ENVELOPE TRACKING PA...Envelope Elimination and Restoration •A slight variation on the ET scheme is to attempt to operate the PA at a peak amplitude. •The phase and envelope are

ENVELOPE TRACKING PA

©James Buckwalter 1

Average Efficiency

• We recognize the importance of average efficiency.

• However, PA design –to now- has focused on peak efficiency.

• Other techniques should be developed to provide peak efficiency over a range of power levels.


Insight into Loadline for High Efficiency

• For Doherty and outphasing, the loadline is increased to improve the efficiency.

• The loadline can be kept constant if both the voltage swing and the current swing are changed proportionally.


Envelope Tracking

• Dynamic adjustment of drain voltage

• Maximizes efficiency by keeping transistor near saturation under all amplitude levels

• PA operates in a quasi linear regime.


Qualitative Explanation of Efficiency in ET System

• Continuously operating at different Vccs allows the peak efficiency at different power levels.


Qualitative Explanation of Efficiency in ET System

• Power dissipation is wasted in the fixed supply voltage. If the supply can be modulated the dissipation is minimized since the transistor can be kept within a few dB of the peak output power.


Envelope Elimination and Restoration

• A slight variation on the ET scheme is to attempt to operate the PA at a peak amplitude.

• The phase and envelope are separated and independently used to drive the gate and drain.


Envelope Tracking Envelope Elimination and Restoration

Comparison of Efficiency for ET and EER


Comparison of Techniques

• EER– Potentially highest

efficiency– Requires very accurate

dynamic supply– Input drive power is higher– Leakage of signal at low

output power is problematic

– Time alignment is critical between phase and amplitude

– Phase modulation has high bandwidth.

• ET– Good efficiency– Reduced supply accuracy– Lower input driver power– Less leakage of signal at

low output power– Requires linearization or

digital correction.


Bridging ET and EER

• Burden of accurate output is placed on PA and supply modulator.


Advantages:1) High efficiency2) Excellent thermal management3) Broadly tunable

Challenges:1) Supply amplifier must be high

efficiency, broad bandwidth, and low cost

2) RF stage should have high gain under a large range of supplies

Example: ET PA with Switcher

• Consider a 6.6 dB WCDMA signal with an average efficiency of 62%. The PAE of the amplifier is above 80%.


Instantaneous Supply Monitoring

• Tracking the drain voltage can be used to compare the signal to


Overall Efficiency

• PA efficiency is only part of the story

• Supply modulator efficiency is also relevant.


h =hPAhMOD

Modulator Efficiency

• Modulator efficiency depends on the time scales of the signals

• Fast tracking involves tracking high-frequency envelope components

• Slow tracking involves lower frequency components.

• What is signal component?


Signal Spectral Behavior

• Most of envelope power is at DC• Portionts of envelope power extend well beyond

2-3x BW.


Share burden

• Use a linear and switching stage.

• Linear stage accurately reproduces the high-frequency signal components

• Switching stage reproduces the low-frequency signal components.


Switcher Waveform

Envelope Waveform

CMOS Implementation

• Dual switching amplifiers and linear amplifier

• CMOS and SOS ICs


Documents

ENVELOPE TRACKING PA...Envelope Elimination and Restoration •A slight variation on the ET scheme is to attempt to operate the PA at a peak amplitude. •The phase and envelope are