Envelope Tracking Basics: Amplifier, Power Supply, Advantages & Disadvantages

This tutorial covers the fundamentals of envelope tracking, explaining the concept with an envelope tracking power supply used for an envelope tracking power amplifier. It provides power according to the amplitude of the time domain input waveform. We will explore the advantages, disadvantages, and applications of envelope tracking.

About Envelope Tracking

Envelope tracking is a power supply technology. The system tracks the input of the amplifier and adjusts the supply voltage to be fed to the amplifier to obtain the required output power at any instant with optimum peak efficiency.

Constant DC supply voltage is used in conventional RF amplifiers, which usually operate in the compression region to achieve higher efficiency. This is suitable when the amplitude of the amplifier signal input is not varying, as is possible in frequency and phase modulated signals (FM and PM). In FM and PM modulation types, the amplitude of the modulated signal is constant.

In modern wireless communication systems such as 4G/LTE and higher versions of WLAN such as 802.11ac and 802.11ad, complex modulation schemes such as QAM are employed along with multi-carrier OFDM techniques to achieve higher data rates. In this situation, it is not desirable to operate the amplifier in the compression region as it leads to clipping of waveform amplitudes. This clipping results in a loss of information carried by the amplitude part of the time domain envelope. Therefore, running the amplifier in the compression region is not an option.

In this case, supply voltage is fed as per the time domain input waveform to the PA (Power Amplifier), i.e., a high supply is used during maxima points and a low supply during minima points. This results in the peak efficiency of the Power Amplifier, as desired.

Let’s understand the basic concept of envelope tracking with its system block diagram.

4G LTE Envelope Tracking Power Amplifier and Envelope Tracking Power Supply

Figure 1: Envelope Tracking System Block Diagram

Figure 1 depicts the envelope tracking system block diagram. As shown, it consists of a baseband IQ transmitter, envelope detector, shaping table as per LUT, RF upconverter, PA (Power amplifier), and envelope tracking power supply.

Figure 2: Waveform Without Envelope Tracking

Figure 2 depicts a waveform without envelope tracking. As shown, the DC supply voltage does not follow the RF power amplifier input, and hence there will be a gap between the supply and the amplifier signal input. This results in unnecessary voltage being pumped into the amplifier for its operation, leading to wasted power and consecutively lower efficiency of the PA (Power Amplifier). This is because efficiency is expressed as the ratio of the output power of the amplifier to the DC power input.

To avoid this situation, the circuit as shown in Figure 1 is used. In envelope tracking, the power supply input is constantly varied according to the envelope of the input power going to the PA. Due to envelope tracking, only the appropriate power supply is fed at the input as required, which makes the amplifier operate at peak efficiency for the desired instantaneous power output specifications.

Both the DC supply voltage and amplifier input should be time synchronized in order to obtain alignment; otherwise, there will be a mismatch between the supply needed and the amplifier input. This is achieved by providing a trigger input to the RF VSG from the same AWG used to generate the variable supply voltage for the power amplifier.

Figure 3: DPD LUT technique

As we know that due to AM-AM and AM-PM distortion, the PA does not always have a linear response. This results in the system not meeting the EVM and ACPR specifications as desired. Due to this, pre-distortion compensation is essential, and this is achieved using an LUT-based approach.

Figure 3 depicts the DPD (Digital Pre Distortion) LUT concept. As shown, LUT coefficients are derived based on the estimated PA nonlinear response. As the LUT output response negates the nonlinear response of the PA, a linear response can be achieved.

Keysight provides test equipment which can be used for testing the envelope tracking concept. The models 33500 or 33600 series are used as AWG. AWG is a baseband IQ generator. An X-series generator or PXIe-M9381A is used as VSG (Vector Signal Generator). The output of the ETPA (Envelope Tracking Power Amplifier) can be measured using the M9391A PXIe VSA hardware and software application (89600 VSA).

Let us go through the advantages and disadvantages of this technique as part of the envelope tracking tutorial.

Benefits or Advantages of Envelope Tracking

Following are the advantages of envelope tracking:

• Improves the efficiency of the amplifier.
• Increases battery life as the battery drains slowly.
• Reduces running costs of wireless system infrastructure due to improvement in efficiency.
• Meets ACPR and EVM requirements of complex systems such as 4G LTE, LTE Advanced, and 5G having complex OFDM/OFDMA multi-carrier systems along with QAM modulation types such as 1024-QAM, 2048-QAM, etc.

Disadvantages of Envelope Tracking

The envelope tracking power supply used in the envelope tracking should meet the following requirements:

• The bandwidth (BW) of the supply should be 2 to 3 times greater than the modulation bandwidth of the RF signal. Due to this, a 4G LTE system needs a supply BW of about 50 MHz.
• The supply output should have very low noise, as this directly affects the adjacent channel noise. This causes noise in the receive band, which masks the wanted signal.
• It should have higher efficiency; otherwise, the gains in using envelope tracking are lost.
• Other than the above power supply requirements, the supply control signal and RF input envelope should be time-aligned to achieve perfect envelope tracking. To achieve time synchronization, a delay balancer is introduced in the chain.
• The above-mentioned rigorous envelope tracking power supply requirements are essential in an envelope tracking power amplifier-based wireless system.

Applications

Envelope tracking is used in a wide variety of applications.

• 4G LTE low power Mobile phone handsets
• High power cellular Base Stations or eNodeBs or eNBs
• RF power Amplifiers