new development is the Texas Instruments ET DC/DC converter for smart phones that will get more life out of the handset batteries. See Figure 3.
The LM3290 with integrated boost converter along with the high speed linear amplifier LM3291, make up an RF envelope supply modulator for ET.
(Source: Texas Instruments)
So we see that essentially, the heart of the design for an ET system is an efficient power supply modulator that controls the RF Power Amplifier supply voltage with the envelope of the input signal that is being transmitted from the baseband/RF up-converter.
I have found that there are some other techniques within this ET framework. For example, using two high efficiency buck switching stages which are coordinated to provide the wideband envelope power to the RF stage. A wide bandwidth linear regulator is also used at low power to maintain the envelope signal accuracy1. See Figure 4.
GaN on diamond
A block diagram of the ET system from Reference 
In April 2013, Triquint Semiconductor announced that it had produced the first GaN-on-diamond wafers that greatly reduce the transistor temperature while maintaining high speed performance. Envelope tracking applications as well as other high speed/high power uses will benefit from this effort. While this is a great improvement in thermal design, can it be integrated into the more complete ET architecture? That remains to be seen. Innovations are happening every day in our industry and old paradigms are being cast aside.
Feedforward architecture for an ET system2
Another possible method is an ET power supply architecture composed of a multilevel converter in series with a linear amplifier. The multilevel converter provides the bulk of the load power with its high efficiency, and the linear amplifier, with high bandwidth, improves the linearity of the output voltage. A feed-forward scheme like this may cancel the spikes in the output voltage caused by the step-wave voltage of the multilevel converter. See Figure 5.
Envelope Amplifier (EA) using multiple linear regulators for ET
Schematic and control method for the ET system in reference 
For a larger view, click here.
An interesting idea is shown in Reference  in which an EA can achieve a higher efficiency over a wide envelope range utilizing multiple linear regulators. The linear regulators are biased as multiple supply voltages and are responsible for delivering a partial envelope current, ensuring high efficiency. Figure 6 shows a linear regulator array and a DC feeding line or bus.
The EA with multiple linear regulators from Reference 
For a larger view, click here.
The paralleled linear regulators (PLRs) are biased separately with the appropriate supply voltage converted by high efficient buck converter. The voltage drop across any one LR is small so power dissipation is low and efficiency is high.
There are many other techniques to improve the highly inefficient PA in base stations and handsets. Some functions can be integrated and some may not be so conducive to integration depending upon the optimum process for a particular function. What experience have you had with PA efficiency enhancement? Please share your thoughts.
- Wideband High Efficiency Digitally-Assisted Envelope Amplifier with Dual Switching Stages for Radio Base-Station Envelope Tracking Power Amplifiers, Chin Hsia, Donald F. Kimball, Sandro Lanfranco and Peter M. Asbeck. 2010
- Feed-Forward Scheme Considering Bandwidth Limitation of Operational Amplifiers for Envelope Tracking Power Supply Using Series-Connected Composite Configuration, Huan Xi, Qian Jin, and Xinbo Ruan, September 2013
- Envelope Amplifier With Multiple-Linear Regulator for Envelope Tracking Power Amplifier,
Joon Hyung Kim, Hyuk Su Son, Woo Young Kim, Chul Soon Park. 2013.