envelope tracking pa...envelope elimination and restoration •a slight variation on the et scheme...
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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.
©James Buckwalter 2
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.
©James Buckwalter 3
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.
©James Buckwalter 4
Qualitative Explanation of Efficiency in ET System
• Continuously operating at different Vccs allows the peak efficiency at different power levels.
©James Buckwalter 5
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.
©James Buckwalter 6
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.
©James Buckwalter 7
Envelope Tracking Envelope Elimination and Restoration
Comparison of Efficiency for ET and EER
©James Buckwalter 8
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.
©James Buckwalter 9
Bridging ET and EER
• Burden of accurate output is placed on PA and supply modulator.
©James Buckwalter 10
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%.
©James Buckwalter 11
Instantaneous Supply Monitoring
• Tracking the drain voltage can be used to compare the signal to
©James Buckwalter 12
Overall Efficiency
• PA efficiency is only part of the story
• Supply modulator efficiency is also relevant.
©James Buckwalter 13
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?
©James Buckwalter 14
Signal Spectral Behavior
• Most of envelope power is at DC• Portionts of envelope power extend well beyond
2-3x BW.
©James Buckwalter 15
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.
©James Buckwalter 16
Switcher Waveform
Envelope Waveform
CMOS Implementation
• Dual switching amplifiers and linear amplifier
• CMOS and SOS ICs
©James Buckwalter 17
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