Envelope Tracking

Unlike fixed-supply amplifiers that must maintain a high constant voltage to accommodate peak power levels, ET modulates the supply voltage (Vcc) in real-time. The supply follows the signal's instantaneous envelope magnitude, ensuring the transistor operates near its compression point where efficiency is highest. This requires a high-bandwidth, high-efficiency supply modulator capable of tracking envelope variations with minimal distortion.

ET fundamentally addresses the power amplifier efficiency-linearity dilemma. By reducing the voltage headroom during low-amplitude signal periods, ET minimizes the power dissipated as heat. Key metrics improved include:

• Power-Added Efficiency (PAE): Can increase from 20-30% to over 50% in handset PAs
• Average power consumption: Reduced by 30-50% for high-PAPR signals like OFDM
• Thermal management: Lower junction temperatures extend device lifetime

However, the dynamic supply introduces its own non-linearities, often requiring a companion Digital Pre-Distortion (DPD) system for full linearization.

While both are dynamic supply techniques, they differ fundamentally:

• Envelope Tracking: The supply voltage tracks the envelope shape but the RF input signal retains its full amplitude and phase modulation. The PA operates in a quasi-linear mode.
• Envelope Elimination and Restoration (EER): The RF input is a constant-amplitude, phase-modulated signal (envelope eliminated), and the amplitude modulation is entirely applied through the supply voltage (restored).

ET is more tolerant of supply modulator bandwidth limitations and PA non-idealities, making it the preferred approach for modern wideband communications.

The supply modulator is the critical enabling component of an ET system. It must efficiently convert a fixed battery or system voltage to the dynamically varying PA supply. Key specifications include:

• Bandwidth: Must exceed the signal's envelope bandwidth, typically 1.5-3x the RF modulation bandwidth
• Efficiency: The modulator's own power loss directly subtracts from system gains; >80% efficiency is typically required
• Output voltage swing: Must cover the full range from minimum to peak PA voltage
• Slew rate: Determines the ability to track rapid envelope transitions

Hybrid architectures combining a linear amplifier with a switching converter are common to balance accuracy and efficiency.

The relationship between the instantaneous envelope magnitude and the applied supply voltage is defined by a shaping function. This is not a simple linear mapping. Common shaping strategies include:

• Iso-gain shaping: Maintains constant PA gain across all envelope levels by mapping supply voltage to keep the transistor at a fixed compression point
• Iso-ACLR shaping: Optimizes the mapping to meet spectral mask requirements while maximizing efficiency
• Peak-power tracking: A simplified approach where supply steps between discrete levels based on average power

The shaping function is stored in a Look-Up Table (LUT) and is calibrated per device to account for process variation.

ET introduces dynamic non-linearities that are functions of both the instantaneous input amplitude and the instantaneous supply voltage. This creates a multi-dimensional distortion problem. Modern systems employ joint ET-DPD architectures where:

• The DPD model includes the supply voltage as an additional input dimension
• Volterra-based models or neural networks capture the cross-dependence between signal envelope and supply modulation
• The DPD adapts to compensate for both the PA's inherent non-linearity and the distortion introduced by imperfect supply tracking

This co-optimization is essential for meeting stringent 5G and Wi-Fi 7 spectral requirements.

The definitive metric for quantifying ET benefits. PAE measures the net efficiency of a power amplifier by calculating the ratio of added RF output power (P_RF_out - P_RF_in) to DC input power.

• Formula: PAE = (P_RF_out - P_RF_in) / P_DC
• ET directly boosts PAE by reducing the DC power wasted as heat during low-envelope periods
• A typical Class-AB amplifier might achieve 20% PAE at 6 dB back-off; ET can push this beyond 45%
• PAE is the primary optimization target for battery-powered handsets and massive MIMO arrays

The fundamental signal characteristic that necessitates envelope tracking. PAPR is the ratio of a signal's peak power to its average power, expressed in dB.

• Modern OFDM waveforms (5G, Wi-Fi 6) exhibit PAPR values of 8-12 dB
• High PAPR forces fixed-supply amplifiers to operate at severe back-off, crushing efficiency
• ET decouples the supply voltage from the peak requirement, allowing the amplifier to operate near compression while the supply tracks the envelope
• Crest Factor Reduction (CFR) is often paired with ET to further reduce PAPR before the amplifier

The essential linearization partner to envelope tracking. While ET improves efficiency, the dynamic supply modulation introduces new non-linear distortion mechanisms that must be corrected.

• AM-AM and AM-PM distortion vary as a function of the instantaneous supply voltage
• Joint ET-DPD systems use 2D look-up tables or Volterra-based models indexed by both input amplitude and supply voltage
• Neural network DPD architectures excel at modeling the complex, multi-dimensional non-linearity of ET PAs
• Without DPD, the spectral regrowth from an ET system would violate Adjacent Channel Leakage Ratio (ACLR) regulatory masks

The dominant high-efficiency amplifier architecture that competes with and complements envelope tracking. A Doherty PA uses a main (carrier) amplifier and a peaking amplifier with an impedance inverter to achieve high efficiency at back-off.

• Doherty achieves efficiency peaks at two power levels; ET provides continuous efficiency improvement across the entire dynamic range
• ET-enhanced Doherty architectures combine both techniques for extreme efficiency in 5G base stations
• Doherty amplifiers exhibit severe memory effects and non-linearity, requiring advanced DPD regardless of ET implementation
• The choice between standalone ET, Doherty, or hybrid architectures depends on bandwidth, linearity, and cost constraints

The critical design constraint for ET systems. The envelope modulator must track the instantaneous envelope of the RF signal with high fidelity, requiring a bandwidth typically 3-5x the signal bandwidth.

• For a 100 MHz 5G NR carrier, the modulator may need 300-500 MHz tracking bandwidth
• Hybrid envelope tracking uses a slow, efficient switching converter in parallel with a fast, linear amplifier to meet bandwidth demands
• Modulator efficiency directly impacts overall system PAE; a 90% efficient modulator is essential for net efficiency gains
• Envelope shaping techniques deliberately reduce tracking bandwidth to trade marginal efficiency for practical modulator design

Dynamic supply modulation introduces unique memory effects beyond those found in fixed-supply amplifiers. The power amplifier's response depends not just on the instantaneous input and supply voltage, but on their recent history.

• Thermal memory: Transistor junction temperature varies with dissipated power, which changes dynamically with the supply voltage
• Electrical memory: Bias network impedances and decoupling capacitors create frequency-dependent supply paths
• Trapping effects: Semiconductor charge traps in GaN and LDMOS devices respond to changing drain voltages on microsecond timescales
• Modeling these effects requires generalized memory polynomial or recurrent neural network structures that include supply voltage as an additional input dimension