Envelope Tracking for Efficient Power Amplifiers
A neat way to save your cell phone battery...
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We’ve all been there.
Our phone battery drops below 10%, and we put our phones on airplane mode to save up some juice for when we need it.
The power amplifier (PA) that is used to boost the radio signal before transmission is a major battery hog, and airplane mode shuts it down.
Envelope Tracking (ET) is a way to make these power amplifiers energy efficient.
In this article, you will learn the following:
Why conventional power amplifiers are inefficient when power is backed-off
What is the need for ET in modern communications
How ET conceptually works (without gory details)
What are the challenges in implementing ET
Let’s dive in!
Let's say we have a modulated RF signal going into a power amplifier (PA). Modulation is the process of encoding information into a high frequency signal, and is usually done by changing the amplitude or phase (or both) of the high frequency signal according to the information being encoded. The PA has a supply voltage that provides the DC power needed for amplification. The amplified signal is sent out to the antenna to be broadcast. Think of the power amplifier as a karaoke machine with a DC power adapter.
A key performance metric of a PA is its efficiency. It is a measure of how much output power is obtained relative to the DC power provided to it. The more the better. Nature usually takes its energy tax and efficiency is always lesser than 100%.
Consider a signal which is only phase modulated. Such signals were common in early communication systems such as GSM. A time-sampled versions of such a waveform is shown in Fig. 1(a) below. The waveform is said to have a constant envelope because you can draw an imaginary line connecting the peaks of the waveform that is a flat line. This signal is sent to the PA for amplification.
Compression and back-off efficiency
Notice that the peaks of the waveform stay within the limits of the power supply voltage. The advantage of the envelope not touching the power supply voltage is that the amplification will be linear. If the envelope touches the power supply voltage limit, the output waveform will appear squished at the top and bottom, and the result will be nonlinear. Too much nonlinearity and you may start to sound like the Terminator to the person on the other end.
The disadvantage of the extra room between the envelope and power supply voltage is that the excess is wasted power not being used for amplification and being dissipated as heat.
A simple solution is to increase the level of the input signal so that there is minimum overhead between the envelope and power supply, but still not distorting the output as shown in Fig. 1(b). Now you have minimum energy dissipated as heat, and efficiency is better. This is exactly what was done in early communication systems. The PA is said to running in compression, that is, a signal level that gives maximum efficiency.
Unfortunately, it is not possible to guarantee that the input signal is always at the optimum level. Sometimes it drops, and in that case, the PA is said to be operating in a power "back-off" mode. Now we again have wasted energy from excess overhead, and the PA efficiency drops.
Mo’ data rate, mo’ problems
But wait, there's more. Modern communication standards like 4G LTE target high data rates and this requires modulation of both amplitude and phase. Depending on the modulation scheme and end-application, the signal can have large peaks relative to the average level. The metric that describes this property of a modulated signal is Peak to Average Power Ratio (PAPR). It is the power of the signal peaks relative to the average power level, and can be as high as 10 times.
If you connect the peaks of this waveform with an imaginary line, the envelope is no longer flat. Such a continuously varying envelope is shown in Fig. 2(a). You can imagine the high frequency carrier signal to vary rapidly in the white region on the graph, such that by connecting the peaks of the signal, you get the envelope signal shown.
In this situation, increasing the level to accommodate the signal peaks (by running it in compression) is still applicable, but for portions of the waveform with low signal levels, there is still wasted energy. You also cannot risk distorting the signal by clipping the peaks too much to minimize wasted power.
Enter Envelope Tracking (ET).
Pushing the envelope by tracking it
Here's an analogy to ET. The best way to avoid wasted water while brushing your teeth is to shut off the faucet (or reduce it to a drip) and turn it on fully only when needed. The water is our energy and the faucet is the power supply to the PA. Turn on as much of the supply you need, when you need it - and you will keep your efficiency high.
We can no longer have a constant power supply voltage to the PA.
The only way to minimize wasted energy is to continuously change the supply level to just about accommodate the envelope signal. The power supply is now "tracking the envelope" of the input signal.
An important feature of ET is that even when the input signal level drops, the efficiency does not degrade like in the case of a fixed power supply PA because the supply voltage will track the lower envelope level ensuring minimal wasted energy. Generally an efficiency of 70-75% can be achieved with ET and it remains relatively constant in power back-off.
To continuously change the PA power supply, we need an "Envelope Amplifier" that takes the digital envelope information directly from the baseband signal processor (the thing thats doing the modulation to begin with) and provides a voltage level that tracks the envelope. Often, the envelope is reshaped for improved performance. Designing an envelope amplifier is a challenge all on its own. You have to guarantee that the voltage it provides can actually keep up with how quickly the envelope varies. If the envelope signal is rapidly changing, it is said to have a high signal bandwidth and the envelope amplifier design gets correspondingly harder.
Staying in sync
It is also absolutely critical that the envelope and the modulated input signal are perfectly synchronized. If they are not, then the envelope amplifier will end up providing too little voltage needed to cleanly amplify the input signal, or too much voltage degrading the efficiency. The linearity and efficiency of the PA are both affected.
To ensure time alignment, we can take a digitized sample of the amplified output signal envelope and compare it with the required input signal envelope. If there are differences, then it is possible to digitally alter the input signal envelope so that the output signal matches the desired input signal envelopes. This is called adaptive digital predistortion (DPD). Usually, this adds cost and complexity to the PA system, but as digital technology scales down, the energy overhead and overall cost of DPD is constantly dropping.
ET PAs have become a reality in cell phones ever since the envelope amplifier could be implemented in CMOS technology. Also, most cell phones today use a simplified form of ET called Average Power Tracking (APT) that tracks the average power instead of the envelope thus easing the requirements of the envelope amplifier. The power supply of your cell phone PA changes even on a second-to-second basis using a DC-DC converter chip to make sure your battery life is prolonged as much as possible.
⭐️Key Takeaways
For modulation schemes with relatively flat envelope, running a PA at compression gives good efficiency. In power back-off, efficiency drops.
Modern communication standards have highly dynamic signals, and simply running an amplifier in compression does not give good efficiency.
The power supply of the PA is continuously varied to track the envelope of the signal and minimize energy dissipation / increase efficiency.
It is challenging to track rapidly varying signal envelopes efficiently, and methods such as digital pre-distortion are employed for performance enhancements.
📚Resources
ET Comes of Age by Peter Asbeck and Zoya Popovic: A microwave magazine article from 2017 that gives a good explanation of why ET is required, how it works, and what the future for ET looks like. There is significant additional discussion on envelope amplifier design, and how design approaches differ for cellular versus basestation applications.
Push the Envelope by Bumman Kim et al.: A microwave magazine from 2013 that goes into significant detail about how to design ET PA systems with a focus on supply modulator design techniques, and provides design guidelines for ET PAs.
The views, thoughts, and opinions expressed in this newsletter are solely mine; they do not reflect the views or positions of my employer or any entities I am affiliated with. The content provided is for informational purposes only and does not constitute professional or investment advice.
Well written article and quite informative.
Thanks Vikram, this is a very good refreshing topic since college. Your points are very clear and easy to understand. I really appreciate it.